TRANSFORMATION BY MIDDLE T-ANTIGENS

The polyomaviruses, members of the papovavirus group of DNA tumor viru ses, encode in the 'early' region of the genome, three proteins involv ed in transformation, the tumor (or T) antigens, called large (LT), mi ddle (mT) and small (sT) T. LT is required for establishment of primar y fibroblasts, and sT promotes the efficiency of transformation both i n vivo and in vitro, but it is mT that carries the transforming abilit y. The mTs of the two known polyomaviruses, from mouse and hamster, po ssess no intrinsic catalytic activity, but rather interact with and ch ange the activity of several cellular proteins, including Src family p rotein tyrosine kinases, protein phosphatase 2A, phosphatidylinositol 3-kinase and Shc. Same of these proteins are also involved in signal t ransduction events elicited by growth factors. Like activated growth r eceptors, mT brings its associated proteins to a membranous environmen t. Transformation by mT might result not only from allosteric effects of mT on its interacting proteins but also as a result of their subcel lular relocalization.