INTRACELLULAR-TRANSPORT OF INVARIANT CHAIN MHC CLASS-II COMPLEXES TO THE PEPTIDE-LOADING COMPARTMENT

Th cells recognize peptide fragments of foreign Ags bound to MHC class II molecules. Upon synthesis in the endoplasmic reticulum, the alpha- and beta-chains of the class II molecules rapidly associate with inva riant chains (Ii). The dissociation of Ii from class II molecules prec edes binding of processed Ag and the formation of SDS-stable cup dimer s, We previously showed that functional, processed Ag-class II complex es are assembled in a dense lysosome-like compartment that contains st able class II molecules, but no Ii, referred to in this work as the pe ptide-loading compartment. We also identified a separate compartment t hat contains predominantly SDS-unstable Ii-class II complexes. Because we were unable to identify known organelle markers associated with th is compartment, we refer to it as the X compartment, In this work, we provide results that indicate that the X compartment is composed of tr ansport vesicles that move Ii-class II complexes to the peptide-loadin g compartment, where all events in the assembly of processed Ag-class II complexes occur.