Upon binding of an extracellular growth factor, receptor tyrosine kinases (RTKs) undergo dimerization, bringing their intracellular domains into close proximity. What is the direct and immediate consequence of this dimerization for the RTK itself?
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The answer
Correct answer: D. The juxtaposed kinase domains of the receptor subunits phosphorylate each other on specific tyrosine residues.
Tested concept: Receptor tyrosine kinases
Explanation
Upon ligand binding and subsequent dimerization, the intracellular kinase domains of receptor tyrosine kinases (RTKs) are brought into close proximity. This allows them to phosphorylate each other on specific tyrosine residues, a process known as autophosphorylation. These newly phosphorylated tyrosine residues then serve as docking sites for various intracellular signaling proteins, initiating downstream signaling cascades. The option suggesting that the receptor directly binds and hydrolyzes GTP to initiate a G protein signaling cascade is incorrect because RTKs do not directly interact with G proteins or hydrolyze GTP; that is characteristic of G protein-coupled receptors. The option proposing that a separate cytoplasmic kinase is recruited to phosphorylate the receptor on serine and threonine residues is incorrect because RTKs possess intrinsic tyrosine kinase activity and autophosphorylate on tyrosine residues, not serine or threonine, and do not require a separate cytoplasmic kinase for this initial step. The option stating that the receptor immediately recruits and activates downstream effector proteins without prior phosphorylation is incorrect because autophosphorylation is a prerequisite for the recruitment and activation of most downstream effector proteins, as it creates the necessary binding sites.
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