Function of pRb in the Cell Cycle

The pRb protein is a nuclear phosphoprotein of ca. 100 kDa. The domain structure of pRb is shown in Fig. 13.13 (review Riley et al., 1994 Weinberg, 1995). The pRb proteins possesses numerous Ser/Thr phosphorylation sites, binding sites for the transcription factor E2F, for the viral oncoproteins TAg, ElA and E7 (see Chapter 14) and a non-specific DNA binding site. Furthermore, an N-terminal sequence section has been identified in pRb protein that is required for oligomerization. 

The pRb protein has the characteristics of a tumor suppressor protein. Loss of its function is associated with deregulation of cell division and favors tumor formation (see Chapter 14). 

A central function of pRb in control of the cell cycle (review Sherr, 1996) is illustrated by the following  

The restriction point marks the transition of the cell from a growth-factor-dependent state to a mostly growth-factor-independent state. The fact that changes in the degree 

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