Jr. Newcomb et al., TRIMERIC INTERACTIONS OF THE INVARIANT CHAIN AND ITS ASSOCIATION WITHMAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II ALPHA-BETA DIMERS, The Journal of biological chemistry, 271(39), 1996, pp. 24249-24256
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.