HLA-B polymorphism affects interactions with multiple endoplasmic reticulum proteins

To explore the nature of amino acid substitutions that influence associatio n with TAP, we compared a site-directed mutant of HL4-B*0702 (Y116D) to unm utated HLA-B7 in regard to TAP interaction. We found that the mutant had st ronger association with TAP, and, in addition, with tapasin and calreticuli n. These data confirm the importance of position 116 for TAP association, a nd indicate that (1) an aspartic acid at the 116 position can facilitate th e interaction, and (2) association with tapasin and calreticulin is affecte d along with TAP. Furthermore, we tested three natural subtypes of HLA-B15, and found that a B15 subtype with a tyrosine at position 116 (B*1510) was strongly associated not only with TAP, but also with tapasin and calreticul in. In contrast, two B15 subtypes with a serine at position 116 (B*1518 and B*1501) exhibited very little or no association with any of these proteins . Thus, very closely related HLA-B subtypes can differ in regard to interac tion with the entire assembly complex. interestingly, when their surface ex pression was tested by flow cytometry, the HL4-B15 subtypes with little to no detectable intracellular assembly complex association had a slightly, ye t consistently, higher level of the open heavy chain form than did the B15 subtype with intracellular assembly complex association. These data suggest that the relatively low strength or short length of interaction between en doplasmic reticulum proteins and natural HLA class I molecules can decrease their surface stability.