A region of tapasin that affects L^d binding and assembly

Tapasin has been shown to stabilize TAP and to link TAP to the MHC class I H chain. Evidence also has been presented that tapasin influences the loading of peptides onto MHC class I. To explore the relationship between the ability of tapasin to bind to TAP and the MHC class I H chain and the ability of tapasin to facilitate class I assembly, we have created novel tapasin mutants and expressed them in 721.220-L^d cells. One mutant has a deletion of nine amino acid residues (tapasin Δ334-342), and the other has amino acid substitutions at positions 334 and 335. In this report we describe the ability of these mutants to interact with L^d and their effects on L^d surface expression. We found that tapasin Δ334-342 was unable to bind to the L^d H chain, and yet it facilitated L^d assembly and expression. Tapasin Δ334-342 was able to bind and stabilize TAP, suggesting that TAP stabilization may be important to the assembly of L^d. Tapasin mutant H334F/H335Y, unlike tapasin Δ334-342, bound to L^d. Expression of tapasin H334F/H335Y in 721.220-L^d reduced the proportion of cell surface open forms of L^d and retarded the migration of L^d from the endoplasmic reticulum. In total, our results indicate that the 334-342 region of tapasin influences L^d assembly and transport.