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The class lb antigen HLA-G is expressed as a membrane-bound protein like classical class la molecules (WHLA-G) but, unlike typical class I, isalso expressed as a soluble protein (SaHLA-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 by the ability to immunoprecipitate HLA-G class I heavy chain with TAP antisera. In contrast, the soluble G protein did not appear to associate with TAP in the same manner, since similar immunoprecipltation experiments failed to detect soluble G complex. A detailed analysis of peptides bound to the soluble and membrane HLA-G proteins expressed in the B lymphoblastoid cell line 721.221 showed that, like class la complexes, both HLA-G proteins consist of heavy and light chains complexed with nonameric peptides in a 1: l: l ratio. The two proteins bind essentially the …