Final answer:
A kinase, particularly cyclin-dependent kinases (Cdks) in complex with cyclins, is a protein that acts as a positive regulator by phosphorylating other proteins upon activation, playing a key role in cell cycle progression and cellular metabolism.
Step-by-step explanation:
The protein that acts as a positive regulator by phosphorylating other proteins when activated is a kinase, specifically cyclin-dependent kinases (Cdks) when associated with cyclins. Proteins known as kinases facilitate the process of phosphorylation, which is the addition of a phosphate group to proteins, thus altering their function. This can lead to changes in cellular metabolism and progression through different cell cycle stages.
Positive regulators, such as cyclins and Cdks, are crucial for the progression of cells through various checkpoints in the cell cycle. Their levels increase in response to signals causing the cell to progress to the next stage, where they then phosphorylate specific proteins that are involved in advancing the cell to the subsequent phase.
One example of positive regulation in action is with the retinoblastoma protein (Rb) which monitors cell size. When Rb is phosphorylated, it releases transcription factors like E2F, which then activate genes necessary for cell cycle progression, like the G₁/S transition, showing how phosphorylation can activate pathways necessary for cell growth and division.