Showing papers on "Alternative splicing published in 1989"
TL;DR: Current progress in the immunological measurement of the IGF is reviewed including different strategies for avoiding binding protein interference and an attempt is made to indicate circumstances in which the IGFs are exerting their actions in an autocrine/paracrine mode and when endocrine actions predominate.
Abstract: There is currently widespread interest in the IGFs (IGF-I and IGF-II) and their roles in the regulation of growth and differentiation of an ever increasing number of tissues are being reported. This selective review focused on the current state of our knowledge about the structure of mammalian IGFs and the multiple forms of mRNAs which arise from alternative splicing and promoter sites which arise from gene transcription. Current progress in the immunological measurement of the IGF is reviewed including different strategies for avoiding binding protein interference. The results of measurements of serum IGF-I and IGF-II in fetus and mother and at various stages of postnatal life are described. Existing knowledge of the concentration of these peptides in body fluids and tissues are considered. Last, an attempt is made to indicate circumstances in which the IGFs are exerting their actions in an autocrine/paracrine mode and when endocrine actions predominate. In the latter context it was concluded that an important role for GH action on skeletal tissues via hepatic production of IGF-I and endocrine action of IGF-I on growth cartilage is likely.
1,852 citations
TL;DR: It is shown that the gene for the D2 receptor produces two receptor isoforms by alternative messenger RNA splicing, providing a route to receptor diversity in this family of receptors.
Abstract: Dopamine receptors are classified into D1 and D2 subtypes on the basis of their pharmacological properties and the intracellular responses they mediate. The cerebral D2 dopamine receptor is the target of drugs used to alleviate the main symptoms of schizophrenia. Although it is considered to be a single molecular entity, there is evidence that multiple D2-receptor subtypes exist. A complementary DNA encoding a D2 receptor has recently been cloned and the deduced 415-amino-acid sequence indicates that it belongs to the large superfamily of receptors coupled to G proteins, and that its topology consists of seven transmembrane domains. In this family, the genes are frequently without introns and each is believed to encode a unique polypeptide product. Here we show that the gene for the D2 receptor produces two receptor isoforms by alternative messenger RNA splicing, providing a route to receptor diversity in this family. One isoform corresponds to the D2(415) receptor, but the second contains an additional sequence encoding a 29-amino-acid fragment, defining a novel D2(444) receptor isoform. Expression of the two isoforms is tissue-specific, and both are regulated by guanyl nucleotides. As the extra sequence is located within a putative cytoplasmic loop that binds to G proteins, the two isoforms might interact with different G proteins and thereby initiate distinct intracellular signals.
705 citations
TL;DR: In this review, the discussion of mechanisms attempts to be exhaustive, drawing not only upon direct experimental investigations of alternatively spliced genes, but also upon relevant themes derived from the study of constitutive splicing.
Abstract: In this review, we focus upon the mechanistic, functional, and evolutionary aspects of alternative splicing. The discussion of mechanisms attempts to be exhaustive, drawing not only upon direct experimental investigations of alternatively spliced genes, but also upon relevant themes derived from the study of constitutive splicing
641 citations
TL;DR: The identification and cloning of a cDNA encoding an RNA splice variant of the rat D2 receptor cDNA is reported, which codes for a receptor isoform which is predominantly expressed in the brain, and contains an additional 29 amino acids in the third cytoplasmic loop, a region believed to be involved in G protein coupling.
