HLA-E SURFACE EXPRESSION DEPENDS ON BINDING OF TAP-DEPENDENT PEPTIDESDERIVED FROM CERTAIN HLA CLASS-I SIGNAL SEQUENCES

Previous studies showed that HLA-E was expressed in lymphoblastoid cel l line (LCL) 721.221 cells, but surface expression was lacking. To det ermine the signals controlling surface expression, we constructed a se ries of hybrid genes using complementary portions derived from the HLA -E and HLA-A2 genes. In this manner, a hybrid of HLA-E was identified, designated AEH, which differed from HLA-E by having the HLA-AZ signal sequence substituting for the HLA-E leader peptide. Transfection of L CL 721.221 cells,vith AEH induced HLA-E surface expression. Analysis o f peptides bound to HLA-E revealed that a nonamer peptide derived from the A2 signal sequence was the predominant peptide bound. LCL 721.221 cells transfected with certain class I genes, including HLA-G, were a lso sufficient to promote peptide binding and HLA-E surface expression without increasing the level of HLA-E heavy chain synthesis, Peptides bound to HLA-E consisted of nine amino acids, with methionine at posi tion 2 and leucine in the carboxyl-terminal position, and were nearly identical to the leader sequence-derived peptide previously shown to b e a predominant peptide bound to the murine Qa-l Ag, Signal peptides d erived from certain HLA-B proteins with threonine in position 2 only m arginally up-regulated FILA-E surface expression in .221 cells, An exa mination of HLA-E peptide binding in the TAP negative cell line .134 i ndicated that peptide binding to HLA-E was dependent on a functional T AP heterodimer regardless of whether peptide was available in cis, as in the AEH construct, or in trans, as in the class I transfectants of .221 cells.