title: Silencing of mammalian Sar1 isoforms reveals COPII-independent protein sorting and transport.
abstract: The Sar1 GTPase coordinates the assembly of coat protein complex-II (COPII) at specific sites of the endoplasmic reticulum (ER). COPII is required for ER-to-Golgi transport, as it provides a structural and functional framework to ship out protein cargoes produced in the ER. To investigate the requirement of COPII-mediated transport in mammalian cells, we used small interfering RNA (siRNA)-mediated depletion of Sar1A and Sar1B. We report that depletion of these two mammalian forms of Sar1 disrupts COPII assembly and the cells fail to organize transitional elements that coordinate classical ER-to-Golgi protein transfer. Under these conditions, minimal Golgi stacks are seen in proximity to juxtanuclear ER membranes that contain elements of the intermediate compartment, and from which these stacks coordinate biosynthetic transport of protein cargo, such as the vesicular stomatitis virus G protein and albumin. Here, transport of procollagen-I is inhibited. These data provide proof-of-principle for the contribution of alternative mechanisms that support biosynthetic trafficking in mammalian cells, providing evidence of a functional boundary associated with a bypass of COPII.
authors: Department of Cellular and Translational Pharmacology, Consorzio Mario Negri Sud, Via Nazionale 8/A, 66030 Santa Maria Imbaro, Chieti, Italy. meritxell@negrisud.it
authors: Meritxell B
authors: MB
authors: Cutrona
authors: Galina V
authors: GV
authors: Beznoussenko
authors: Aurora
authors: A
authors: Fusella
authors: Oliviano
authors: O
authors: Martella
authors: Pedro
authors: P
authors: Moral
authors: Alexander A
authors: AA
authors: Mironov
mesh_terms: D022181:COP-Coated Vesicles / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D020868:Gene Silencing; D006056:Golgi Apparatus / Q000378:metabolism; D006367:HeLa Cells; D056945:Hep G2 Cells; D006801:Humans; D020559:Monomeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011347:Procollagen / Q000378:metabolism; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport; D034741:RNA, Small Interfering / Q000235:genetics; D055571:Secretory Pathway / Q000235:genetics*
chemical_list: D011347:Procollagen; D020033:Protein Isoforms; D034741:RNA, Small Interfering; C493083:SAR1A protein, human; D020559:Monomeric GTP-Binding Proteins