Low pH puts proteasomes in storage

Low pH puts proteasomes in storage P eters et al. describe how starving yeast cells regulate the formation of proteasome storage granules (PSGs). The proteasome is a large, multisubunit protease essential for many cellular functions including proliferation. In the absence of glucose or other carbon sources, budding yeast aggregate their proteasomes in PSGs, which may help protect the complexes until conditions improve. When glucose becomes available again, PSGs disperse, allowing cells to quickly reenter the cell cycle without waiting for new proteasomes to be built from scratch. How glucose levels regulate PSG assembly is unclear, however. By screening a library of yeast deletion mutants, Peters et al. found that yeast lacking components of the vacuolar ATPase (V-ATPase) formed PSGs more quickly upon glucose starvation but took longer to disassemble the granules when glucose supplies were restored. The V-ATPase pumps protons out of the cytosol and into the vacuole, so yeast lacking this complex have a lower cyto-plasmic pH. Lowering intracellular pH in other ways—by mutating the plasma membrane proton pump Pma1, for example—also promoted PSG formation, even in the presence of normal glucose levels. Low pH also stimulated the assembly of other cytoplasmic granules, such as actin bodies, that form in starving yeast cells. Glucose deprivation is known to induce disassembly of the V-ATPase, which would acidify the cytosol and stimulate PSG formation. Senior author Shay Ben-Aroya now wants to determine how low pH promotes PSG assembly and to investigate whether proteasomes are inactive inside PSGs or whether they continue to degrade proteins during cell starvation. F erreira et al. reveal how Au-rora B and the microtubule plus-end binding protein EB3 coordinate cytokinesis and daughter cell adhesion at the end of mitosis. EB3 and its relative EB1 regulate microtubule dynamics in inter-phase cells, but they seem to be less important during mitosis. Indeed, Ferreira et al. confi rmed that, although cells lacking EB1 and EB3 had problems orienting their mitotic spindles, they passed through mitosis with little delay. EB3-defi cient cells ran into trouble as they exited mitosis, however. In the absence of EB3, the microtubules that form the mid-body between separating daughter cells were unstable, causing the midbody to oscillate and partially inhibit cytokinesis. The mitotic kinase Aurora B, which concentrates at the midbody during anaphase, phosphorylated EB3 and promoted its ability to stabilize midbody microtubules. Most cells lacking EB3 still completed cytokinesis but their daughter cells often failed …