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Published 31 January 2005. doi:10.1083/jcb.200407141
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 168, Number 3, 401-414
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Article

 The vacuolar kinase Yck3 maintains organelle fragmentation by regulating the HOPS tethering complex

Tracy J. LaGrassa and Christian Ungermann

Biochemie-Zentrum der Universität Heidelberg, 69120 Heidelberg, Germany

Correspondence to Christian Ungermann: cu2{at}ix.urz.uni-heidelberg.de

The regulation of cellular membrane flux is poorly understood. Yeast respond to hypertonic stress by fragmentation of the normally large, low copy vacuole. We used this phenomenon as the basis for an in vivo screen to identify regulators of vacuole membrane dynamics. We report here that maintenance of the fragmented phenotype requires the vacuolar casein kinase I Yck3: when Yck3 is absent, salt-stressed vacuoles undergo fission, but reassemble in a SNARE-dependent manner, suggesting that vacuole fusion is disregulated. Accordingly, when Yck3 is deleted, in vitro vacuole fusion is increased, and Yck3 overexpression blocks fusion. Morphological and functional studies show that Yck3 modulates the Rab/homotypic fusion and vacuole protein sorting complex (HOPS)-dependent tethering stage of vacuole fusion. Intriguingly, Yck3 mediates phosphorylation of the HOPS subunit Vps41, a bi-functional protein involved in both budding and fusion during vacuole biogenesis. Because Yck3 also promotes efficient vacuole inheritance, we propose that tethering complex phosphorylation is a part of a general, switch-like mechanism for driving changes in organelle architecture.

Abbreviations used in this paper: ALP, alkaline phosphatase; CKI, casein kinase I; GAP, GTPase activating protein; GEF, guanine nucleotide exchange factor; HOPS, homotypic fusion and vacuole protein sorting complex; PIC, protease inhibitor cocktail; PS, 10 mM Pipes-KOH, pH 6.8, 200 mM sorbitol; PSK, PS with 150 mM KCl; PtdIns(3,5)P2, phosphatidylinositol 3,5-bisphosphate; wt, wild-type.


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