PHOSPHORYLATION REGULATES THE DELIVERY OF MHC CLASS-II INVARIANT CHAIN COMPLEXES TO ANTIGEN-PROCESSING COMPARTMENTS

Transport of newly synthesized MHC class II glycoproteins to endosomal Ag processing compartments is mediated by their association with the invariant chain Oil, Targeting to these compartments is dependent upon recognition of leucine-based endosomal/lysosomal targeting motifs in the Ii cytosolic domain. Ii, like many molecules that contain leucine- based endosomal targeting motifs, is phosphorylated in vivo. In this r eport we demonstrate that the cytosolic domain of the p35 Ii isoform i s phosphorylated in class II Ii complexes isolated from human B lympho blastoid cell lines or freshly obtained PBMC, Mutation of serine resid ue 6 or 8 prevents phosphorylation of Ii-p35 expressed in HeLa cells, Treatment of B lymphoblastoid cell lines with the serine/threonine kin ase inhibitor staurosporine prevented Ii phosphorylation and significa ntly delayed trafficking of newly synthesized class II Ii complexes to endosomal Ag processing compartments. By contrast, staurosporine had no effect on the rate of transport of class I or class II glycoprotein s through the Golgi apparatus and did not inhibit the delivery of the chimeric molecule Tac-DM beta to endocytic compartments, suggesting th at staurosporine does not nonspecifically inhibit protein transport to the endocytic pathway. These results demonstrate that phosphorylation regulates the efficient targeting of MHC class II Ii complexes to Ag processing compartments and strongly suggest that this effect is media ted by phosphorylation of the MHC class II-associated Ii chain.