Final answer:
The cyclin component of the Cdk complex determines the substrate specificity by binding to Cdks and indicating which complexes form at specific points in the cell cycle.
Step-by-step explanation:
The feature of the cyclin-dependent kinase (Cdk) complex that determines the substrate specificity of the kinase is the cyclin component. Cyclins are regulatory proteins that bind to Cdks, and their levels rise and fall during the cell cycle, indicating which Cdk/cyclin complexes are formed at specific points in the cycle.
These different Cdk/cyclin complexes, once activated through phosphorylation, target and phosphorylate other proteins that are specific to advancing the cell to the next phase of the cycle. This regulatory mechanism ensures that cell cycle events occur in a precise and controlled manner.