title: Functional specificity of the mammalian Beclin-Vps34 PI 3-kinase complex in macroautophagy versus endocytosis and lysosomal enzyme trafficking.
abstract: Beclin 1 was originally identified as a novel Bcl-2-interacting protein, but co-immunoprecipitation studies suggest that the major physiological partner for Beclin 1 is the mammalian class III phosphatidylinositol 3-kinase (PI 3-kinase) Vps34. Beclin 1 has been proposed to function as a tumor suppressor by promoting cellular macroautophagy, a process that is known to depend on Vps34. However, an alternative role for Beclin 1 in modulating normal Vps34-dependent protein trafficking pathways has not been ruled out. This possibility was examined in U-251 glioblastoma cells. Immunoprecipitates of endogenous Beclin 1 contained human Vps34 (hVps34), but not Bcl-2. Suppression of Beclin 1 expression by short interfering (si)RNA-mediated gene silencing blunted the autophagic response of the cells to nutrient deprivation or C2-ceramide. However, other PI 3-kinase-dependent trafficking pathways, such as the post-endocytic sorting of the epidermal growth factor receptor (EGFR) or the proteolytic processing of procathepsin D en route from the trans-Golgi network (TGN) to lysosomes, were not affected. Depletion of Beclin 1 did not reduce endocytic internalization of a fluid phase marker (horseradish peroxidase, HRP) or cause swelling of late endosomal compartments typically seen in cells where the function of hVps34 is impaired. These findings argue against a role for Beclin 1 as an essential chaperone or adaptor for hVps34 in normal vesicular trafficking, and they support the hypothesis that Beclin 1 functions mainly to engage hVps34 in the autophagic pathway.
authors.affiliation: Department of Biochemistry and Cancer Biology, Medical University of Ohio, Toledo, OH 43614, USA.
authors.forename: Xuehuo
authors.initials: X
authors.lastname: Zeng
authors.forename: Jean H
authors.initials: JH
authors.lastname: Overmeyer
authors.forename: William A
authors.initials: WA
authors.lastname: Maltese
mesh_terms: D000818:Animals; D051017:Apoptosis Regulatory Proteins; D001343:Autophagy / Q000502:physiology*; D000071186:Beclin-1; D045744:Cell Line, Tumor; D004705:Endocytosis / Q000502:physiology*; D006801:Humans; D008247:Lysosomes / Q000201:enzymology*; D008565:Membrane Proteins; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism; D046912:Multiprotein Complexes; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport / Q000502:physiology; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D022161:Transport Vesicles / Q000378:metabolism*; D021601:trans-Golgi Network / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; D000071186:Beclin-1; D008565:Membrane Proteins; D008869:Microtubule-Associated Proteins; D046912:Multiprotein Complexes; D019869:Phosphatidylinositol 3-Kinases; D011506:Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D034741:RNA, Small Interfering; D011993:Recombinant Fusion Proteins; C491997:BECN1 protein, human; C477012:MAP1LC3A protein, human