A structural pocket mutation of pRb increases its affinity for E2F4, which is coupled with activation of muscle differentiation.

Coordination of proliferation and differentiation in cells committed to muscle fate requires the interaction of the retinoblastoma gene product (pRb) with transcription factors of the E2F family, pRb having different affinities for distinct E2Fs. It was found that pRb carrying a small deletion at the end of the T antigen-binding region (ΔS/N) and unable to interact with the SV40 large T antigen could induce an acute cell cycle block, stable prolongation of the cell cycle in G0/G1 and G2/M phases, and suppression of the growth of tumor cells. ΔS/N showed increased affinity for E2F4, bound hyperphosphorylated forms of E2F4, and induced its nuclear compartmentalization. The ability of ΔS/N to form complexes with E2F4 on DNA was associated with an increased formation of “free” E2F4 and trans-suppression of a specific reporter through preferential binding to E2F4, but not to E2F1. Stable expression of ΔS/N in multipotent fibroblasts promoted early muscle commitment. It was assumed that a mutation in the T antigen-binding region increases pRb affinity for E2F4 combined with activation of muscle differentiation. [ABSTRACT FROM AUTHOR]