title: Localization of the AP-3 adaptor complex defines a novel endosomal exit site for lysosomal membrane proteins.
abstract: The adaptor protein (AP) 3 adaptor complex has been implicated in the transport of lysosomal membrane proteins, but its precise site of action has remained controversial. Here, we show by immuno-electron microscopy that AP-3 is associated with budding profiles evolving from a tubular endosomal compartment that also exhibits budding profiles positive for AP-1. AP-3 colocalizes with clathrin, but to a lesser extent than does AP-1. The AP-3- and AP-1-bearing tubular compartments contain endocytosed transferrin, transferrin receptor, asialoglycoprotein receptor, and low amounts of the cation-independent mannose 6-phosphate receptor and the lysosome-associated membrane proteins (LAMPs) 1 and 2. Quantitative analysis revealed that of these distinct cargo proteins, only LAMP-1 and LAMP-2 are concentrated in the AP-3-positive membrane domains. Moreover, recycling of endocytosed LAMP-1 and CD63 back to the cell surface is greatly increased in AP-3-deficient cells. Based on these data, we propose that AP-3 defines a novel pathway by which lysosomal membrane proteins are transported from tubular sorting endosomes to lysosomes.
authors.affiliation: Genentech Inc., South San Francisco, CA 94080, USA.
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Peden
authors.forename: Viola
authors.initials: V
authors.lastname: Oorschot
authors.forename: Boris A
authors.initials: BA
authors.lastname: Hesser
authors.forename: Cary D
authors.initials: CD
authors.lastname: Austin
authors.forename: Richard H
authors.initials: RH
authors.lastname: Scheller
authors.forename: Judith
authors.initials: J
authors.lastname: Klumperman
mesh_terms: D033963:Adaptor Protein Complex 3; D033966:Adaptor Protein Complex beta Subunits; D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000502:physiology; D011992:Endosomes / Q000502:physiology* / Q000648:ultrastructure; D007700:Kinetics; D008247:Lysosomes / Q000502:physiology* / Q000648:ultrastructure; D008565:Membrane Proteins / Q000378:metabolism*; D021381:Protein Transport; D018808:Transcription Factor AP-1 / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism*; D021601:trans-Golgi Network / Q000502:physiology / Q000648:ultrastructure
chemical_list: D033963:Adaptor Protein Complex 3; D033966:Adaptor Protein Complex beta Subunits; C113910:Ap3d1 protein, mouse; D008565:Membrane Proteins; D018808:Transcription Factor AP-1; D014157:Transcription Factors