Bembenek Lab

Eggshells, Membranes and Chromosomes

Genome-wide screens identified many genes that gave rise to similar phenotypes. A particularly strange observation was that numerous cell cycle regulators caused eggshell defects when knocked out in C. elegans. The eggshell was considered to be the ultimate non-specific phenotype because everything from ribosome subunits to cell cycle genes gave rise to defects in its formation, suggesting that a sick cell can’t properly make an eggshell. Motivated to eliminate this artifact to study the function of cell cycle genes in the embryo, we discovered that the eggshell is formed in part by cargo delivered outside the egg by cortical granule exocytosis. Cortical granule exocytosis is observed in oocytes of many species and is required to modify the covering of oocytes to prevent polyspermy.

A number of 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 polward journey. Several key regulators of the metaphase to anaphase transition such as the Anaphase Promoting Complex/ Cyclosome, Securin and Separase all disrupt cortical granule trafficking. This pathway is key to chromosome segregation by ensuring that Separase, a large protease, becomes active only at the onset of anaphase when it cleaves the cohesin glue holding sister chromatids together. Unexpectedly, 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. These observations form the basis for our NIH R01 funded research project seeking to understand how the core cell cycle machinery coordinates chromosome segregation with membrane trafficking during division. We have three main areas of investigation on this problem:

Investigating the Role of the Protease Activity of Separase

We have sought to understand how separase promotes exocytosis by first asking whether the protease activity is required. On the surface, this is an obvious question. However, in addition to cleaving substrates, separase also regulates signaling pathways that control cytokinesis independently of the protease activity in other systems. To test this, we expressed protease dead separase fused to GFP and unexpectedly found that it is dominant negative. Our results indicate that protease dead separase is a substrate trapping enzyme that interferes with cleavage of endogenous substrates and indicates that separase has a substrate independent of cohesin that it must cleave in order to promote exocytosis. This sets the stage for the next phase of the project to identify the relevant substrates of separase using immunoprecipitation of the substrate trapping enzyme. We are collaborating with John Yates at Scripps to identify candidate interacting proteins. Despite the fundamental importance of separase, few substrates are known, therefore, identification of novel substrates would have a significant impact.

Genetic Screening for Separase Suppressors

            In order to identify new players that are involved in separase regulation and function, we conducted a suppressor screen with our hypomorphic mutants, hoping to identify vesicle trafficking machinery related to separase function. We found 11 intragenic suppressors within the N-terminal domain of separase that may provide new insight into the function of this regulatory region. We also identified numerous alleles of PPH-5, a previously identified suppressor, and an allele of HSP-90 that has a mutation near the C-terminal domain that is known to bind and activate PPH-5. These results suggest that the PPH-5/HSP-90 signaling complex might be important for regulating separase function. Finally, we found at least four unknown suppressors and are using genome wide sequencing to identify them. In the future we will continue our genetic analysis of separase and expect to generate important insights into the regulation and function of this key cell cycle regulator.

Investigating Cell Cycle Regulation of Separase

Separase is a central component in the regulatory network that governs the metaphase to anaphase transition. We have begun investigating how this pathway regulates the function of separase during exocytosis. A number of players in this pathway regulate several aspects of vesicle trafficking and also affect separase localization. This project will provide insight into how key regulatory components of the cell cycle control SEP-1 localization and activity to promote timely exocytosis. These results will also expand the number of metaphase to anaphase transition regulators involved in regulating membrane trafficking to ensure disparate cellular functions are coordinated during cell division.