TAP associates with a unique class I conformation, whereas calnexin associates with multiple class I forms in mouse and man

To define the rules governing de novo assembly of the trimeric class I complex, we have identified the class I folding/assembly intermediates associated with calnexin or TAP, using both human and mouse cell lines. To better characterize the class I H chain structure associated with TAP, mouse mAb that distinguish open (64-3-7⁺) vs folded (30-5-7⁺) L(d) heavy (H) chains were used. We report here that open forms of L(d) are uniquely and specifically associated with TAP and that the conformational change in the class I H chain coincident with peptide binding induces TAP release. Chimeric L(d)/Q10 displayed TAP association, demonstrating that soluble class I molecules can bind TAP. As previously reported, β₂m was found to be required for H chain association with TAP. Interestingly, β₂m was associated with TAP in the human class I-negative cell line LCL 721.221, suggesting that β₂m can bind to TAP before class I H chain. In contrast to TAP, which binds a specific class I conformation, calnexin was detected in association with multiple forms of both mouse and human class I. Most significantly, we show for the first time that β₂m-assembled forms of human as well as mouse class I molecules interact with calnexin. Based on these findings, we propose a model for the sequential assembly of class I heterotrimers and their respective interactions with TAP and calnexin.