CHARACTERIZATION OF TRANSCRIPTION FACTOR E2F COMPLEXES DURING MUSCLE AND NEURONAL DIFFERENTIATION

The activities of E2F transcription factors are inhibited by interacti ons with members of the retinoblastoma (RE) tumor suppressor family, p 105(RB), p107 and p130, In cycling cells p107 and p130 also interact w ith heterodimers comprised of Cdk2 and either A or E cyclins. We chara cterized E2F complexes present in C2C12 and P19 mouse cells induced to differentiate into muscle and neuronal cells, respectively. In both u ndifferentiated C2C12 and P19 cells, in addition to free species, E2F was found in complexes containing p107 or p130 and Cdk2. No E2F-pRB co mplexes were detected by electrophoretic mobility shift assays even th ough such cells were shown to contain pRB and E2F species capable of i nteracting in vitro. These results suggested that although present, pR B was unable to interact with E2F. Following differentiation of C2C12 cells into myotubes, E2F was present in at least two complexes which c ontained p130, but not in those containing p107 or Cdk2. Low levels of E2F-pRB complexes were also detected in fully differentiated C2C12 my otubes and in freshly isolated skeletal muscle. In the case of differe ntiated P19 neuronal cells, E2F was found in complexes containing pRB, p107 and p130. However, such cells may not be representative of fully differentiated neurons, as studies with rodent brain extracts indicat ed that only pRB-E2F complexes and those recognized by a p130-specific serum were present. These results suggested that in both muscle and n eurons, pRB and p130 may play specific roles in the development or mai ntenance of terminal differentiation.