title: The adaptor protein AP-4 as a component of the clathrin coat machinery: a morphological study.
abstract: The four members of the AP (adaptor protein) family are heterotetrameric cytosolic complexes that are involved in the intracellular trafficking of cargo proteins between different organelles. They interact with motifs present in the cytoplasmic tails of their specific cargo proteins at different intracellular locations. While AP-1, AP-2 and AP-3 have been investigated extensively, very few studies have focused on the fourth member, AP-4. In the present study, we report on the intracellular localization of AP-4 in the MDCK (Madin-Darby canine kidney) and MelJuSo cell lines after immunogold labelling of ultrathin cryosections. We find that AP-4 is localized mainly in the Golgi complex, as well as on endosomes and transport vesicles. Interestingly, we show for the first time that AP-4 is localized with the clathrin coat machinery in the Golgi complex and in the endocytic pathway. Furthermore, we find that AP-4 is localized with the CI-MPR (cation-independent mannose 6-phosphate receptor), but not with the transferrin receptor, LAMP-2 (lysosomal-associated membrane protein-2) or invariant chain. The difference in morphology between CI-MPR/AP-4-positive vesicles and CI-MPR/AP-1-positive vesicles raises the possibility that AP-4 acts at a location different from that of AP-1 in the intracellular trafficking pathway of CI-MPR.
authors.affiliation: Department of Molecular Cell Biology, University of Oslo, PB 1041 Blindern, N-0316 Oslo, Norway.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Barois
authors.forename: Oddmund
authors.initials: O
authors.lastname: Bakke
mesh_terms: D033964:Adaptor Protein Complex 4 / Q000378:metabolism*; D000818:Animals; D015703:Antigens, CD / Q000378:metabolism; D000944:Antigens, Differentiation, B-Lymphocyte / Q000378:metabolism; D002412:Cations / Q000378:metabolism; D002460:Cell Line; D045744:Cell Line, Tumor; D022163:Clathrin-Coated Vesicles / Q000737:chemistry*; D004285:Dogs; D011992:Endosomes / Q000737:chemistry; D006056:Golgi Apparatus; D000949:Histocompatibility Antigens Class II / Q000378:metabolism; D006801:Humans; D007668:Kidney / Q000737:chemistry / Q000166:cytology; D052119:Lysosomal-Associated Membrane Protein 2; D051907:Lysosomal Membrane Proteins; D008545:Melanoma / Q000737:chemistry / Q000473:pathology; D017527:Receptor, IGF Type 2 / Q000378:metabolism; D011990:Receptors, Transferrin / Q000378:metabolism
chemical_list: D033964:Adaptor Protein Complex 4; D015703:Antigens, CD; D000944:Antigens, Differentiation, B-Lymphocyte; D002412:Cations; D000949:Histocompatibility Antigens Class II; D052119:Lysosomal-Associated Membrane Protein 2; D051907:Lysosomal Membrane Proteins; D017527:Receptor, IGF Type 2; D011990:Receptors, Transferrin; C045084:invariant chain