Cell Division in a Model Archaeon

The University Of Texas Health Science Center At Houston, Houston TX

Investigators

Abstract

How does a single cell divide to become two cells? This is a fundamentally important and yet unsolved biological question. In prokaryotes, this essential cellular process is initiated and directed by the tubulin-like FtsZ protein, which forms a polymeric ring, called the Z-ring, at the division site during the initial stages of cytokinesis. The Z-ring then contracts, persisting at the leading edge of the invaginating cytoplasmic membrane throughout the division process. FtsZ is present in the three major biological kingdoms-bacteria, archaea, and organelles.in eukaryotes. Because very little is known about cell division or cellular organization in the archaea, the goal of this research program is to gain insight into the mechanism of archaeal cell division and how FtsZ coordinates this process. Halobacterium salinarum is the system of choice to study FtsZ-based cell division in the archaea for several reasons. First, relatively sophisticated genetic and cytological tools, such as a salt-resistant green fluorescent protein (GFP), have been recently developed for this organism. Second, H. salinarum grows at convenient temperatures in air, and thus can be cultivated without any special equipment. Third, its complete genome sequence has recently been completed, making H. salinarum the best archaeon in which to apply functional genomics. Fourth, H. salinarum harbors five copies of ftsZ: ftsZ1 and ftsZ2 are highly similar to ftsZ orthologs from other species, whereas the other three are more divergent but are nevertheless clearly ftsZ homologs. This unprecedented complexity promises to reveal interesting and novel aspects of FtsZ function. Fifth, despite the potential complexity of FtsZ interactions, there are no other homologs of bacterial cell division proteins in this organism (which is also true for other archaea). This, along with the presence of a relatively simple S-layer instead of a peptidoglycan cell wall, suggests that cell division in H. salinarum may be more streamlined than that of the bacterial model systems and therefore potentially easier to solve. Finally, the rod shape and relatively large size of H. salinarum cells greatly facilitates cytological analysis. The objectives of this project are to (i) define the roles of the five FtsZ homologs, in particular the two most conserved copies, in H. salinarum cell division; (ii) understand what regulates assembly and placement of the Z-ring in H. salinarum, with potential candidates including the two MinD homologs present in the proteome; and (iii) identify other factors involved in cell division and in a potentially novel cytoskeleton in this organism. These objectives will be addressed by construction of gene knockouts, protein overexpression and purification, assays for GTP binding and hydrolysis, self-assembly measurements, and in vivo protein localization and quantitation using GFP fusions and immunodetection. These studies should make a major contribution to our understanding of cell division mechanisms, protein targeting and dynamics, and cellular organization of archaea, and should complement studies of cell division in bacterial and organellar model systems. Moreover, genome sequence of H. salinarum, coupled with its accessibility, affords a timely opportunity for students to develop an in-depth research project on the cell biology of an archaeon.