GTPases are conserved molecular switches, built according to a common structural design. Rapidly accruing knowledge of individual GTPases--crystal structures, biochemical properties, or results of molecular genetic experiments--support and generate hypotheses relating structure to function in other members of the diverse family of GTPases.

The GTPase superfamily: conserved structure and molecular mechanism

Analysis of 13,023 genes in 11 breast and 11 colorectal cancers revealed that individual tumors accumulate an average of ˜90 mutant genes but that only a subset of these contribute to the neoplastic process. Using stringent criteria to delineate this subset, we identified 189 genes (average of 11 per tumor) that were mutated at significant frequency. The vast majority of these genes were not known to be genetically altered in tumors and are predicted to affect a wide range of cellular functions, including transcription, adhesion, and invasion. These data define the genetic landscape of two human cancer types, provide new targets for diagnostic and therapeutic intervention and monitoring.

/pdf/consensus-coding-sequences-of-human-breast-and-colorectal-15edxp2i8x.pdf

Consensus coding sequences of human breast and colorectal cancers

Oncogenes and signal transduction

Recent studies suggest that one or more genes on chromosome 5q21 are important for the development of colorectal cancers, particularly those associated with familial adenomatous polyposis (FAP). To facilitate the identification of genes from this locus, a portion of the region that is tightly linked to FAP was cloned. Six contiguous stretches of sequence (contigs) containing approximately 5.5 Mb of DNA were isolated. Subclones from these contigs were used to identify and position six genes, all of which were expressed in normal colonic mucosa. Two of these genes (APC and MCC) are likely to contribute to colorectal tumorigenesis. The MCC gene had previously been identified by virtue of its mutation in human colorectal tumors. The APC gene was identified in a contig initiated from the MCC gene and was found to encode an unusually large protein. These two closely spaced genes encode proteins predicted to contain coiled-coil regions. Both genes were also expressed in a wide variety of tissues. Further studies of MCC and APC and their potential interaction should prove useful for understanding colorectal neoplasia.

https://science.sciencemag.org/content/sci/253/5020/661.full.pdf

Identification of FAP locus genes from chromosome 5q21

Proteins that bind and hydrolyse GTP are being discovered at a rapidly increasing rate. Each of these many GTPases acts as a molecular switch whose 'on' and 'off' states are triggered by binding and hydrolysis of GTP. Conserved structure and mechanism in myriad versions of the switch--in bacteria, yeast, flies and vertebrates--suggest that all derive from a single primordial protein, repeatedly modified in the course of evolution to perform a dazzling variety of functions.

The GTPase superfamily: a conserved switch for diverse cell functions

A subset of growth hormone-secreting human pituitary tumours carries somatic mutations that inhibit GTPase activity of a G protein alpha chain, alpha(s) The resulting activation of adenylyl cyclase bypasses the cells' normal requirement for trophic hormone Amino acids substituted in the putative gsp oncogene identify a domain of G protein alpha-chains required for intrinsic ability to hydrolyse GTP This domain may serve as a built-in counter-part of the separate GTPase-activating proteins required for GTP hydrolysis by small GTP-binding proteins such as p21ras

GTPase inhibiting mutations activate the α chain of G s and stimulate adenylyl cyclase in human pituitary tumours

One or more of three Gi proteins, Gi1-3, mediates hormonal inhibition of adenylyl cyclase. Whether this inhibition is mediated by the alpha or by the beta gamma subunits of Gi proteins is unclear. Mutations inhibiting the intrinsic GTPase activity of another G protein, the stimulatory regulator of adenylyl cyclase (Gs), constitutively activate it by replacing either of two conserved amino acids in its alpha subunit (alpha s). These mutations create the gsp oncogene which is found in human pituitary and thyroid tumours. In a second group of human endocrine tumours, somatic mutations in the alpha subunit of Gi2 replace a residue cognate to one of those affected by gsp mutations. This implies that the mutations convert the alpha i2 gene into a dominantly acting oncogene, called gip2, and that the mutant alpha i2 subunits are constitutively active. We have therefore assessed cyclic AMP accumulation in cultured cells which stably or transiently express exogenous wild-type alpha i2 complementary DNA or either of two mutant alpha i2 cDNAs. The results show that putatively oncogenic mutations in alpha i2 constitutively activate the protein's ability to inhibit cAMP accumulation.

Mutant alpha subunits of Gi2 inhibit cyclic AMP accumulation.

In a recently discovered class of oncogenes, GTPase-inhibiting mutations constitutively activate alpha subunits of signal-transducing guanine nucleotide-binding proteins (G proteins). Somatic mutations in a subclass of endocrine tumors are found in the arginine-179 codon of the alpha subunit of Gi2 (alpha i2), creating the putative gip2 oncogene. We have tested the ability of gip2 to mediate neoplastic transformation of Rat-1 and NIH 3T3 fibroblasts in tissue culture. Expression of a mutant alpha i2 cDNA encoding cysteine in place of arginine-179 (alpha i2-R179C) caused Rat-1 cells to grow to a higher density in monolayer culture, to lose anchorage dependence, and to form tumors when injected subcutaneously into nude mice. In contrast, expression of alpha i2-R179C failed to alter growth or tumorigenicity of NIH 3T3 cells. We conclude that gip2 is an oncogene, by the criterion that it induces neoplastic transformation of Rat-1 cells. Failure of gip2 to transform NIH 3T3 cells is in keeping with clinical indications that gip2 is a tissue-selective oncogene.

https://www.pnas.org/content/pnas/88/16/7031.full.pdf

Ann M. Pace

Papers

GTPase inhibiting mutations activate the α chain of G s and stimulate adenylyl cyclase in human pituitary tumours

Mutant alpha subunits of Gi2 inhibit cyclic AMP accumulation.

A mutant alpha subunit of Gi2 induces neoplastic transformation of Rat-1 cells.