Cell Biology MCAT Practice Question
A 14-year-old boy presents with recurrent severe infections, lymphadenopathy, and hepatosplenomegaly. Flow cytometry reveals a marked expansion of immature T cells with increased tyrosine phosphorylation on intracellular proteins. Genetic sequencing identifies a heterozygous gain-of-function mutation in LCK, a Src family tyrosine kinase expressed in T cells. The mutation enhances phosphorylation of downstream substrates but does NOT alter the ability of the SH2 domain to bind phosphotyrosine residues. In contrast, a different patient with recurrent infections has a missense mutation that abolishes SH2 domain phosphotyrosine-binding capacity while preserving normal kinase catalytic activity and membrane localization. This second patient's T cells fail to respond to antigen receptor stimulation. Which of the following best explains why the SH2 domain is critical for Src family kinase function in T cell receptor signaling?
Answer choices
- AThe SH2 domain directly catalyzes phosphorylation of protein tyrosine residues on substrate proteins
- BThe SH2 domain acts as the primary mechanism for localizing Lck to the plasma membrane through lipid interactions
- CThe SH2 domain binds to phosphorylated tyrosine residues on other signaling proteins, bringing Lck into proximity with its substratesCorrect answer
- DThe SH2 domain inhibits the kinase domain until antigen receptor crosslinking occurs
- EThe SH2 domain provides the ATP-binding pocket essential for catalytic turnover
- FThe SH2 domain undergoes autophosphorylation to activate the kinase catalytic domain
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations â plus adaptive practice that targets your weak areas â with a free MedBoardPRO account.