GTP-Binding Protein alpha Subunits

About: GTP-Binding Protein alpha Subunits is a research topic. Over the lifetime, 304 publications have been published within this topic receiving 19915 citations.

Nicotiana tabacum cDNAs encoding α and β subunits of a heterotrimeric GTP-binding protein isolated from hairy root tissues

Abstract: Heterotrimeric GTP-binding proteins (G-proteins) play important roles in signal transduction pathways in eukaryotic cells. Through differential screening of a hairy root cDNA library of tobacco (Nicotiana tabacum L.) against transcripts from non-root tissues of normal cuttings, we obtained a partial cDNA clone that showed abundant expression and high homology to the alpha subunit gene of plant G-protein. After RACE-PCR, a full-length cDNA clone was obtained, which was 1,677-bp in length and contained an open reading frame encoding a protein of 384 amino acids. A cDNA clone encoding a beta subunit of G-protein was also isolated from the same cDNA library based on PCR amplification and library screening. The clone was 1,600-bp in length and contained an open reading frame encoding 377 amino acids. The deduced amino acid sequences of these clones showed high homology (75.5 to 99.8% amino acid identity) with alpha and beta subunits of other plant G-proteins. Genomic Southern blot analysis showed that the amphidiploid tobacco genome possessed two major copies of both alpha and beta subunit genes and some minor homologous copies. Northern blot analysis showed that the transcript of alpha subunit gene was abundant in the root tissues, particularly in the hairy root tissues. In contrast, the level of expression of the beta subunit gene was equivalent in all the tissues studied. Possible function of tobacco G-protein was discussed.

Gα–Gβγ dissociation may be due to retraction of a buried lysine and disruption of an aromatic cluster by a GTP-sensing Arg–Trp pair

Abstract: The heterotrimeric G protein α subunit (Gα) functions as a molecular switch by cycling between inactive GDP-bound and active GTP-bound states. When bound to GDP, Gα interacts with high affinity to a complex of the β and γ subunits (Gβγ), but when bound to GTP, Gα dissociates from this complex to activate downstream signaling pathways. Gα's state is communicated to other cellular components via conformational changes within its switch I and II regions. To identify key determinants of Gα's function as a signaling pathway molecular switch, a Bayesian approach was used to infer the selective constraints that most distinguish Gα and closely related Arf family GTPases from distantly related translational and metabolic GTPases. The strongest of these constraints are imposed on seven residues within or near the switch II region. Likewise, constraints imposed on Gα but not on other, closely related molecular switches correspond to four nearby residues. These constraints are explained by a proposed mechanism for GTP-induced dissociation of Gα from Gβγ where an Arg–Trp pair senses the presence of bound GTP leading to conformational retraction of a nearby lysine and to disruption of an aromatic cluster. Within a complex of Giα, Giβγ, and GDP, this lysine establishes greater surface contact with Giβ than does any other residue in Giα, whereas the aromatic cluster packs against a highly conserved tryptophan in Giβ that establishes greater surface contact with Giα than does any other residue in Giβ. Other structural features associated with Gα functional divergence further support the proposed mechanism.