The key that allows a cell to progress beyond the restriction point is the phosphorylation of specific proteins (for example RB) by Cdk. The cyclin-dependent kinases (CDKs) are able to phosphorylate different substrates.
Cyclins regulate the progression of the cell cycle by their interaction with cyclin-dependent kinases (Cdks).
This binding activates the Cdks, thereby converting them into functional enzymes that phosphorylate target proteins and thus regulate cell cycle progression by bypassing restriction points of the cell cycle.
CDKs can phosphorylate different substrates by transferring phosphate groups from adenosine triphosphate (ATP) to specific amino acidic residues in these substrates.
For example, CDKs phosphorylate Rb proteins and thus regulate their activity during the cell cycle.
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