Final answer:
Cyclin-dependent kinase (Cdk) is the positive regulator that phosphorylates other proteins when activated, essential for progressing the cell cycle past various checkpoints.
Step-by-step explanation:
The protein that is a positive regulator and phosphorylates other proteins when activated is Cyclin-dependent kinase (Cdk). Cyclins and Cdks are vital for the cell cycle progression, with cyclins required to activate Cdks. Once activated, Cdks phosphorylate other proteins to advance the cell cycle past checkpoints. The levels of cyclins fluctuate throughout the cell cycle, and their binding to Cdks is essential. The Cdk/cyclin complex must be phosphorylated at specific locations to be fully functional. The Cdk/cyclin complex phosphorylates other proteins, changing their shapes and thereby activating them to push the cell into the next phase of the cycle. In summary, Cdks are the positive regulators that phosphorylate other proteins upon activation.