Trimeric interactions of the invariant chain and its association with major histocompatibility complex class II alpha beta dimers.
The invariant chain (I chain) associates with major histocompatibility complex class II alphabeta heterodimers upon synthesis, preventing them from binding peptides and unfolded proteins in the endoplasmic reticulum and directing class II transport to post-Golgi endosomal compartments. To assess which regions of the I chain are involved in binding class II molecules, we have studied proteolytic fragments of the I chain generated both by natural proteolytic degradation of alphabeta dimer-invariant chain complexes (alphabeta.I) within human B cells and by in vitro digestion of purified alphabeta middle dotI complexes with proteinase K. The 18-kDa luminal I chain fragment generated by proteinase K, called K3, remains associated with alphabeta dimers and retains the complex (alphabeta.K3) in a high molecular mass nonameric configuration. The N terminus of the K3 fragment was identified as glycine 110. This indicates that the K3 fragment lies outside of the class II-associated invariant chain peptide region (amino acids 81-104) of the I chain, shown to be important for initial alphabeta.I assembly. An N-terminal 12-kDa I chain fragment called p12, generated intracellularly, was also analyzed and was found to remain associated with alphabeta dimers in a high molecular mass form analogous to the nonameric alphabeta.I complex. These results demonstrate that at least two class II contact points exist along the length of the I chain and that different regions of the I chain can stabilize the alphabeta.I nonamer. Additional evidence suggests that the O-linked glycan(s) characteristic of the I chain is added to the short C-terminal region absent from the K3 fragment.