Cellular proteins that associate with the middle and small T antigens of polyomavirus

Abstract

We have used two-dimensional gel electrophoresis to analyze in more detail the cellular proteins which associate with the middle and small tumor antigens (MT and ST, respectively) of polyomavirus. Proteins with molecular masses of 27, 29, 36, 51, 61, 63, and 85 kilodaltons (kDa) that specifically coimmunoprecipitated with MT were identified on these gels. The 36-, 51-, 61-, 63-, and 85-kDa proteins are probably the same as the proteins of similar sizes previously reported by a number of groups, whereas the 27and 29-kDa proteins represent proteins that are heretofore undescribed. The 27and 29-kDa proteins were abundant cellular proteins, whereas the others were minor cellular constituents. The association of each of these proteins with MT was sensitive to one or more mutations in MT that rendered it transformation defective. The association of the 85-kDa protein was the most sensitive indicator of the transformation competence of MT mutants. In addition, the 85-kDa protein was the only associated protein whose association with MT changed consistently in parallel with MT-associated phosphatidylinositol kinase activity. Furthermore, the fraction of the 85-kDa protein which was found associated with the MT complex contained 15 to 20% of its phosphate content on tyrosine. The 36and 63-kDa proteins complexed with both polyomavirus MT and ST and comigrated on two-dimensional gels with two simian virus 40 ST-associated proteins originally described by Rundell and coworkers (K. Rundell, E. 0.

Phosphatidylinositol metabolism in cells transformed by polyomavirus middle T antigen

“…All transforming (class I) and nontransforming (class II) middle T antigen mutants bind pp6Oc-src, but nontransforming class III mutants have lost the ability to bind pp6Oc-src. According to this classification, only class I and certain class II mutants retain p81 binding and have associated PtdIns 3-kinase activity (6,8,20,30). It has been recently reported by Auger et al (1) that PtdIns 3-kinase activity associated with the PDGF receptor was capable of utilizing Ptdlns(4)P and PtdIns(4,5)P2 as substrates.…”

“…with these middle T antigen-tyrosine kinase complexes is an 81-kilodalton phosphoprotein (6, 15; S.A.C., unpublished data) whose presence correlates with PtdIns 3-kinase activity and which is most likely identical to the putative 85kilodalton Ptdlns 3-kinase which binds the PDGF receptor (20). Ptdlns 3-kinase activity and p81 are associated in immune precipitates containing all transforming and certain nontransforming middle T antigen mutants (e.g., d11015) but not with other nontransforming middle T antigen mutants which retain the ability to bind pp6-src (6,30). Association of PtdIns 3-kinase activity with middle T antigen, therefore, appears necessary but not sufficient for cell transformation.…”

A completely transformation-defective point mutant of polyomavirus middle T antigen which retains full associated phosphatidylinositol kinase activity

“…Two proteins with molecular masses of 36 and 63 kilodaltons (kDa) present in MTAg immunoprecipitates have been recently determined to be the catalytic and regulatory subunits of protein phosphatase 2a (37). Again, genetic analysis suggests that the presence of these proteins is necessary but TRANSFORMATION-DEFECTIVE MIDDLE T ANTIGEN MUTANT not sufficient for transformation by MTAg (19,30,35,41,45). However, this conclusion is based on data regarding the association of the 36and 63-kDa proteins with various MTAg mutants, rather than measurements of phosphatase activity.…”

The Merkel Cell Polyomavirus and Its Involvement in Merkel Cell Carcinoma

“…The ST protein begins transcription at the same start site as LT but undergoes differential splicing. The ST encodes a protein phosphatase 2A interaction domain that does not appear to affect ST's properties as an oncogene . Although SV40 ST alone did not transform cells but rather enhanced LT's function, MCV ST was found to be an independent oncoprotein allowing for rodent fibroblasts to transform into cancer cell lines.…”