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Science 24 November 1995:
Vol. 270. no. 5240, p. 1277
DOI:

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Major histocompatibility complex (MHC) class II molecules present peptides derived from exogenous antigens. Newly synthesized class II molecules are prevented from binding endogenous peptides by a polypeptide known as the invariant chain. In particular, the CLIP fragment (for class II-associated invariant chain peptides) blocks the peptide-binding groove. The mechanism whereby CLIP is removed to allow binding of exogenous peptide is unclear. Recent evidence pointed to a role for the nonclassical MHC molecule HLA-DM, but a fraction of molecules in DM-negative cells are expressed normally. Kropshofer et al. (p. 1357) show that self-release of CLIP occurs under appropriate conditions. Self-release occurs at endosomal pH and is catalyzed by the amino-terminal segment, which lies outside the groove.




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Science. ISSN 0036-8075 (print), 1095-9203 (online)