Imaging-Based Identification of a Critical Regulator of FtsZ Protofilament Curvature in Caulobacter

FtsZ is an essential bacterial GTPase that polymerizes at midcell, recruits the division machinery, and may generate constrictive forces necessary for cytokinesis. However, many of the mechanistic details underlying these functions are unknown. We sought to identify FtsZ-binding proteins that influence FtsZ function in Caulobacter crescentus. Here, we present a microscopy-based screen through which we discovered two FtsZ-binding proteins, FzIA and FzIC. FzIA is conserved in alpha-proteobacteria and was found to be functionally critical for cell division in Caulobacter. FzIA altered FtsZ structure both in vivo and in vitro, forming stable higher-order structures that were resistant to depolymerization by MipZ, a spatial determinant of FtsZ assembly. Electron microscopy revealed that FzIA organizes FtsZ protofilaments into striking helical bundles. The degree of curvature induced by FzIA depended on the nucleotide bound to FtsZ. Induction of FtsZ curvature by FzIA carries implications for regulating FtsZ function by modulating its superstructure.