Title: Mechanisms Underlying Mitochondrial Quality Control and Cytokinesis in Budding Yeast
Description: This work discusses both mechanisms underlying mitochondrial quality control and cytokinesis in the budding yeast Saccharomyces cerevisiae. As these topics are quite different, their presentation has been divided into two parts, "Part I: Mitochondrial Remodeling Through the Proteasome is Critical for Mitochondrial Quality Control in Budding Yeast" and "Part II: Aim44p Regulates Phosphorylation of Hof1p to Promote Contractile Ring Closure During Cytokinesis in Budding Yeast." In Part I, we show that the proteasome is critical for cellular fitness in response to chronic, low levels of mitochondrial reactive oxygen species (ROS) in budding yeast. Deleting DOA1, which is required for ubiquitin-mediated degradation, UFD5, which promotes proteasome gene expression, or NAS2, which promotes proteasome regulatory particle assembly, increases the sensitivity of yeast to chronic, low levels of mitochondrial ROS. In contrast, deleting ATG32, a gene required for mitophagy, other autophagy genes, non-essential chaperones including prohibitins, or mitochondrial proteins including the Lon protease (Pim1p) or YME1, does not affect cellular fitness under these conditions.

Doa1p binds with Cdc48p and Vms1p, which associates with mitochondria and promotes extraction of ubiquitinated proteins from the organelle for proteasomal degradation in a pathway called mitochondria-associated degradation (MAD). Elevated mitochondrial ROS increases protein ubiquitination, ubiquitination of the mitochondrial protein aconitase and expression of key MAD proteins. Interestingly, down-regulating ER-associated degradation (ERAD), which shares some common proteins with MAD, can promote cell growth under conditions of elevated mitochondrial ROS. Finally, deletion of DOA1 results in increased sensitivity of yeast and yeast mitochondria to oxidative stress. Mitochondria in doa1 null cells are more oxidized than mitochondria in wild-type or atg32 null cells under conditions of elevated mitochondrial ROS. Moreover, deletion of DOA1 results in a decrease in chronological lifespan. These findings support a critical role for the proteasome and MAD in mitochondrial quality control, which in turn affects cellular fitness, in response to chronic, low levels of mitochondrial ROS.

In Part II, we show that the protein product of YPL158C, Aim44p, undergoes septin-dependent recruitment to the site of cell division. Aim44p co-localizes with Myo1p, the type II myosin of the contractile ring, throughout most of the cell cycle. The Aim44p ring does not contract when the actomyosin ring closes. Instead, it forms a double ring that associates with septin rings on mother and daughter cells after cell separation. Deletion of AIM44 results in defects in contractile ring closure. Aim44p co-immunoprecipitates with Hof1p, a conserved F-BAR protein that binds both septins and type II myosins and promotes contractile ring closure. Deletion of AIM44 results in a delay in Hof1p phosphorylation, and altered Hof1p localization. Finally, overexpression of Dbf2p, a kinase that phosphorylates Hof1p and is required for re-localization of Hof1p from septin rings to the contractile ring and for Hof1p-triggered contractile ring closure, rescues the cytokinesis defect observed in aim44 null cells.

Our studies reveal a novel role for Aim44p in regulating contractile ring closure through effects on Hof1p.