Abstract: DOPAMINE receptors belong to a large class of neurotransmitter and hormone receptors that are linked to their signal transduction pathways through guanine nucleotide binding regulatory proteins (G proteins). Pharmacological, biochemical and physiological criteria have been used to define two subcategories of dopamine receptors1 referred to as D1 and D2. D1 receptors activate adenylyl cyclase2 and are coupled with the Gs regulatory protein3. By contrast, activation of D2 receptors results in various responses including inhibition of adenylyl cyclase4, inhibition of phosphatidylinositol turnover5, increase in K+ channel activity6 and inhibition of Ca2+ mobilization7. The G protein(s) linking the D2 receptors to these responses have not been identified, although D2 receptors have been shown to both copurify8,9 and functionally reconstitute10,11 with both Gi and Go related proteins3. The diversity of responses elicited by D2-receptor activation could reflect the existence of multiple D2 receptor subtypes, the identification of which is facilitated by the recent cloning of a complementary DNA encoding a rat D2 receptor12. This receptor exhibits considerable amino-acid homology with other members of the G protein-coupled receptor superfamily12,13. Here we report the identification and cloning of a cDNA encoding an RNA splice variant of the rat D2 receptor cDNA12. This cDNA codes for a receptor isoform which is predominantly expressed in the brain, and contains an additional 29 amino acids in the third cytoplasmic loop, a region believed to be involved in G protein coupling.
612 citations
TL;DR: Results from polymerase chain reactions as well as in situ hybridization revealed that mRNA encoding both receptor forms is present in pituitary and brain of both rat and man, and the two receptor forms are generated by differential splicing possibly to permit coupling to different G proteins.
Abstract: Cloned human dopamine D2 receptor cDNA was isolated from a pituitary cDNA library and found to encode an additional 29 amino acid residues in the predicted intracellular domain between transmembrane regions 5 and 6 relative to a previously described rat brain D2 receptor. Results from polymerase chain reactions as well as in situ hybridization revealed that mRNA encoding both receptor forms is present in pituitary and brain of both rat and man. The larger form was predominant in these tissues and, as shown in the rat, expressed by dopaminergic and dopaminoceptive neurons. Analysis of the human gene showed that the additional peptide sequence is encoded by a separate exon. Hence, the two receptor forms are generated by differential splicing possibly to permit coupling to different G proteins. Both receptors expressed in cultured mammalian cells bind [3H]spiperone with high affinity and inhibit adenylyl cyclase, as expected of the D2 receptor subtype.
601 citations
TL;DR: Hydropathy analysis suggests the existence of seven transmembrane domains that show homology with the corresponding regions of other G protein-coupled receptors.
Abstract: Complementary DNA clones, encoding the LH-hCG (luteinizing hormone-human choriogonadotropic hormone) receptor were isolated by screening a lambda gt11 library with monoclonal antibodies. The primary structure of the protein was deduced from the DNA sequence analysis; the protein contains 696 amino acids with a putative signal peptide of 27 amino acids. Hydropathy analysis suggests the existence of seven transmembrane domains that show homology with the corresponding regions of other G protein-coupled receptors. Three other types of clones corresponding to shorter proteins were observed, in which the putative transmembrane domain was absent. These probably arose through alternative splicing. RNA blot analysis showed similar patterns in testis and ovary with a major RNA of 4700 nucleotides and several minor species. The messenger RNA was expressed in COS-7 cells, yielding a protein that bound hCG with the same affinity as the testicular receptor.
570 citations
TL;DR: The results support the conclusion that para encodes a functionally predominant class of sodium channels in Drosophila neurons and the para transcript appears to undergo alternative splicing to produce several distinct subtypes of this channel.
519 citations
TL;DR: The finding that the pattern of FN splicing during wounds resembles an embryonic pattern suggests that alternative splicing may be used during wound healing as a mechanism to generate forms of FN that may be functionally more appropriate for the cell migration and proliferation associated with tissue repair.
