Investigation of the structures of ZapA and ZapD from Escherichia coli and their roles in bacterial cell division
Bacterial cell division is an essential, highly coordinated process that requires multiple proteins. FtsZ is the main scaffolding protein required for division and its mid-cell localization, membrane anchoring, and polymerization are all needed for the formation of the bacterial Z-ring. The Z-ring is a highly dynamic ring-like structure made up of closely associated FtsZ filaments. It acts as the base for recruitment of all downstream proteins into a complex known as the divisome. The bacterial divisome is comprised of 10 essential and 7 non-essential proteins, in Escherichia coli, which are recruited to mid-cell in a hierarchical manner and relies on the Z-ring for stability. A group of proteins referred to as the FtsZ associated proteins (ZapA, ZapB, ZapC and ZapD) act to stabilize the Z-ring prior to the on-set of division. This has been visualized in vitro as FtsZ filament bundling. Individually these proteins are non-essential, but it is our hypothesis that collectively the FtsZ associated proteins play an essential role in cell division. In this thesis research the crystal structures of ZapA and ZapD are described. ZapA was shown to interact with FtsZ through a charged α-helix located on each of the four protomers in the ZapA tetramer and bundle FtsZ filaments in vitro. ZapD was shown to interact with FtsZ through several charged residues located in a proposed binding pocket on each protomer comprising the ZapD dimer. While the FtsZ bundles formed by ZapD appear slightly more organized than those formed in the presence of ZapA, co-bundling and competition assays revealed that they likely share a common binding site on FtsZ. However, ZapA and ZapD did not show any competition for binding FtsZ. The overall findings of this research indicate that ZapA and ZapD have overlapping functions and they share a binding site on FtsZ, while their structures remain distinct from one another. Taken together, the results pave the way for future studies on Zap proteins and open doors for the further analysis of the ZapA-FtsZ and ZapD-FtsZ interactions.