Which protein directly phosphorylates target proteins in the context of RTKs?

Tyrosine kinases play a critical role in the signaling pathways initiated by receptor tyrosine kinases (RTKs). When a ligand binds to an RTK, it typically causes the receptor to dimerize and autophosphorylate on specific tyrosine residues. This autophosphorylation activates the tyrosine kinase activity of the receptor itself, leading to the phosphorylation of downstream target proteins on tyrosine residues.

The phosphorylation of these target proteins is a key mechanism for transmitting the signal inside the cell, influencing various cellular processes, such as growth, differentiation, and metabolism. Other proteins mentioned, such as PKA (protein kinase A) and GAPs (GTPase-activating proteins), have distinct roles in cellular signaling but do not directly phosphorylate targets in the context of RTKs. PKA is primarily activated by cyclic AMP and phosphorylates serine and threonine residues rather than tyrosine. GAPs primarily facilitate the inactivation of small GTPases by promoting the hydrolysis of GTP to GDP, rather than directly phosphorylating proteins.

In summary, the direct phosphorylation of target proteins in the context of RTKs is the specific function of tyrosine kinases, making it the