title: Involvement of clathrin and AP-2 in the trafficking of MHC class II molecules to antigen-processing compartments.
abstract: Major histocompatibility complex class II (MHC-II) molecules are composed of two polymorphic chains, alpha and beta, which assemble with an invariant chain, Ii, in the endoplasmic reticulum. The assembled MHC-II complexes are transported to the Golgi complex and then to late endosomes/lysosomes, where Ii is degraded and alphabeta dimers bind peptides derived from exogenous antigens. Targeting of MHC-II molecules to these compartments is mediated by two dileucine-based signals in the cytoplasmic domain of Ii. These signals bind in vitro to two adaptor protein (AP) complexes, AP-1 and AP-2, which are components of clathrin coats involved in vesicle formation and cargo sorting. The physiological roles of these proteins in MHC-II molecule trafficking, however, remain to be addressed. Here, we report the use of RNA interference to examine the involvement of clathrin and four AP complexes (AP-1, AP-2, AP-3, and AP-4) in MHC-II molecule trafficking in vivo. We found that depletion of clathrin or AP-2 caused >10-fold increases in Ii expression on the cell surface and a concomitant decrease in Ii localization to endosomal/lysosomal vesicles. In addition, depletion of clathrin or AP-2 delayed the degradation of Ii and reduced the surface expression of peptide-loaded alphabeta dimers. In contrast, depletion of AP-1, AP-3, or AP-4 had little or no effect. These findings demonstrate that clathrin and AP-2 participate in MHC-II molecule trafficking in vivo. Because AP-2 is only associated with the plasma membrane, these results also indicate that a significant pool of MHC-II molecules traffic to the endosomal-lysosomal system by means of the cell surface.
authors.affiliation: Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: McCormick
authors.forename: José A
authors.initials: JA
authors.lastname: Martina
authors.forename: Juan S
authors.initials: JS
authors.lastname: Bonifacino
mesh_terms: D033962:Adaptor Protein Complex 2 / Q000235:genetics / Q000378:metabolism*; D017951:Antigen Presentation / Q000276:immunology*; D000944:Antigens, Differentiation, B-Lymphocyte / Q000378:metabolism; D002402:Cathepsin D / Q000378:metabolism; D002966:Clathrin / Q000378:metabolism*; D005434:Flow Cytometry; D006367:HeLa Cells; D000949:Histocompatibility Antigens Class II / Q000378:metabolism*; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D008247:Lysosomes / Q000378:metabolism; D008856:Microscopy, Fluorescence; D021381:Protein Transport / Q000502:physiology; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000235:genetics; D015398:Signal Transduction / Q000276:immunology*
chemical_list: D033962:Adaptor Protein Complex 2; D000944:Antigens, Differentiation, B-Lymphocyte; D002966:Clathrin; D000949:Histocompatibility Antigens Class II; D034741:RNA, Small Interfering; C045084:invariant chain; D002402:Cathepsin D