The transforming protein of a primate sarcoma virus and a platelet-derived growth factor are derived from the same or closely related cellular genes. This conclusion is based on the demonstration of extensive sequence similarity between the transforming protein derived from the simian sarcoma virus onc gene, v-sis, and a human platelet-derived growth factor. The mechanism by which v-sis transforms cells could involve the constitutive expression of a protein with functions similar or identical to those of a factor active transiently during normal cell growth.

In the second of a series of experiments designed to identify p47 WASP is mainly present in the cytosol fraction, although significant amounts are also present in membrane and nuclear fractions. The main p47 nck region implicated in the association with p66WASP was found to be the carboxy-terminal SH3 domain.Signal transduction pathways that connect the cell surface with the nucleus utilize protein-protein interactions. Two of the most studied elements that participate in the recognition of specific protein targets are the Src homology 2 (SH2) and Src homology 3 (SH3) domains, first described as noncatalytic components of the Src family of protein tyrosine kinases (36). SH2 domains associate with proteins phosphorylated on tyrosine residues within a specific amino acid sequence motif (4, 44). In contrast, SH3 domains recognize proline-rich segments (8) without any requirements for protein modification, allowing for the constitutive association of this domain with its target proteins.No common functional features have been found among either the SH3-containing proteins or the SH3-binding proteins. For instance, the SH3 domains of the tyrosine kinases Lck, Fyn, and Abl associate with the p85 subunit of pI3 kinase (23); the adaptor molecule Grb2, which possesses two SH3 domains, links receptor tyrosine kinases to the SH3-binding protein Sos, a regulator of small G proteins (12); and the SH3 domain of the cytoskeletal protein ␣-spectrin binds to the epithelial sodium channel (41). Such heterogeneity makes it difficult to predict the putative ligands for each of the known SH3 domains.The nck gene was initially isolated from a human melanoma cDNA library, and its product is a 47-kDa cytosolic protein exclusively composed of one SH2 and three SH3 domains (30). This protein is ubiquitously expressed, and its function has not been defined. Although activated tyrosine kinase receptors (6,31,34,35) and insulin receptor substrate-1 (29) have been shown to bind to the SH2 domain of p47 nck

We have evaluated the muscarinic acetylcholine family of G protein-coupled receptors (mAChRs) for their oncogenic potential. These receptors are preferentially expressed in postmitotic cells, transducing signals specified by their endogenous agonist, the neurotransmitter acetylcholine. Cells transfected with individual human mAChR genes were morphologically indistinguishable from parental NIH 3T3 cells in the absence of agonist. In contrast, when cultures were supplemented with carbachol, a stable analog of acetylcholine, foci of transformation readily appeared in ml, m3, or m5 but not in m2 or m4 mAChRs transfectants. Receptor expression was verified by ligand binding and was similar for each transfected culture. Transformation was dose-dependent and required only low levels of receptor expression. In transformation-competent cells, agonist induced phosphatidylinositol hydrolysis, whereas in m2 or m4 transfectants, receptors were coupled to the inhibition of adenylyl cyclase. These findings demonstrate that mAChRs linked to phosphatidylinositol hydrolysis can act as conditional oncogenes when expressed in cells capable of proliferation.Receptors for polypeptides, such as epidermal growth factor and platelet-derived growth factor, can induce cellular transformation when constitutively activated (1-4). Structural mutations or unregulated availability of ligand are mechanisms known to account for their transforming activity. These receptors are prototypes of a class that mediate signal transduction by virtue of an intrinsic protein-tyrosine kinase activity (2,5,6). When the mas oncogene was discovered, a class of cell-surface receptors lacking protein-tyrosine kinase domains was also implicated in cellular transformation. The mas oncogene product has a structural motif characteristic of receptors that mediate signal transduction by coupling to GTP-binding proteins (G proteins) (7). Although mas has a weak focus-inducing activity in vitro, cells transfected with this gene are highly tumorigenic in nude mice (7). More recently, G protein-coupled serotonin receptors have been shown to convert fibroblasts to a tumorigenic state (8). Because in these latter cases exogenous ligand is not required for transformation, either these genes encode aberrant receptors or endogenous ligands are responsible for their activation. Thus, ligand independence has limited the study of the mechanism by which these receptors mediate transformation.In the present study, we have directly tested the hypothesis that normal G-protein-coupled receptors can induce agonistdependent neoplastic transformation. We chose a family of cell-surface neurotransmitter receptors, human muscarinic acetylcholine receptors (mAChRs), which possess sequence homology with both mas and the serotonin receptor (9-12). Muscarinic receptors are preferentially expressed in neurons and other postmitotic cells, and they transduce signals specified by their endogenous agonist, the neurotransmitter acetylcholine. The mAChR family consists of five distinct but high...

Polyadenylated RNAs of certain human tumour cell lines are shown to contain transcripts related to the cell-derived transforming onc genes of molecularly cloned primate, murine or avian transforming retrovirus genomes. Thus, analogues of retroviral transforming genes are both present and frequently expressed in human neoplastic cells.