Abstract: The adhesive extracellular matrix glycoprotein fibronectin (FN) is thought to play an important role in the cell migration associated with wound healing. Immunolocalization studies show abundant FN in healing wounds; however, these studies cannot define the cellular site(s) of FN synthesis, nor do they distinguish the different and potentially functionally distinct forms of FN that can arise from alternative splicing of the primary gene transcript. To examine these questions of FN synthesis and splicing during wound healing, we have performed in situ hybridization with segment-specific probes on healing wounds in adult rat skin. We find that the FN gene is expressed at increased levels after wounding both in the cells at the base of the wound and in subjacent muscle and dermis lateral to the wound. Interestingly, however, the pattern of splicing of FN mRNA was different in these areas. In adjacent dermis and muscle, the splicing pattern remains identical with that seen in normal adult rat skin, with two of the three spliced segments (EIIIA and EIIIB) excluded from FN mRNA. In contrast, these two segments are included in the FN mRNA present in the cells at the base of the wound. As a result, the mRNA in this region is spliced in a pattern identical with that found during early embryogenesis. The finding that the pattern of FN splicing during wound healing resembles an embryonic pattern suggests that alternative splicing may be used during wound healing as a mechanism to generate forms of FN that may be functionally more appropriate for the cell migration and proliferation associated with tissue repair.
483 citations
TL;DR: In this paper, a human D2 dopamine receptor was sequenced from a pituitary cDNA library and sequenced with the deduced protein sequence is 96% identical with that of the cloned rat receptor with one major difference: the human receptor contains an additional 29 amino acids in its putative third cytoplasmic loop.
Abstract: A clone encoding a human D2 dopamine receptor was isolated from a pituitary cDNA library and sequenced. The deduced protein sequence is 96% identical with that of the cloned rat receptor with one major difference: the human receptor contains an additional 29 amino acids in its putative third cytoplasmic loop. Southern blotting demonstrated the presence of only one human D2 receptor gene. Two overlapping phage containing the gene were isolated and characterized. DNA sequence analysis of these clones showed that the coding sequence is interrupted by six introns and that the additional amino acids present in the human pituitary receptor are encoded by a single exon of 87 base pairs. The involvement of this sequence in alternative splicing and its biological significance are discussed.
475 citations
TL;DR: In the fruit-fly Drosophila melanogaster, differential splicing of a hierarchy of regulatory genes determines sex, and thus the molecular biology of sex determination in the Fruit-fly may lead to insights into the mechanisms by which alternative splicing is regulated.
Abstract: The discovery that the primary transcripts of many genes are processed to produce more than one kind of messenger RNA focuses attention on the control of RNA processing as a developmental regulatory mechanism. In the fruit-fly Drosophila melanogaster, differential splicing of a hierarchy of regulatory genes determines sex, and thus the molecular biology of sex determination in the fruit-fly may lead to insights into the mechanisms by which alternative splicing is regulated.
462 citations
TL;DR: Alternative splicing of the erbAα transcript thus generates products with opposing biological activities, suggesting a novel mechanism for the modulation of hormonal responsiveness, and shows here that expression of r-erbAα2 inhibits the T3-dependent inductive effect of either r- DerbAβ or r-urbAαl on expression of a T 3-responsive test gene.
Abstract: Thyroid hormone (T3) binds to a nuclear receptor protein which regulates gene expression by binding to specific DNA sequences near hormone-responsive genes. Proteins encoded by two cellular proto-oncogenes, c-erbA alpha and beta, bind T3 and can act as functional T3 receptors. In rats, alternative splicing of the alpha-gene transcript generates at least two distinct protein products, termed r-erbA alpha 1 and r-erbA alpha 2. Although these proteins bind to the same DNA sequence, r-erbA alpha 2 does not bind T3. We show here that expression of r-erbA alpha 2 inhibits the T3-dependent inductive effect of either r-erbA beta or r-erbA alpha 1 on expression of a T3-responsive test gene. Alternative splicing of the erbA alpha transcript thus generates products with opposing biological activities, suggesting a novel mechanism for the modulation of hormonal responsiveness.
TL;DR: The results suggest that the mechanism for production of the rat serum GH-binding protein is by alternative splicing of the gene for the rat GH receptor.
