Local structure of a peptide contact site on a(Formula presented)

We have sought to determine how much amino acid diversity is tolerable at position 69 of the Ak chain, a position previously implicated as a peptide contact site. Slot-machine mutagenesis was used to create a set of 11 mutant Ak cDNAs, each specifying a different amino acid at position 69. These cDNAs were individually expressed In L cells together with a wild-type Ak cDNA to produce a panel of mutant antigen-presenting cell lines. The ability of each member of this panel to present a hen egg lysozyme and a bovine ribonucléase peptide to various T hybrldomas was assessed. We found that a surprising degree of amino acid diversity is tolerable at Ak position 69: Even charged (Glu, Arg) or bulky (Trp, Tyr) residues can be accommodated without abrogating cell-surface expression of Ak, peptide binding to it, or T cell recognition of it. We discuss the implications of these findings for models of T cell recognition of the class II molecule/antlgen duplex.