Influence of the tapasin C terminus on the assembly of MHC class I allotypes

Abstract

Several endoplasmic reticulum proteins, including tapasin, play an important role in MHC class I assembly. In this study, we assessed the influence of the tapasin cytoplasmic tail on three mouse MHC class I allotypes (H2-K b , -K d , and -L d ), and demonstrated that the expression of truncated mouse tapasin in mouse cells resulted in very low K b , K d , and L d surface expression. The surface expression of K d also could not be rescued by human soluble tapasin, suggesting the surface expression phenotype of the mouse MHC class I molecules in the presence of soluble tapasin was not due to mouse/human differences in tapasin. Notably, soluble mouse tapasin was able to partially rescue HLA-B8 surface expression on human 721.220 cells. Thus, the cytoplasmic tail of tapasin (either mouse or human) has a stronger impact on the surface expression of murine MHC class I molecules on mouse cells than on the expression of HLA-B8 on human cells. A K408W mutation in the mouse tapasin transmembrane/cytoplasmic domain disrupted K d folding and release from tapasin, but not interaction with TAP, indicating that the mechanism whereby the tapasin transmembrane/cytoplasmic domain facilitates MHC class I assembly is not limited to TAP stabilization. Our findings indicate that the C-terminus of mouse tapasin plays a vital role in enabling murine MHC class I molecules to be expressed at the surface of mouse cells.

Productive association between MHC class I and tapasin requires the tapasin transmembrane/cytosolic region and the tapasin C-terminal Ig-like domain

“…Furthermore, the HLA molecules that are assembled in the presence of soluble tapasin are less stable than those assembled in the presence of wild type tapasin (Tan et al, 2002). In striking contrast, we have previously reported that both soluble mouse tapasin and soluble human tapasin do not enable the cell surface expression of murine MHC class I molecules (Simone et al, 2009a; Simone et al, 2009b). Rather, soluble tapasin suppressed murine MHC class I surface expression below the level present on cells without tapasin, suggesting that soluble tapasin impedes murine MHC class I assembly (Simone et al, 2009b).…”

Newly Discovered Viral E3 Ligase pK3 Induces Endoplasmic Reticulum-associated Degradation of Class I Major Histocompatibility Proteins and Their Membrane-bound Chaperones

“…Interestingly, we found that it is only the integral membrane components of the PLC, TAP, and Tpn that are dependent upon MHCI expression. Consistently, the TM domain of Tpn is known to be important for both its interaction with MHCI (45,53,54) and TAP (55). Although TM interactions are known to be important for the assembly of multimeric complexes such as the TCR-CD3 complex (56), our findings suggest TM interactions are also critical for monitoring the disassembly of MHC-PLC complexes and targeting them for ERAD.…”

Calreticulin in the immune system: ins and outs

“…Tapasin binds to MHC class I molecules, and this interaction is enhanced by ERp57 and calreticulin [7, 8, 37]. Additionally, the transmembrane region of tapasin, which binds TAP, increases the efficiency of tapasin-MHC class I binding [37, 38]. The protein complex comprising TAP, tapasin, ERp57, calreticulin and MHC class I is termed the peptide loading complex (PLC).…”