Discussion
Peptide-binding cleft in MHC-I is formed by
Answer: A
The peptide-binding cleft of MHC class I is formed by the \(\alpha 1\) and \(\alpha 2\) domains.
The MHC I heavy chain has three extracellular domains. The \(\alpha 1\) and \(\alpha 2\) domains together build a platform of eight β-strands topped by two long α-helices, and the groove between the helices holds a peptide of about 8 to 10 residues for display to CD8+ T cells.
The \(\alpha 3\) domain and the separate light chain \(\beta 2\)-microglobulin are immunoglobulin-like domains that sit beneath the platform and support it; \(\alpha 3\) is also where CD8 binds. Neither forms the cleft, so the options pairing \(\alpha 1\) or \(\alpha 2\) with \(\alpha 3\) or \(\beta 2\)-microglobulin are wrong. (In MHC II the cleft is built from \(\alpha 1\) and \(\beta 1\), one from each chain.)
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