Salt stress fragments wt (top) and yck3Δ (bottom) vacuoles (middle panels), but yck3Δ vacuoles then prematurely reassemble (right panels).

On page 401, LaGrassa and Ungermann show that divided vacuolar membranes are kept apart by a kinase that disables their tethering complexes. Similar tethers may be the regulatory center of fission/fusion cycles at various membranes.

Yeast vacuoles divide, or fragment, during budding or in response to salt stress. Mutants have been isolated that are unable to fragment, but the new article identifies a regulatory mutant whose vacuoles, although initially fragmented correctly, reassemble prematurely.

This mutant lacks the Yck3 casein kinase. In vitro and in vivo, abundant Yck3 inhibited vacuole fusion, whereas its absence improved it. The authors find that the effects are tied to Yck3′s ability to inhibit tethering, which brings two membranes in close enough proximity for their SNAREs to zip them together.

On vacuoles, tethering is controlled by a Rab GTPase and the HOPS complex. The authors show that a HOPS component is phosphorylated during salt stress and that this modification requires Yck3. Phosphorylation increased HOPS dynamics and may interfere with tethering by loosening its association with the Rab. But without Yck3, unphosphorylated HOPS was tightly bound to vacuole membranes, allowing membranes to snap back together.

Tethering complexes may be a common target for inhibiting fusion. Phosphorylation of GM130 tethering protein prevents refusion of Golgi fragments. Tethering complexes of other organelles, such as the exocyst, may be similarly targeted by kinases—to slow protein secretion, for example.