Abstract: A cDNA clone isolated from rat liver was demonstrated to encode a soluble, secreted growth hormone (GH)binding protein consistent with the properties of the newly discovered serum GH-binding protein. The protein coding region of this cDNA was identical in sequence to the extracellular domain of the rat liver GH receptor up to three amino acids before the putative transmembrane domain. At this point, an additional 17 amino acids were encoded in the GH-binding protein before a stop codon was encountered. This cDNA clone was shown to be representative of the structure of the mRNA present in rat liver. These results suggest that the mechanism for production of the rat serum GH-binding protein is by alternative splicing of the gene for the rat GH receptor.
TL;DR: A human thyroid cDNA library was screened by hybridization with a dog thyrotropin receptor (TSHr) cDNA, suggesting that alternative splicing could generate multiple forms of human TSHr.
TL;DR: The results show that while in normal, adult, human tissues total FN has a widespread distribution, the B-FN isoform is restricted only to synovial cells, to some vessels and areas of the interstitium of the ovary, and to the myometrium, and the results demonstrate that, in vivo, different FN isoforms have a differential distribution.
Abstract: Fibronectin (FN) represents the mixture of a number of structurally different molecules (isoforms) whose make-up varies depending on the FN sources. FN from cultured transformed human cells has a very different isoform composition with respect to its normal counterpart. In fact, SV-40-transformed WI-38VAI3 human fibroblasts produce high levels of a FN isoform (B-FN) which is very poorly expressed in their normal, WI-38, counterpart. We have recently demonstrated that the B-FN isoform derives from a differential splicing pattern of the FN primary transcript which leads, in transformed cells, to a high level expression of the exon ED-B (Zardi, L., B. Carnemolla, A. Siri, T. E. Petersen, G. Paolella, G. Sebastio, and F. E. Baralle. 1987. EMBO (Eur. Mol. Biol. Organ.) J. 6:2337-2342). Here we report on the production and characterization of a monoclonal antibody (BC-1) which recognizes an epitope within the protein sequence coded for by the ED-B exon. This monoclonal antibody makes it possible to carry out immunohistochemical analysis of the distribution of the ED-B-containing FN isoform (B-FN) in human tissues. The results show that while in normal, adult, human tissues total FN has a widespread distribution, the B-FN isoform is restricted only to synovial cells, to some vessels and areas of the interstitium of the ovary, and to the myometrium. On the contrary, the B-FN isoform has a much greater expression in fetal and tumor tissues. These results demonstrate that, in vivo, different FN isoforms have a differential distribution and indicate that the B-FN isoform may play a role in ontogenesis and oncogenetic processes.
TL;DR: Analysis of the clones indicates that bovine vascular endothelial growth factor can exist in two forms, probably due to alternative RNA splicing, and shows that VEGF shares homologies of about 21% and 24% respectively with the A and B chains of human platelet-derived growth factor (PDGF).
TL;DR: It is found that the first exon of the human dystrophin transcript is different in brain and muscle, indicating that dyStrophin expression could be differentially regulated in these tissues by usage of distinct promoters.
Abstract: DYSTROPHIN is the protein product of the Duchenne/Becker muscular dystrophy locus. It has a relative molecular mass of 427,000 and is encoded by a large RNA transcript processed from more than 65 exons spread over two million base pairs of the human X chromosome1–5. We have used the polymerase chain reaction6 to see whether any of these exons are used alternatively in the different tissues that express dystrophin. As reported for rat dystrophin7, we find that the first exon of the human dystrophin transcript is different in brain and muscle, indicating that dystrophin expression could be differentially regulated in these tissues by usage of distinct promoters. The 3′ end of the dystrophin transcript can be alternatively spliced to create numerous isoforms differing at their carboxy 1 domains; this is the only domain of dystrophin that does not share any similarity with the related cytoskeletal α-actinins3. These alternative transcripts yield dystrophin molecules which may interact with different proteins of the tissues expressing dystrophin.
