FIG. 7.

Models for the requirement of both the J domain and pRb family binding motif of T antigen to block apoptosis. (A) In the absence of EGF, one or more members of the Rb family function to induce both the G₁ arrest and apoptosis of SFME cells. T antigen binds and acts on one or more members of the Rb family, blocking apoptosis and allowing proliferation in the absence of EGF. The J domain and Rb binding motif of T antigen are sufficient to block apoptosis and allow SFME cell proliferation in the absence of EGF. (B) An alternate hypothesis is that while one or members of the Rb family are involved in G₁ arrest, it is some other cellular factor (termed DNA K here) that induces apoptosis in the absence of EGF. T antigen must interact with both (sets of) factors, the Rb family through CR2 and DNA K through the J domain, to allow SFME cell survival and proliferation in the absence of EGF. EGFR, EGF receptor.