The transmembrane segment of invariant chain mediates binding to MHC class II molecules in a CLIP-independent manner

Abstract

Invariant chain (Ii) association with MHC class II molecules is strongly dependent upon interaction of CLIP (Ii exon 3, residues 81 – 104) with the peptide binding groove of the class II dimer. This dominant interaction does not adequately explain, however, the efficient association of Ii with class II molecules of diverse allelic and isotypic origin, which have markedly different affinities for synthetic peptides corresponding to CLIP. In agreement with other recent observations, we demonstrate here that class II molecules with occupied binding sites unable to engage CLIP maintain association with Ii in mild detergent. The association is direct and not mediated through unoccupied class II chains bound to properly assembled and loaded class II dimers, nor is it mediated through chaperones. The site of this CLIP‐independent binding has been mapped using truncation mutants and an Ii‐human transferrin receptor chimeric protein to the transmembrane segment of Ii. The existence of multiple low‐affinity sites of interaction between MHC class II and Ii helps explain how effective occupancy of all newly synthesized class II molecules can occur despite substantial variations in the strength of CLIP‐dependent association that arise from class II binding domain polymorphism. These data establishing a site of Ii‐MHC class II association N‐terminal to CLIP also provide new insight into the possible functional relationship between the sequential endocytic proteolysis of Ii from its C terminus and a series of contact sites with MHC class II molecules spread from the transmembrane region through to the tip of the lumenal segment of Ii.

Differential Transmembrane Domain GXXXG Motif Pairing Impacts Major Histocompatibility Complex (MHC) Class II Structure

“…This idea is consistent with our previous finding that the addition of a ␤ chain tethered peptide (which would prevent/alter class II-Ii associations) blocks formation of Ia.2 ϩ class II (5) and with the work of Germain and co-workers (21), who demonstrated an interaction between the TM domains of Ii and class II, and suggest that the TM domain of Ii may interact with that of class II to guide GXXXG motif pairing. Studies are currently underway to investigate this intriguing possibility.…”

Dissecting MHC Class II Export, B Cell Maturation, and DM Stability Defects in Invariant Chain Mutant Mice

“…A strong argument can be made that Ii chain cannot possibly bind as for conventional class II via its CLIP segment. The Ii chain has also been shown to weakly interact with conventional class II transmembrane domains, and these contacts depend on detergent and isolation conditions (54,114,115). Interestingly, DM transmembrane/cytoplasmic domains are known to contribute to its peptide editing functions (116).…”

DM Loss in k Haplotype Mice Reveals Isotype-Specific Chaperone Requirements

“…Wiley and co-workers (64) previously noted that the conserved CLIP Ala 94 and Pro 96 side chains hardly fit inside the DR3 P4 and P6 acceptor pockets, but these small residues avoid destabilizing contacts in the central portion of the groove. Interactions with adjacent residues outside the groove, possibly class II contacts mapped to transmembrane domains (81,82), potentially strengthen associations necessary for coassembly of oligomeric complexes. DM has been viewed as an enzyme that catalyzes peptide exchange, but the structural basis of its activities remains ill defined.…”