Involvement of clathrin and AP-2 in the trafficking of MHC class II molecules to antigen-processing compartments
- Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892
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Edited by Peter Cresswell, Yale University School of Medicine, New Haven, CT, and approved April 21, 2005 (received for review March 17, 2005)
Abstract
Major histocompatibility complex class II (MHC-II) molecules are composed of two polymorphic chains, α and β, 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 αβ 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 αβ 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.
Footnotes
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↵ † To whom correspondence should be addressed at: Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, Building 18T, Room 101, National Institutes of Health, Bethesda, MD 20892. E-mail: bonifacinoj{at}mail.nih.gov.
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↵ * Present address: Laboratory of Cell Biology, National Heart, Lung, and Blood Institute, Building 50, Room 2308, National Institutes of Health, Bethesda, MD 20892.
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Author contributions: P.J.M. and J.S.B. designed research; P.J.M. and J.A.M. performed research; P.J.M., J.A.M., and J.S.B. analyzed data; and P.J.M. and J.S.B. wrote the paper.
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This paper was submitted directly (Track II) to the PNAS office.
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Abbreviations: MHC-II, MHC class II; MHC-I, MHC class I; AP, adaptor protein; TfR, transferrin receptor; TGN, trans-Golgi network; Ii, invariant chain; CHC, clathrin heavy chain; siRNA, small interfering RNA.
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Note Added in Proof. An article by Dugast et al. (44) showing similar findings is in press.
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Freely available online through the PNAS open access option.
