THE BLOCK OF ADIPOCYTE DIFFERENTIATION BY A C-TERMINALLY TRUNCATED, BUT NOT BY FULL-LENGTH, SIMIAN-VIRUS-40 LARGE TUMOR-ANTIGEN IS DEPENDENT ON AN INTACT RETINOBLASTOMA SUSCEPTIBILITY PROTEIN FAMILY BINDING DOMAIN

Simian virus 40 (SV40) can promote cell transformation and suppress di fferentiation, It does this partly by targeting tumor suppressors such as p53 and members of the retinoblastoma susceptibility protein (Rb) family, This work concentrates on mechanisms by which SV40 large tumor antigen (SVLT) suppresses adipocyte differentiation, We created cell lines derived from murine 3T3-L1 preadipocytes expressing different ve rsions of SV40 early-region sequences, SVLT-expressing cells failed to exhibit adipocyte morphology, to induce glycerophosphate dehydrogenas e activity, and to induce differentiation-dependent mRNA for adipocyte P2. SVLT alone was sufficient, in the absence of SV40 small tumor ant igen, to inhibit differentiation, A truncated SVLT containing only the N-terminal 121 amino acids (SVLT1-121) blocked differentiation, thus mapping at least one differentiation blocking function to the N-termin al region, K1 (Glu-107-->Lys) point mutants of SVLT, which are unable to bind to the Rb protein family or induce neoplastic transformation, are defective for blocking differentiation in the case of SVLT1-121 bu t retain the ability to block differentiation in the case of full-leng th SVLT, This finding demonstrates that Rb family proteins are importa nt in regulating adipocyte differentiation but that other functions of full-length SVLT can block adipocyte differentiation independently of RE family binding and transformation.