Segregation of open Major Histocompatibility Class I conformers at the plasma membrane and during endosomal trafficking reveals conformation-based sorting in the endosomal system.

Fully conformed Major Histocompatibility Class I molecules are complexes of heavy chain non-covalently associated with the peptide and beta-2-microglobulin. Conformational change in the extracellular domain of heavy chain leads to their disassembly and formation of open conformers, a process that physiologically occurs in normal cells and results in their presence at the cell surface. In this study we characterized endosomal trafficking of open conformers of a murine class I allele in order to examine whether conformational change in the extracellular domain of a membrane glycoprotein determines its endosomal sorting. Open conformers segregated from their fully conformed counterparts at the plasma membrane and in endosomes by sequestration in lipid-organized membrane environment. Consequently, open conformers constitutively internalized via distinct clathrin-independent endocytic carriers and converged into "classical" early endosomes together with transferrin receptor and cholera-toxin B subunit. In early endosomes, open conformers were excluded from recycling and diverted towards late endosomes. Due to lack of recycling, open conformers were constitutively internalized at a higher rate than full conformed proteins. Concanamycin A, methyl-β-cyclodextrin and sphingomyelinase treatment prevented segregation of open conformers in vacuolar early endosomes indicating that acidic endosomal environment and membrane composition are critical for the maintenance of the sorting mechanism. In the absence of endosomal acidification open conformers partitioned into lipid disordered membrane composition of early endosomes. Thus, our data suggest for the existence of a lipid-dependent mechanism in the endosomal system that distinguish membrane proteins based on conformation of their extracellular domain.