TL;DR: Alternative splicing of a 36 base pair exon, exon 11, generates hINSR transcripts encoding receptor isoforms which differ in sequence at the C-terminal end of the insulin-binding alpha-subunit.
TL;DR: Four major tau protein isoforms isolated from bovine brain were identified by comparison with translation products of cDNA constructs and the use of antisera raised against synthetic peptides to provide reagents and a basis for analyzing potentially altered forms of tau proteins in brains of patients with Alzheimer's disease.
Abstract: Tau, a major class of microtubule-associated proteins, consists of a family of proteins that are heterogeneous in molecular weight. The presence of internal deletions in previously described cDNA clones for murine and bovine tau suggested that alternative splicing of transcripts could account for the protein size heterogeneity. Analysis of the exon-intron structure of the bovine tau gene provided sequence information necessary to detect new variants of tau transcripts by in vitro amplification techniques. The variant transcripts found corresponded to mRNA species missing one or more exons, which suggested that by skipping various exons during mRNA splicing, a family of proteins is generated. Four major tau protein isoforms isolated from bovine brain were identified by comparison with translation products of cDNA constructs and the use of antisera raised against synthetic peptides. These studies provide reagents and a basis for analyzing potentially altered forms of tau proteins in brains of patients with Alzheimer's disease.
TL;DR: Although the functional significance of alternative splicing of the insulin receptor mRNA is unknown, differential expression of these two receptor mRNAs may provide a structural basis for previously observed tissue-specific differences in insulin binding and action.
Abstract: Two previously reported insulin receptor cDNA sequences differ by 36 base pairs (bp) in the distal alpha-subunit, suggesting that alternative mRNA splicing within the coding region may occur (two insulin receptor isoforms). We developed a quantitative modification of the polymerase chain reaction technique in order to detect and characterize differential mRNA splicing at this site within the distal alpha-subunit. Using RNA derived from a variety of human cell types, we detected two polymerase chain reaction-amplified cDNA species reflecting the presence or absence of the above 36 nucleotides. Identity of the two cDNA species was confirmed by Southern blots, the use of a BANI restriction site present only in the 36 base pair segment and dideoxy sequencing. The relative expression of the two mRNA forms varied markedly in a tissue-specific manner. Buffy coat leukocytes and Epstein-Barr virus-transformed lymphocytes express only the shorter mRNA. Placenta expresses both species equally; muscle, isolated adipocytes and cultured fibroblasts express somewhat more of the longer mRNA (relative ratios of mRNA abundance of 1.51, 3.18, and 2.77, respectively); liver expresses mostly the longer mRNA (relative ratio of 9.8). In RNA derived from cultured and fresh cells from patients with several states of insulin resistance, the relative expression of the two mRNA species was similar to results obtained with comparable normal tissues. Although the functional significance of alternative splicing of the insulin receptor mRNA is unknown, differential expression of these two receptor mRNAs may provide a structural basis for previously observed tissue-specific differences in insulin binding and action.
TL;DR: The structural heterogeneity of the human low affinity receptor for IgG, FcRII(CD32), has been elucidated through the isolation, characterization, and expression of cDNA clones derived from myeloid and lymphoid RNA that predict amino acid sequences consistent with integral membrane glycoproteins with single membrane spanning domains.
Abstract: The structural heterogeneity of the human low affinity receptor for IgG, FcRII(CD32), has been elucidated through the isolation, characterization, and expression of cDNA clones derived from myeloid and lymphoid RNA. These clones predict amino acid sequences consistent with integral membrane glycoproteins with single membrane spanning domains. The extracellular domains display sequence homology to other Fc gamma Rs and members of the Ig supergene family. A minimum of three genes (Fc gamma RIIa, IIa', and Fc gamma RIIb) encode these transcripts, which demonstrate highly related extracellular and membrane spanning domains. IIa/IIa' differ substantially in the intracytoplasmic domain from IIb. Alternative splicing of the IIb gene generates further heterogeneity in both NH2- and COOH-terminal domains of the predicted proteins. Comparison to the murine homologues of these molecules reveals a high degree of conservation between the products of one of these genes, Fc gamma RIIb, and the murine beta gene in primary sequence, splicing pattern, and tissue distribution. In contrast, the sequence of IIa' indicates its relationship to the beta-like genes, with mutation giving rise to a novel cytoplasmic domain, while IIa is a chimera of both alpha- and beta-like genes. Expression of these cDNA molecules by transfection results in the appearance of IgG binding molecules that bear the epitopes defined by the FcRII(CD32) mAbs previously described.
