Answer:
One example of an extracellular signal protein that can trigger a wave of G1/S-CDK (Cyclin-Dependent Kinase) activity is the growth factor known as Platelet-Derived Growth Factor (PDGF).
Step-by-step explanation:
PDGF is a potent mitogen that plays a crucial role in regulating cell proliferation, particularly in connective tissues and blood vessels.
When PDGF binds to its corresponding receptor on the cell surface, it initiates a signaling cascade that leads to the activation of G1/S-CDKs. This activation promotes the progression of the cell cycle from the G1 phase to the S phase, where DNA synthesis occurs.
PDGF stimulates the expression of cyclins, which are regulatory proteins that bind and activate CDKs. In the context of G1/S transition, PDGF induces the expression of cyclin D, which binds to and activates CDK4 and CDK6. The activated G1/S-CDKs then phosphorylate and inactivate the retinoblastoma protein (Rb), allowing for the release of transcription factors that drive the expression of genes required for DNA replication.
Therefore, PDGF is an example of an extracellular signal protein that can trigger a wave of G1/S-CDK activity, promoting cell cycle progression and proliferation. It is worth noting that there are other extracellular signals and pathways involved in regulating G1/S transition, and the specific signals and proteins may vary depending on the cell type and context.