TRIMERIC INTERACTIONS OF THE INVARIANT CHAIN AND ITS ASSOCIATION WITHMAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II ALPHA-BETA DIMERS

The invariant chain (I chain) associates with major histocompatibility complex class II alpha beta heterodimers upon synthesis, preventing t hem from binding peptides and unfolded proteins in the endoplasmic ret iculum and directing class II transport to post-Golgi endosomal compar tments. To assess which regions of the I chain are involved in binding class II molecules, we have studied proteolytic fragments of the I ch ain generated both by natural proteolytic degradation of alpha beta di mer-invariant chain complexes (alpha beta . I) within human B cells an d by in vitro digestion of purified alpha beta . I complexes with prot einase K. The 18-kDa luminal I chain fragment generated by proteinase K, called K3, remains associated with alpha beta dimers and retains th e complex (alpha beta . K3) in a high molecular mass nonameric configu ration. The N terminus of the K3 fragment was identified as glycine 11 0. This indicates that the K3 fragment lies outside of the class II-as sociated invariant chain peptide region (amino acids 81-104) of the I chain, shown to be important for initial alpha beta . I assembly. An N -terminal 12-kDa I chain fragment called p12, generated intracellularl y, was also analyzed and was found to remain associated with alpha bet a dimers in a high molecular mass form analogous to the nonameric alph a beta . I complex. These results demonstrate that at least two class II contact points exist along the length of the I chain and that diffe rent regions of the I chain can stabilize the alpha beta . I nonamer. Additional evidence suggests that the O-linked glycan(s) characteristi c of the I chain is added to the short C-terminal region absent from t he K3 fragment.