Final answer:
pRb guards the R-point by inhibiting E2F when dephosphorylated and allowing cell cycle progression once phosphorylated to ensure cells only divide when ready. Other proteins like p53 and p21 also contribute to cell cycle regulation.
Step-by-step explanation:
The retinoblastoma protein (pRb) acts as a guard at the R-point, or the restriction point, in the cell cycle. pRb functions by binding to transcription factors such as E2F when it is dephosphorylated, which inhibits the transcription of genes required for progressing from the G1 phase to the S phase. As the cell grows, pRb is slowly phosphorylated, leading to the release of E2F and subsequent activation of the required genes for the cell cycle to continue. Hence, the proper functioning of pRb ensures that cells only pass the restriction checkpoint and enter the S phase when they have reached an appropriate size and all systems are ready for DNA replication.
Roles of other proteins like p53 and p21 are also pivotal in the checking mechanism, where p53 can halt the cell cycle upon detecting damaged DNA, and p21 works to enforce this halt and inhibit the activity of the Cdk/cyclin complexes. The phosphorylation state of pRb thereby serves as a key regulator of cell cycle progression through its interaction with critical transcription factors and other regulatory proteins.