Trafficking of major histocompatibility complex class II molecules in human B-lymphoblasts deficient in the AP-3 adaptor complex.
The major histocompatibility complex class II subunits (MHC-II) alpha and beta assemble with the invariant chain (Ii) in the endoplasmic reticulum and are transported to endosomal-lysosomal organelles known as MHC class II compartments (MIICs). Although it has been shown that two dileucine-based signals in the cytosolic tail of Ii, as well as a dileucine-based signal in the tail of the beta chain mediate sorting to MIICs, the molecular mechanisms by which alphabetaIi complexes are sorted have yet to be resolved fully. The AP-3 adaptor complex stands out as a particularly good candidate for mediating this targeting because: (i) it has a proven role in the trafficking of membrane proteins to lysosome-related organelles; and (ii) it has the ability to interact with dileucine-based signals in vitro. To investigate the potential role of AP-3 in transport of MHC-II to MIICs, we have examined MHC-II trafficking in human B-lymphoblast lines from patients with Hermansky-Pudlak syndrome type 2 (HPS-2), which are deficient in the AP-3 complex. Pulse-chase analyses revealed no significant alteration in the kinetics of synthesis and degradation of either MHC-II subunits or Ii. Moreover, we observed neither impairment of the formation of compact SDS-resistant alphabeta dimers, nor delay in the appearance of a conformational epitope indicative of a mature, Ii-free alphabeta dimer. Finally, we demonstrated that in HPS-2 patients' cells, there was no delay in the expression of the alphabeta dimers on the cell surface. Thus, AP-3 does not seem to be essential for normal trafficking of MHC-II. These findings have important implications for HPS-2 patients, because they suggest that the recurrent bacterial infections suffered by these patients are not likely due to impaired antigen processing and presentation by MHC-II.
Caplan
S
S
Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Building 18T Room 101, 18 Library Dr. MSC 5430, 20892-5430, Bethesda, MD, USA.
Dell'Angelica
E C
EC
Gahl
W A
WA
Bonifacino
J S
JS
D033965:Adaptor Protein Complex alpha Subunits; D033942:Adaptor Proteins, Vesicular Transport; D016115:Albinism, Oculocutaneous / Q000235:genetics* / Q000276:immunology* / Q000378:metabolism; D017951:Antigen Presentation / Q000235:genetics; D001402:B-Lymphocytes / Q000276:immunology / Q000378:metabolism*; D002461:Cell Line, Transformed; D000949:Histocompatibility Antigens Class II / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D008213:Lymphocyte Activation / Q000235:genetics; D008565:Membrane Proteins / Q000172:deficiency* / Q000235:genetics; D034041:Monomeric Clathrin Assembly Proteins
D033965:Adaptor Protein Complex alpha Subunits; D033942:Adaptor Proteins, Vesicular Transport; D000949:Histocompatibility Antigens Class II; D008565:Membrane Proteins; D034041:Monomeric Clathrin Assembly Proteins; C439023:clathrin assembly protein AP180