CAREER: Cytokinesis without an actomyosin ring and its coordination with organelle division

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Cytokinesis is a fundamental cellular process in which a mother cell is divided into two or more daughter cells. All cells today are connected to the first cell that emerged several billion years ago, through the continuous events of cytokinesis. Despite this continuity, the mechanisms of cytokinesis as seen in today’s animals, plants, and prokaryotes appear unrelated without underlying principles, because most cytokinesis research has focused on a small number of organisms and cell types. As a result, we currently lack insights into the evolution of cytokinesis mechanisms. In this project, we will address the current knowledge gap and inherent bias in our understanding of cytokinesis by using the green alga Chlamydomonas reinhardtii. Chlamydomonas, divides by ingressing the plasma membrane like animals, yet it lacks the canonical animal cell-division machinery. Its division involves a hybrid of plant-division machinery and other components conserved in many eukaryotes, including humans. Furthermore, Chlamydomonas possesses a single chloroplast, which must be divided and segregated into daughter cells in coordination with the cell division process. Chloroplasts derive from a cyanobacterium that was engulfed by an early eukaryote and became a stable endosymbiont, and therefore possess their own division machinery of bacterial origin. Thus, by investigating cytokinesis and its coordination with chloroplast division in Chlamydomonas, this project will elucidate (i) the ancestral and fundamental mechanism of cytokinesis that is conserved in the majority of eukaryotes and (ii) the communication between the eukaryotic and bacterial division systems crucial for the establishment of endosymbiosis and subsequently the maintenance this essential organelle for photosynthesis. The Broader Impacts of the work include the intrinsic nature of the research as all cells undergo the process of cytokinesis. In addition, the project will provide research opportunities to high school students and their teachers, and educate undergraduate and graduate students about the current bias in the use of model organisms and the potential future opportunities emerging models can bring.The overall research goals of this project are to elucidate (i) the mechanism of actomyosin-independent, microtubule-dependent cleavage-furrow ingression in Chlamydomonas and (ii) the role of actin cytoskeleton in the spatiotemporal coordination of cytokinesis and chloroplast division. To this end, one part of this research will address how microtubules are organized at the division site using light and electron microscopy, and investigate the mechanisms of augmin-mediated formation of furrow-associated microtubules by genetics and proteomics. In another part, the project will investigate how nuclear-encoded proteins required for chloroplast division are transported to and translocated into the chloroplast in an F-actin-dependent manner, and identify novel genes and proteins involved in chloroplast division through genetic screens and proximity-based proteomics. Identifying the actomyosin-independent mechanisms of cleavage-furrow ingression is critical to elucidate the generalizable, fundamental rules governing the seemingly diverse and specialized modes of cytokinesis found across the eukaryotic tree, as well as to understand how these modern cells evolved from the ancestral ones that did not have an actomyosin ring. This research is also relevant to investigating the evolutionary events in early eukaryotes, because spatiotemporal coordination of division is a critical step in endosymbiosis between an ancestral eukaryotic cell and a cyanobacterial cell. Chlamydomonas provides an excellent system to fill these gaps in our current understanding. This project represents a crucial paradigm shift in the existing literature and classroom education.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.