The cyclin D-CDK4/6 complex is involved in the regulation of progression through the G1 phase of the cell cycle. Which of the following represents a major target of this kinase complex?
A) cyclin E-CDK2
B) E2F
C) p27
D) p53
E) pRb

Question

The cyclin D-CDK4/6 complex is involved in the regulation of progression through the G1 phase of the cell cycle. Which of the following represents a major target of this kinase complex? 
A) cyclin E-CDK2
B) E2F
C) p27
D) p53
E) pRb

Quizplus · Accepted Answer

The cyclin D-CDK4/6 complex phosphorylates the retinoblastoma protein (pRb), leading to the release of E2F transcription factors and progression through the G1 phase of the cell cycle.  In the context of cell cycle regulation, the transcription factor E2F activates the expression of cyclin A, cyclin E, and CDK2. The activity of E2F is itself controlled via interaction with pRB. When pRB binds E2F it can no longer function as a transcription factor as it is sequestered in the cytosol. Interaction of pRB and E2F correlates to the state of phosphorylation of pRB and the affinity between the 2 proteins is highest when pRB is hypophosphorylated. Phosphorylation of pRB is maximal at the start of S-phase and lowest after mitosis and entry into G1. When pRB is phosphorylated by G1 cyclin-CDK complexes (cyclin D-CDK4/6), it releases E2F allowing E2F to transcriptionally activate its target genes.

The Cyclin D-CDK4/6 Complex Is Involved in the Regulation of Progression