The MHC class II β chain cytoplasmic tail overcomes the invariant chain p35‐encoded endoplasmic reticulum retention signal

The human‐specific p35 isoform of the invariant chain (Ii) includes an R‐X‐R endoplasmic reticulum (ER) retention motif that is inactivated upon HLA‐DR binding. Although the masking is assumed to involve the cytoplasmic tails of class II molecules, the mechanism underlying this function remains to be investigated. Moreover, in light of the polymorphic nature of the class II cytosolic tails, little is known about the capacity of various isotypes or alleles to overcome the retention signal of Iip35. To gain further insights into these issues, we first addressed the proposed role of the HLA‐DR cytoplasmic tails. As shown by flow cytometry, the presence of Iip35 in transfected HeLa cells prevented surface expression of HLA‐DR molecules lacking their cytoplasmic tails (DRαTM/βTM). These truncated class II molecules and Iip35 accumulated in the ER, and co‐localized with calnexin, as determined by confocal microscopy. Sensitivity of DRαTM/βTM to endoglycosidase H treatment confirmed that these molecules do not reach the trans‐Golgi network when associated with Iip35. Further characterization revealed that the β chain cytosolic tail is critical for efficient ER egress of class II/Iip35 complexes. Interestingly, our results clearly demonstrate for the first time that DP and DQ isotypes can also overcome the retention motif of Iip35 through a mechanism involving their very distinctive polymorphic β chain cytoplasmic tails. Altogether, these results further dissect the masking of di‐basic retention signals, and emphasize the interplay between class II molecules and Ii for the transport of the complex to the endocytic pathway.