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© The Rockefeller University Press, 0021-9525/1997//299 $5.00
The Journal of Cell Biology, Volume 136, Number 2, , 1997 299-306

Article

A Heterodimer of Thioredoxin and IB2 Cooperates with Sec18p (NSF) to Promote Yeast Vacuole Inheritance



Zuoyu Xu, Andreas Mayer, Eric Muller*, and William Wickner

Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755-3844; and * Department of Biochemistry, University of Washington, Seattle, Washington 98195

Early in S phase, the vacuole (lysosome) of Saccharomyces cerevisiae projects a stream of vesicles and membranous tubules into the bud where they fuse and establish the daughter vacuole. This inheritance reaction can be studied in vitro with isolated vacuoles. Rapid and efficient homotypic fusion between saltwashed vacuoles requires the addition of only two purified soluble proteins, Sec18p (NSF) and LMA1, a novel heterodimer with a thioredoxin subunit. We now report the identity of the second subunit of LMA1 as IB2, a previously identified cytosolic inhibitor of vacuolar proteinase B. Both subunits are needed for efficient vacuole inheritance in vivo and for the LMA1 activity in cell extracts. Each subunit acts via a novel mechanism, as the thioredoxin subunit is not acting through redox chemistry and LMA1 is still needed for the fusion of vacuoles which do not contain proteinase B. Both Sec18p and LMA1 act at an early stage of the in vitro reaction. Though LMA1 does not stimulate Sec18p-mediated Sec17p release, LMA1 cannot fulfill its function before Sec18p. Upon Sec17p/Sec18p action, vacuoles become labile but are rapidly stabilized by LMA1. The action of LMA1 and Sec18p is thus coupled and ordered. These data establish LMA1 as a novel factor in trafficking of yeast vacuoles.

Abbreviations used in this paper: NSF, N-ethylmaleimide–sensitive fusion protein; SNAP, soluble NSF attachment protein.

Address all correspondence to W. Wickner, Department of Biochemistry, Dartmouth Medical School, 7200 Vail Building, Hanover, NH 03755-3844. Tel.: (603) 650-1701. Fax: (603) 650-1353.


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