An 85,000-molecular-weight polypeptide (85K polypeptide) has previously been identified as a common substrate for tyrosine phosphorylation upon polyomavirus middle T transformation or upon platelet-derived growth factor stimulation of 3T3 cells. In each case, pp85 has an associated phosphatidylinositol kinase activity. The tissue distribution of pp85 was determined by middle T blotting experiments; the highest levels were found in brain, lung, and spleen tissues. High-resolution examination of 85K by isoelectric focusing demonstrated that there are at least 10 different forms. These were resolved into two families, 85K and 86K; the ratio of the two families changed in different cells. Similar forms were found for pp85 associated with pp60v-src. Individual species within each family differed by phosphorylation. Analysis of pp85 and pp86 by immunoprecipitation with anti-phosphotyrosine antibody showed increasing phosphorylation in response to middle T or pp60v-src transformation. The association of middle T with pp85 and pp60c-src was examined in pulse-chase experiments. Association of middle T with pp60c-src was slow and was accompanied by progressive modification of middle T. pp85 formed a dissociable complex with middle T within 2.5 min.

By using a random mutagenesis procedure combined with a recombinant retrovirus vector, mutants of polyomavirus middle T antigen (MTAg) were generated. Three new MTAg mutants with various degrees of transformation competence were more thoroughly characterized. All of the mutants produced a stable MTAg, as assessed by metabolic labeling or immunoblotting, and each mutant possessed wild-type levels of associated tyrosine kinase activity and associated phosphatidylinositol-3 (PI-3) kinase activity. One of these mutants, with a substitution of leucine for proline at amino acid 248 of MTAg (248m) was completely transformation defective, as measured in a focus-forming assay. Furthermore, the pattern of phosphorylation of 248m in vivo was identical to that of wild-type MTAg, and the kinetics of association of MTAg with an 85-kilodalton protein, the putative PI kinase, was not altered. Similarly, the pattern of PI derivatives obtained in an in vitro kinase assay was not altered by the substitution at amino acid 248. Since the single base pair mutation at amino acid 248 resulted in an MTAg that was completely transformation defective despite possessing wild-type levels of kinase activities, this suggests that neither tyrosine kinase nor PI-3 kinase activity nor the combination of both are sufficient for transformation by MTAg.

A family of 85/86-kDa (85K/86K) polypeptides closely linked to phosphatidylinositol kinase activity is found in polyoma middle-sized tumor antigen (MTAg)/pp60c-src complexes. MTAg and the 85-kDa phosphoprotein (pp85) could be reassociated in solution, or on blots, after denaturation with SDS. Results from such experiments focus attention on phosphorylation in controlling intracellular sorting and activation of pp85. Tyrosine phosphorylation seems important for recruitment of pp85 from cytosol to membrane. By blotting, pp85 is substantially cytosolic, whereas that recognized by anti-phosphotyrosine antibody is almost exclusively in membranes. Tyrosine phosphorylation also determined association of pp85 with MTAg. Manipulation of MTAg tyrosine phosphorylation, for example, by expressing MTAg using baculovirus vectors in the absence or presence of pp60c-src, dramatically affects reassociation. Finally, tyrosine phosphorylation appears to be involved in release of pp85 from MTAg, since vanadate increased its rate of dissociation.

Our recent work has focused on the elucidation of the mechanisms by which membrane-bound tyrosine kinases transmit signals within the cell. To examine the role of tyrosine phosphorylation, we have employed the following strategy. First, we have utilized antibodies to phosphotyrosine (anti-P.Tyr) to identify candidate substrates of various tyrosine kinases, such as pp60c-src, the CSF-1 receptor, or the platelet-derived growth factor (PDGF) receptor (Piwnica-Worms et al. 1987; Morrison et al. 1988). Second, we have attempted to characterize the biochemical properties of the putative substrates and to determine in what manner these properties are modified by phosphorylation on tyrosine residues (Kaplan et al. 1986, 1987; Piwnica-Worms et al. 1987; Morrison et al. 1988 and in prep.). In this endeavor, we are recapitulating the classic biochemical analysis used to study the effect of kinases on metabolism. The final portion of our work consists of using modern molecular biological strategies to clone the...