TL;DR: Results are consistent with a model in which female-specific factors block the function of the non-sex-specific 3' splice site, and eliminate Sex-lethal-induced regulation of RNA from the Drosophila transformer gene.
TL;DR: Shab, Shaw, and Shal genes were identified in Drosophila using a Shaker complementary DNA probe and low-stringency hybridization as mentioned in this paper, which suggests the presence of a family of Shaker-like genes.
Abstract: Mutant flies in which the gene coding for the Shaker potassium channel is deleted still have potassium currents similar to those coded by the Shaker gene. This suggests the presence of a family of Shaker-like genes in Drosophila. By using a Shaker complementary DNA probe and low-stringency hybridization, three additional family members have now been isolated, Shab, Shaw, and Shal. The Shaker family genes are not clustered in the genome. The deduced proteins of Shab, Shaw, and Shal have high homology to the Shaker protein; the sequence identity of the integral membrane portions is greater than 50 percent. These genes are organized similarly to Shaker in that only a single homology domain containing six presumed membrane-spanning segments common to all voltage-gated ion channels is coded by each messenger RNA. Thus, potassium channel diversity could result from an extended gene family, as well as from alternate splicing of the Shaker primary transcript.
TL;DR: It is concluded that expression of SM1 and SM2 are differentially regulated during development and that a third type of MHC isoform may exist in embryonic and perinatal vascular smooth muscles.
TL;DR: The data demonstrate that the change in IGF-I and IGF/I receptor mRNA levels during fasting is quantitatively different in different tissues and suggest that regulation of IGF-i and IGF- I receptor gene expression by fasting is discoordinate.
Abstract: We have examined, in liver and extrahepatic tissues, the effects of fasting on total insulin-like growth factor I (IGF-I) mRNA levels, on levels of different IGF-I mRNAs generated by alternative splicing of the primary IGF-I transcript, and on IGF-I receptor binding and mRNA levels. A 48-h fast decreased total IGF-I mRNA levels by approximately 80% in lung and liver, approximately 60% in kidney and muscle, and only approximately 30-40% in stomach, brain, and testes. In heart, IGF-I mRNA levels did not change. The levels of the different splicing variants, however, were essentially coordinately regulated within a given tissue. Specific 125I-IGF-I binding in lung, testes, stomach, kidney, and heart was increased by fasting by approximately 30-100%, whereas in brain 125I-IGF-I binding did not change in response to fasting. In tissues in which fasting increased IGF-I receptor number, receptor mRNA levels increased approximately 1.6- to 2.5-fold, whereas when IGF-I receptor number was unchanged in response to fasting, receptor mRNA levels did not change. These data demonstrate that the change in IGF-I and IGF-I receptor mRNA levels during fasting is quantitatively different in different tissues and suggest that regulation of IGF-I and IGF-I receptor gene expression by fasting is discoordinate.
TL;DR: The results suggest that the varied actions of PRL may involve multiple receptors that are part of distinct signal transduction pathways.
