Organelle size control – increasing vacuole content activates SNAREs to augment organelle volume through homotypic fusion

Abstract

Cells control the size of their compartments relative to cell volume, but there is also size control within each organelle. Yeast vacuoles neither burst nor do they collapse into a ruffled morphology, indicating that the volume of the organellar envelope is adjusted to the amount of content. It is poorly understood how this adjustment is achieved. We show that the accumulating content of yeast vacuoles activates fusion of other vacuoles, thus increasing the volume-to-surface ratio. Synthesis of the dominant compound stored inside vacuoles, polyphosphate, stimulates binding of the chaperone Sec18/NSF to vacuolar SNAREs, which activates them and triggers fusion. SNAREs can only be activated by lumenal, not cytosolic, polyphosphate ( polyP). Control of lumenal polyP over SNARE activation in the cytosol requires the cytosolic cyclin-dependent kinase Pho80-Pho85 and the R-SNARE Nyv1. These results suggest that cells can adapt the volume of vacuoles to their content through feedback from the vacuole lumen to the SNAREs on the cytosolic surface of the organelle.

Ppn2, a novel Zn2+-dependent polyphosphatase in the acidocalcisome-like yeast vacuole

“…This suggests that polyP levels of the cell are mainly controlled through synthesis and not through degradation. This interpretation is in line with our earlier findings showing that hyper-activation of the VTC complex by point mutations or overexpression of the regulatory subunit leads to higher polyP levels in vivo (Desfougeres et al, 2016a) and that the polyP content of vacuoles may be sensed and communicated to the cytosol (Desfougeres et al, 2016b). Studies on the mobilization of this store have so far been hampered by the fact that polyP turnover could be inhibited only partially because only a single vacuolar polyphosphatase was known.…”

“…The effect of Ppn2 was the strongest during early logarithmic phase, when ppn2Δ cells showed a clear shift towards longer chains. Taken together with previous observations (Desfougeres et al, 2016b), which suggested that the amount of polyP may be sensed inside the vacuole, our results suggest that polyP accumulation is mostly controlled by regulating the rate of the synthesis rather than degradation. In line with this, deletion of additional genes, such as (Table S1).…”

SNARE ‐mediated membrane fusion arrests at pore expansion to regulate the volume of an organelle

“…As a second strategy, yeast cells respond to osmotic challenges by fusing their vacuoles or fragmenting them into smaller units. This re‐adjusts the surface‐to‐volume ratio of the vacuoles (Brett & Merz, ; Desfougères et al , ), probably to avoid an overload of these hydrolytic compartments, which might lead to their lysis and presumably catastrophic consequences for the cells. Insufficient loading, on the other hand, might eliminate vacuolar membrane tension, which is a critical parameter for the functioning of many channels and transporters (Hamill & Martinac, ; Sukharev & Sachs, ).…”

“…Fusion is favored when vacuolar content increases, either by accumulation of macromolecules, e.g., during autophagy, or through osmotic water influx in hypotonic media. Fragmentation is triggered by lack of luminal content or hypertonic media (Bonangelino et al, 2002;Zieger & Mayer, 2012;Desfougères et al, 2016a).…”

Vtc5, a Novel Subunit of the Vacuolar Transporter Chaperone Complex, Regulates Polyphosphate Synthesis and Phosphate Homeostasis in Yeast

“…The ADH1 promoter results in moderate overexpression. This explains the fusion of the vacuoles into a single larger organelle, which occurs as a consequence of enhanced polyP production (52). DIC, differential interference contrast.…”