Cdks gain enzymatic function by binding to which regulatory proteins?

Introduction / Context:
Cyclin-dependent kinases (Cdks) are central to cell-cycle regulation. On their own, Cdks are inactive; they require specific regulatory partners that fluctuate throughout the cycle to provide timing and substrate specificity.

Given Data / Assumptions:

• Cdks are protein kinases.
• Activation requires a regulatory subunit.
• The question asks for the binding partner enabling enzymatic activity.

Concept / Approach:
Cyclins are phase-specific proteins that bind Cdks, activating them and directing phosphorylation of key substrates (for example, RB, lamins, replication factors). The Cdk–cyclin complex is sometimes historically called MPF when referring to the mitotic form, but MPF is a complex, not the regulator the Cdk binds to.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical assays demonstrate Cdk kinase activity increases sharply upon cyclin binding and appropriate phosphorylation.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing terminology where the complex name (MPF) is mistaken as a separate regulator instead of the Cdk–cyclin complex itself.

Final Answer:
Cyclins.