Control of FtsZ-ring formation and cell division in Streptomyces venezuelae

Abstract: Cell division is essential for all life forms. In bacteria, this fundamental process is precisely orchestrated by a protein, FtsZ, the ancestral homolog of eukaroytic tubulin. On the onset of division, at the division site, FtsZ assembles into a cytoskeleton structure - the Z ring - that recruits other division proteins and constricts the membrane together with cell wall synthesizing machinery, eventually splitting a cell into two. Despite billions of years of evolution, FtsZ-based division is highly conserved in most bacteria and also found in a major group of Archaea, plant chloroplasts, and mitochondria of many eukaryotes. In recent years, FtsZ has been extensively studied as an attractive antibacterial drug target.Cell division is mainly controlled at the stage of Z-ring assembly, but the molecular mechanisms are not fully understood. For example, in Actinobacteria, the largest bacterial phylum, consisting of human pathogens such as Corynebacterium diphtheriae, Mycobacterium tuberculosis and industrially important Streptomyces spp., proteins involved in the control of Z-ring assembly are not clearly known. The overall aim of this work is to shed light on the regulation of Z-ring assembly using Streptomyces venezuelae as a model. First, we show that a specific region, the β strand S2, of FtsZ, is crucial for it’s assembly and function. We found that a specific amino acid change in this region leads to a decrease in FtsZ’s intrinsic GTPase activity and loss of critical concentration for FtsZ polymerization. This compromised assembly behaviour possibly affects cooperative assembly of treadmilling FtsZ polymers, most likely via the conformational switch. Defects in assembly dynamics and treadmilling of FtsZ may explain the failure of the mutant to form proper Z rings, but the incorrectly shaped FtsZ structures that form in the cells are still able to initiate cell constrictions, leading to perturbed cell shape (Paper I). Second, we show that in S. venezuelae, most of the core division proteins, including FtsZ are non-essential for viability, thereby confirming that the dispensability of division proteins for viability is a conserved cell division aspect in Streptomyces. Additionally, we show that the core divisome proteins are not required for Z-ring assembly (Paper II). Third, we demonstrate that SepF is a putative membrane anchor for the Z ring and is required for sporulation-specific cell division, while SepF2 and SepF3 have accessory role in S. venezuelae cell division (Paper III). Finally, we confirm that a septal factor SepIVA, supposedly to function in mycobacterial cell division, is not involved in Streptomyces cell division. Our results from SepIVA localization in vivo suggest that it may instead have a regulatory role in polar growth or cell wall synthesis in S. venezuelae (Paper IV).Overall, this thesis work provides insights into the control of FtsZ-ring assembly and also on the functionality of several division proteins in sporulation-associated cell division in S. venezuelae.