THE MEMBRANE-BOUND AND SOLUBLE FORMS OF HLA-G BIND IDENTICAL SETS OF ENDOGENOUS PEPTIDES BUT DIFFER WITH RESPECT TO TAP ASSOCIATION

The class Ib antigen HLA-G is expressed as a membrane-bound protein li ke classical class Ia molecules (M . HLA-G) but, unlike typical class I, is also expressed as a soluble protein (S . HLA-G) with a unique C terminus. Our results show that, similar to classical class I proteins , the membrane-bound form of HLA-G associated with TAP, as evidenced b y the ability to immunoprecipitate HLA-G class I heavy chain with TAP antisera. In contrast, the soluble G protein did not appear to associa te with TAP in the same manner, since similar immunoprecipitation expe riments failed to detect soluble G complex. A detailed analysis of pep tides bound to the soluble and membrane HLA-G proteins expressed in th e B lymphoblastoid cell line 721.221 showed that, like class Ia comple xes, both HLA-G protei ns consist of heavy and light chains complexed with nonameric peptides in a 1:1:1 ratio. The two proteins bind essent ially the same set of peptides, which are derived from a variety of in tracellular proteins and define a peptide motif for HLA-G. The peptide s contain Leu at the C terminus and Pro or small hydrophobic amino aci ds in position 3 followed by Pro or Gly in position 4. The complexity of the bound peptides is lower than that found for some class Ia compl exes, but is more similar to class Ia than to the limited repertoire o f some murine class Ib molecules.