The p41 isoform of invariant chain is a chaperone for cathepsin L.

The p41 splice variant of major histocompatibility complex (MHC) class II-associated invariant chain (Ii) contains a 65 aa segment that binds to the active site of cathepsin L (CatL), a lysosomal cysteine protease involved in MHC class II-restricted antigen presentation. This segment is absent from the predominant form of Ii, p31. Here we document the in vivo significance of the p41-CatL interaction. By biochemical means and electron microscopy, we demonstrate that the levels of active CatL are strongly reduced in bone marrow-derived antigen-presenting cells that lack p41. This defect mainly concerns the mature two-chain forms of CatL, which depend on p41 to be expressed at wild-type levels. Indeed, pulse-chase analysis suggests that these mature forms of CatL are degraded by endocytic proteases when p41 is absent. We conclude that p41 is required for activity of CatL by stabilizing the mature forms of the enzyme. This suggests that p41 is not merely an inhibitor of CatL enzymatic activity, but serves as a chaperone to help maintain a pool of mature enzyme in late-endocytic compartments of antigen-presenting cells.

Association of mouse mammary tumor virus superantigen with MHC class II during biosynthesis.

Mouse mammary tumor viruses encode superantigens that interact with MHC class II proteins and stimulate T cells. We show here that presentation of mouse mammary tumor virus superantigen does not require DM. Furthermore, we have identified a strong class II peptide binding motif in the Mtv-7 superantigen, and we show that this motif is necessary for association with class II molecules in in vitro translation and in vivo functional assays. Our results suggest that endogenously synthesized viral superantigen can bind to MHC class II heterodimers during biosynthesis in the endoplasmic reticulum in a manner analogous to that used by the class II-associated invariant chain.