Salt bridge residues between I-A(k) dimer of dimers alpha-chains modulate antigen presentation

Class II dimers of dimers are predicted to have functional significance in antigen presentation. The putative contact amino acids of the I-A(k) class II dimer of dimers have been identified by molecular modeling based on the DR1 crystal structure (Nydam et al., Int. Immunol. 10, 1237,1998). We have previously reported the role in antigen presentation of dimer of dimers con tact amino acids located in the C-terminal domains of the alpha- and beta-c hains of class II. Our calculations show that residues E alpha 89 and R alp ha 145 in the alpha(2)-domain form an inter alpha-chain salt bridge between pairs of alpha beta-heterodimers. Other residues, Q alpha 92 and N alpha 1 15, may be involved in close association in that part of the alpha-chain. W e investigated the role of these amino acids on class II expression and ant igen presentation. Class II composed of an E alpha 89K substituted alpha-ch ain paired with a wt beta-chain exhibited inhibited antigen presentation an d expression of alpha-chain serologic epitopes. In contrast: mutation of R alpha 145E had less affect on antigen presentation and did not affect I-A(k ) serologic epitopes. Interchanging charges of the salt bridge residues by expressing both R alpha 145E and E alpha 89K on the same chain obviated the large negative effect of the E alpha 89K mutation on antigen presentation but not on the serologic epitopes. Our results are similar for those report ed for mutation of DR3's inter-chain salt bridge with the exception that do uble mutants did not moderate the DR3 defect. Interestingly, the amino acid s differences between I-A and DR change the location of the inter-chain sal t bridges. In DR1 these residues are located at positions E alpha 88 and K alpha 111; in I-A(k) these residues are located at position E alpha 89 and R alpha 145. Inter alpha-chain salt bridges are thus maintained in various class II molecules by amino acids located in different parts of the alpha(2 )-domain. This conservation of structure suggests that considerable functio nal importance may attach to the ionic interactions. (C) 1999 Elsevier Scie nce B.V. All rights reserved.