Abstract: We have characterized the PRL receptor (PRL-R) present in mouse liver by purification, cross-linking, and immunological analysis of the protein, and by the isolation of PRL-R cDNA clones. Analysis of the cDNA clones indicates that the liver receptor is actually a family of proteins. Two of these proteins are predicted to be synthesized as precursors of 303 and 292 amino acids, with common signal sequences, extracellular domains, and transmembrane domains; a portion of their cytoplasmic domains are also identical, but these proteins differ markedly in the terminal region of this domain. A third PRL-R protein is predicted to be a truncated form and may be secreted. These multiple PRL-R mRNAs appear to be encoded by at least two genes, with the sequence variation for the two full-length proteins likely due to alternative RNA splicing. These results suggest that the varied actions of PRL may involve multiple receptors that are part of distinct signal transduction pathways.
TL;DR: It is suggested that it is the acceptor site which is specified relative to the branch point, which provides a simple mechanistic basis for strict mutually exclusive splicing.
TL;DR: The results suggest that FN splicing is used during development as a mechanism to create different forms of FN within the extracellular matrix by the inclusion or exclusion of specific segments.
Abstract: The primary gene transcript for the adhesive extracellular matrix glycoprotein fibronectin (FN) is alternatively spliced in three regions (EIIIA, EIIIB and V). At least one of these regions (V) has been shown to encode cell-binding sites, suggesting that splicing represents a mechanism to create functionally different forms of FN at different times and places. In order to test this hypothesis, we have examined the extent of alternative splicing of fibronectin during embryonic development. The distribution of the different spliced forms of FN mRNA in developing chicken embryos was determined using probes specific for the spliced regions in ribonuclease protection and in situ hybridization experiments. At embryonic day 2-4 (E2-4), all three spliced regions were included wherever FN mRNA was detected. At E16, however, we found spatially distinct splicing differences within the embryo, with cell-type-specific splicing excluding EIIIA and/or EIIIB in some tissues. In contrast, we did not detect exclusion of the V region. In a more detailed developmental study of the simplest of these tissues, the chorioallantoic membrane, we found that EIIIB was preferentially excluded after the completion of growth. These results suggest that FN splicing is used during development as a mechanism to create different forms of FN within the extracellular matrix by the inclusion or exclusion of specific segments. The data are consistent with an essential role for one of these segments, EIIIB, in the migration and/or proliferation of embryonic cells prior to their terminal differentiation and also suggest possible roles for the EIIIA segment.
TL;DR: The expression of the smooth/non-muscle isoform in every tissue tested supports the hypothesis that this molecule represents the "housekeeping" endoplasmic reticulum Ca2+-ATPase.
TL;DR: It is reported that the coding region for N‐terminal domains of NCAM also contains at least two sites of alternative splicing, termed alpha and pi, which could mean that 24 different mRNAs could be generated coding for 18 different proteins.
Abstract: The murine neural cell adhesion molecule (NCAM) is known to exist in three isoforms of different size, NCAM-180, -140 and -120 coded for by four transcripts of 6.9, 6.1, 4.8 and 2.7 kb in length. Since the differences between these isoforms are due to alternative splicing in the coding region for the transmembrane and cytoplasmic domains, the extracellular, N-terminal portion of NCAM seemed to be shared by all three protein forms. Here we report that the coding region for N-terminal domains of NCAM also contains at least two sites of alternative splicing, termed alpha and pi. Short additional sequences of 3, 18 and 30 nt in length can be introduced at these sites, which are located in the membrane-proximal 'stem' between the Ig-like domains and the membrane attachment site and within the Ig-like domain IV, respectively. Proof for at least eight different mRNAs has been found by sequencing and S1 nuclease protection assays of selected independent cDNA clones, and Northern blot analyses. If most combination of the splice patterns identified so far in mouse brain occurred, 24 different mRNAs could be generated coding for 18 different proteins. The shortest extra-sequence found inserted at splice site alpha consisted only of the trinucleotide AAG, raising questions about the mechanism of this particular insertion.
TL;DR: Two previously described variant Prostate-specific Antigen cDNAs can now be explained by intron retention and alternative splicing of the primary transcript.