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MBC in Press, published online ahead of print February 2, 2005
Mol. Biol. Cell 10.1091/mbc.E04-08-0725

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Submitted on August 21, 2004
Revised on January 6, 2005
Accepted on January 18, 2005

A Rab Requirement Is Not Bypassed in SLY1-20 Suppression

Nicole Ballew, Yiting Liu, and Charles Barlowe

Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755

Monitoring Editor: Suzanne Pfeffer

The Rab GTPase Ypt1p and the large homodimer Uso1p are both required for tethering ER-derived vesicles to early Golgi compartments in yeast. Loss-of-function ypt1 and uso1 mutations are suppressed by SLY1-20, a dominant allele that encodes the SNARE-associated protein, Sly1p. Here we investigate the mechanism of SLY1-20 suppression. In wild-type strains, Ypt1p can be coimmunoprecipitated with Uso1p however in a ypt1{Delta}/SLY1-20 strain, which lacks this complex, membrane binding of Uso1p was reduced. In spite of Ypt1p depletion, Uso1p-dependent vesicle tethering was not bypassed under the ypt1{Delta}/SLY1-20 condition. Moreover, tethering and fusion assays using ypt1{Delta}/SLY1-20 membranes remained sensitive to Rab GDP dissociation inhibitor (RabGDI). These results indicate that an alternative Rab protein satisfies the Ypt1p requirement in Uso1p-dependent tethering when SLY1-20 is expressed. Further genetic and biochemical tests revealed that a related Rab protein, Ypt6, might substitute for Ypt1p in ypt1{Delta}/SLY1-20 cells. Additional experimentation to address the mechanism of SLY1-20 suppression in a cog2{Delta} [sec35{Delta}] strain indicated that the Cog2p subunit of the conserved oligomeric Golgi (COG) complex is either functionally redundant or is not directly required for anterograde transport to the Golgi complex.

Address correspondence to: Charles Barlowe (barlowe{at}dartmouth.edu)






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