Bembenek Lab

The eggshell as a model for membrane trafficking during cell division

Genome-wide screens categorized many genes according to similar phenotypes, suggesting they regulate similar underlying cell biological processes. A strange observation was that numerous cell cycle regulators caused eggshell defects when knocked out in C. elegans. The “osmotic integrity defective” (OID) phenotype was considered non-specific because depletion of various genes from ribosome subunits to cell cycle genes cause the phenotype. While investigating cell cycle genes in the embryo, we discovered that the eggshell is actually formed in part by secretion of extracellular matrix material. Oocytes of many species produce secretory vesicles called cortical granules that deliver cargo required to modify the covering of oocytes to prevent polyspermy right after fertilization. Cortical granule exocytosis is required for eggshell formation in C. elegans and disruption of this process causes the OID phenotype.

Hidden Membrane Functions of Chromosome Regulators

Several core cell cycle regulators disrupt eggshell formation by causing defects in cortical granule biogenesis, trafficking and exocytosis. The exocytosis of these vesicles occurs during mid-anaphase, after chromosomes initiate their poleward journey. Several key regulators of the metaphase to anaphase transition such as the Anaphase Promoting Complex/ Cyclosome (APC/C), securin and separase all disrupt cortical granule trafficking. This regulatory pathway ensures that separase, a large protease, becomes active only at the onset of anaphase. Separase cleaves the cohesin glue holding sister chromatids together to allow chromosomes to come apart. We discovered that separase also localizes to cortical granules and is required for exocytosis during anaphase. This same pathway operates during mitosis to control RAB-11 vesicle trafficking required for cytokinesis. Separase was the first important clue that the chromosome regulatory system has a function in vesicle trafficking.

The kinetochore is a critical structure assembled on chromosomes to mediate attachment to spindle microtubules for chromosome movements during cell division. Separase activation is regulated by the spindle assembly checkpoint (SAC), which monitors chromosome attachments to the spindle at the kinetochore. In exciting new work, we discovered that outer kinetochore proteins assemble into “linear element” structures in the cortical cytoplasm to regulate vesicle transport during prometaphase I. Our new finding that cytoplasmic assemblies of outer kinetochore proteins regulate vesicle transport is a paradigm shifting observation. This significantly extends the extent of the chromosome regulatory machinery involved in membrane trafficking, which suggests that this system constitutes a dual purpose pathway providing a unified control system for both chromosomes and membranes during division. Numerous additional cell cycle regulators also cause the OID phenotype, and their role in vesicle trafficking remains largely unexplored. Therefore, this system represents an ideal context to understand cell cycle control of membrane trafficking.