Proteins from the MinD/ParA family of ATPases are ubiquitous across the microbial world. Primarily studied in E. coli for their ability to self-organize in space and time, these proteins control a wide range of cellular functions. Indeed, MinD is a critical component of the Min system, which defines the cell division plane, and ParA is a critical component of the Par system, which segregates plasmids, chromosomes and other large macromolecular structures in the cell. The capacity of these proteins to self-organize requires: (i) a biological surface - membranes for MinD and the nucleoid for ParA, (ii) ATP, and (iii) an additional protein to provide site specificity – MinE for MinD and ParB for ParA. Cyanobacteria comprise a phylum of photosynthetic bacteria that have important ecological, bioindustrial and evolutionary significance. Despite their importance, how MinD/ParA family proteins behave in cyanobacteria has been understudied. Furthermore, cyanobacteria have a number of distinct cellular features relative to the classic prokaryotic models in which self-organizing protein systems have been studied. In this thesis, I used a combination of genetics, microscopy, and computational modeling in the model rod-shaped cyanobacterium Synechococcus elongatus PCC 7942 to study several aspects of protein self-organization in cyanobacteria. I first show that the cyanobacterial Min system exhibits a robust self-organizing oscillation even in the face of the complicating architecture of the thylakoid membranes. My work suggests that this is accomplished in part by the capacity of Min proteins to distinguish and minimize interactions with the thylakoid membranes while maintaining the capacity to transiently self-organize selectively on the plasma membrane. Since the oscillation of Min proteins defines the division plane by spatially regulating the assembly of the protein FtsZ, and thus, the site of cytokinesis, my next study explored how FtsZ interacts with the plasma membrane. I show that the protein Ftn2 in cyanobacteria and ARC6 in algae and plant chloroplasts are homologs that tether FtsZ to the inner membrane through an evolutionarily conserved domain. In the absence of Ftn2, FtsZ assembles primarily as filaments throughout S. elongatus cells. In contrast, upon overexpression of Ftn2, numerous FtsZ filaments assemble as zig-zag patterns. These results suggest that Ftn2 stoichiometry is important for properly defining the cell division plane.Lastly, I explored how oscillation of a ParA-like protein, McdA, contributes to the organization of carboxysomes, cyanobacterial carbon-fixing microcompartments. I found that McdA binds to the nucleoid as a biological surface for self-organization of oscillatory patterns. A novel factor, McdB, was found to be required for McdA dynamics on nucleoids. McdB colocalizes with carboxysomes through interaction with carboxysome shell components, and stimulates the inherent ATPase activity of nearby McdA that is bound to the DNA. This effect is similar to the effect of ParB proteins on ParA ATPase activity even though McdB shares no similarity with known ParB proteins. Computational modeling and in vivo microscopy also suggested that McdB-bound carboxysomes produce zones of McdA depletion on nucleoids and that carboxysome movement is directed towards the highest local McdA concentration. Together, this work substantially improves our understanding of how MinD/ParA family proteins self-organize in cyanobacteria and has broader implications for understanding self-organization in other diverse microbes.
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