CHARACTERISTICS OF PEPTIDE AND MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I BETA(2)-MICROGLOBULIN BINDING TO THE TRANSPORTERS ASSOCIATED WITH ANTIGEN-PROCESSING (TAP1 AND TAP2)

The transporter proteins associated with antigen processing (TAP prote ins) transport antigenic peptides across the endoplasmic reticulum mem brane where they can assemble with newly synthesized major histocompat ibility complex (MHC) class I/beta(2)-microglobulin (beta(2)m) dimers. We have shown previously that TAP possesses a peptide-recognition sit e with broad specificity and that MHC class I/beta(2)m dimers physical ly associate with TAP, Here, we further characterize the nature of the peptide-binding site on TAP, and the site of interaction of TAP with MHC class I/beta(2)m dimers. TAP photoaffinity labeling experiments re vealed that both TAP1 and TAP2 are photolabeled by two distinct photop eptide analogues, suggesting that elements of both TAP1 and TAP2 compo se the peptide-recognition site. TAP photolabeling analysis on transfe ctant cell lines that express TAP1 and TAP2 both individually and toge ther revealed that efficient formation of the peptide-binding site occ urs only when TAP1 and TAP2 are coexpressed, which correlates with the finding that peptide translocation via TAP occurs only in the presenc e of both TAP1 and TAP2. These data strongly support the notion that T AP functions as a heterodimer. MHC class I/beta(2)m dimers were shown to associate with individual TAP1 chains but were not detectable with individual TAP(2) chains. This result suggests that the site of intera ction for MHC class I/beta(2)m dimers with TAP is on TAP1.