EAGER: Cytokinesis mechanisms and cytoskeletal dynamics in Chlamydomonas

Stanford University, Stanford CA

Investigators

Abstract

Cytokinesis is the final stage in cell division in which the single mother cell is physically separated into two daughter cells, and is essential for the normal proliferation and differentiation of cells of every form of life. Historically, studies of cytokinesis have focused on just two groups of organisms: the animals, fungi and their relatives, whose cells divide by forming an inward-growing "cleavage furrow", and the land plants, whose cells divide by forming an outward-growing "cell-plate". Thus, there is currently an unresolved paradox at the heart of cell biology: by what mechanism did the common ancestor of modern cells divide, and how did that mechanism evolve into the seemingly very different mechanisms seen today? This project will exploit the experimental advantages and favorable evolutionary position of the green alga Chlamydomonas reinhardtii (which is closely related to land plants but divides by forming a cleavage furrow like animals) to help elucidate the common core mechanisms that underlay cytokinesis in ancestral cells and are still shared in modern organisms despite their superficial differences. Understanding these core mechanism will provide a better understanding not only of animal, fungal, and plant cells, but also of a broad range of other cell types that have been largely ignored by cell biologists but are of enormous ecological and economic importance. In addition to basic knowledge, this project will provide new experimental tools that should be valuable both in basic research and in applied studies (particularly in biofuels development), and it will provide opportunities for postdoctoral, graduate, undergraduate, and high-school students (including students from underrepresented groups) to learn genetics, molecular biology, and cell biology as a prelude to continuing in careers in science and engineering. This project focuses on elucidation of the mechanisms by which Chlamydomonas cells form cleavage furrows for cell division without a contractile actomyosin ring (CAR), a structure formed of filamentous (F) actin, type II myosin, and other proteins, which has long been thought to be necessary for furrow formation in animal and fungal cells. However,it is now clear both that many cells that normally have a CAR can divide without it and that most eukaryotic cells in other phylogenetic groups divide by forming furrows but have no myosin-II, so there must another driver(s) of this process. To elucidate the mechanisms of furrowing in Chlamydomonas and the associated cytoskeletal functions, three interrelated aims will be pursued. (1) Improved methods for expressing tagged proteins of interest and for microscopic visualization of cytokinesis processes and cytoskeletal dynamics will be developed and used to clarify the details of these processes and the questions to be asked at the molecular level. (2) A combination of microscopy, biochemistry, genetics, and transcriptome analyses will be used to investigate the possible role of F-actin in the cleavage furrow and the mechanisms and roles of the recently discovered dramatic transcriptional regulatory response to actin depolymerization. Similar approaches will be used to investigate the function(s) of the single Chlamydomonas septin protein, which is also expected (from comparisons to other organisms) to be involved in cytokinesis. (3) Imaging-based methods and high-throughput platforms will be used to identify mutants defective in cytokinesis and/or cytoskeletal dynamics; characterization of these mutants should help to reveal the underlying mechanisms of furrow formation and cytoskeletal behavior through the cell cycle.