CHARACTERIZATION OF A LYSOZYME MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II MOLECULE-LOADING COMPARTMENT AS A SPECIALIZED RECYCLING ENDOSOME IN MURINE B-LYMPHOCYTES

We have previously identified an intracellular compartment involved in the association between processed lysozyme and IA(k) major histocompa tibility complex class II molecules (called the lysozyme-loading compa rtment (LLC)), Here, we show that She LLC polypeptide composition anal yzed by two-dimensional gel electrophoresis shares similarities with t hat of early endosomes, but not with that of late endosomes, The trans ferrin receptor, a well known marker for both early and recycling endo somes, colocalizes with IA(k) molecules in LLC. Moreover, both transfe rrin and fluid-phase markers have access to LLC after 15 min of intern alization. In the presence of concanamycin B, SDS-stable dimer formati on and transport of class II molecules out of LLC are impaired, In con trast, nocodazole treatment has no effect. These results suggest that LLC is a specialized compartment of the recycling pathway involved in lysozyme loading and in the targeting of lysozyme-major histocompatibi lity class II complexes toward the cell surface.