title: Sec22p export from the endoplasmic reticulum is independent of SNARE pairing.
abstract: Molecularly distinct sets of SNARE proteins localize to specific intracellular compartments and catalyze membrane fusion events. Although their central role in membrane fusion is appreciated, little is known about the mechanisms by which individual SNARE proteins are targeted to specific organelles. Here we investigated functional domains in Sec22p that direct this SNARE protein to the endoplasmic reticulum (ER), to Golgi membranes, and into SNARE complexes with Bet1p, Bos1p, and Sed5p. A series of Sec22p deletion mutants were monitored in COPII budding assays, subcellular fractionation gradients, and SNARE complex immunoprecipitations. We found that the N-terminal "profilin-like" domain of Sec22p was required but not sufficient for COPII-dependent export of Sec22p from the ER. Interestingly, versions of Sec22p that lacked the N-terminal domain were assembled into ER/Golgi SNARE complexes. Analyses of Sec22p SNARE domain mutants revealed a second signal within the SNARE motif (between layers -4 and -1) that was required for efficient ER export. Other SNARE domain mutants that contained this signal were efficiently packaged into COPII vesicles but failed to assemble into SNARE complexes. Together these results indicated that SNARE complex formation is neither required nor sufficient for Sec22p packaging into COPII transport vesicles and subsequent targeting to the Golgi complex. We propose that the COPII budding machinery has a preference for unassembled ER/Golgi SNARE proteins.
authors.affiliation: Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755, USA.
authors.forename: Yiting
authors.initials: Y
authors.lastname: Liu
authors.forename: John J
authors.initials: JJ
authors.lastname: Flanagan
authors.forename: Charles
authors.initials: C
authors.lastname: Barlowe
mesh_terms: D022181:COP-Coated Vesicles / Q000737:chemistry / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D015870:Gene Expression; D005816:Genetic Complementation Test; D006056:Golgi Apparatus / Q000378:metabolism; D015151:Immunoblotting; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D017354:Point Mutation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000502:physiology; D050683:R-SNARE Proteins; D011956:Receptors, Cell Surface / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D050600:SNARE Proteins; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017384:Sequence Deletion; D013347:Subcellular Fractions / Q000378:metabolism; D033921:Vesicular Transport Proteins
chemical_list: D008565:Membrane Proteins; D050683:R-SNARE Proteins; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins; D050600:SNARE Proteins; D029701:Saccharomyces cerevisiae Proteins; C106909:Sec22 protein, S cerevisiae; D033921:Vesicular Transport Proteins