The Human Invariant Chain P35 Favors Formation of Nonameric Complexes with MHC Class II Molecules (107.1)

Abstract The invariant chain (Ii) associates with MHCII alpha beta (αβ) chains after biosynthesis. Although previous studies described (αβIi)3 nonameric complexes, a recent report suggested that Ii trimers could only bind a single MHCII heterodimer at once. (Roche et al, 1991, Koch et al, 2011) We have investigated this discrepancy in the context of two human Ii isoforms, p33 and p35. The latter includes an ER retention motif (RXR) that is masked by MHCII β cytoplasmic tail upon its association with Ii. Using transfected HEK293T cells and flow cytometry, we monitored ER egress of various combinations of MHCII and Ii variants. First, we showed that MHCII only masked Ii RXR motif if associated directly (in cis) to the p35 subunit. Second, our results indicated that while p33 homotrimers may exit the ER as pentamer with one MHCII, p35-p33 mixed-trimers require cis masking of every Ii RXR motif before egress, thereby favoring heptamers and nonamers formation. Indeed, sucrose gradient of cell extracts confirmed larger complexes with p35 compared to p33. Altogether, our data argue for a kinetic model in which p33 homotrimers are free to exit the ER at any time, independently of the number of associated MHCII. In contrast, ER retention of Ii trimer containing p35 allows for binding of multiple MHCII until every RXR motifs are inactivated.