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M110300200v1
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Papers In Press, published online ahead of print November 16, 2001
J. Biol. Chem, 10.1074/jbc.M110300200
Submitted on October 26, 2001
Revised on November 15, 2001
Accepted on November 15, 2001

Functional analysis of tryptophans alpha62 and beta120 on HLA-DM

Amélie Faubert, Angela Samaan, and Jacques Thibodeau

Microbiologie et Immunologie, Université de Montréal, Montréal, Québec H3C 3J7

Corresponding Author: jacques.thibodeau{at}umontreal.ca

In the endocytic pathway of antigen presenting cells, HLA-DM catalyzes the exchange between CLIP and antigenic peptides onto MHC class II molecules. At low pH of lysosomal compartments, both HLA-DM and HLA-DR undergo conformational changes and it was recently postulated that two partially exposed tryptophans on HLA-DM might be involved in the interaction between the two molecules. In order to define contact regions on HLA-DM, we have conducted site-directed mutagenesis on those two hydrophobic residues. HLA-DM aW62A, bW120A (DMW62A/W120A) double mutant was expressed in HLA-DR+ HeLa cells expressing invariant chain and the activity of this DM molecule was assessed. Flow cytometry analysis of cell surface DR/CLIP complexes revealed that DMW62A/W120A removes CLIP as efficiently as its wild-type counterpart. DMW62A/W120A was found in the endocytic pathway by immunofluorescence and DM/DR complexes were immunoprecipitated from these cells at pH 5. Finally, mutations aW62A and bW120A on HLA-DM did not affect the association with HLA-DO. The complex egresses the endoplasmic reticulum and accumulates in endocytic vesicles. Moreover, DO and DMW62A/W120A were co-immunoprecipitated at pH 7. We conclude that the a62 and b120 tryptophan residues are not required for the activity of DM nor are they directly implicated in the interaction with DR or DO.


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