The conserved tubulin homolog FtsZ assembles into GTP-dependent protofilaments that organize the bacterial cell division machinery. Purified Escherichia coli FtsZ (FtsZEc) can form biomolecular condensates in the absence of GTP in macromolecular crowding conditions, but FtsZ condensates have not yet been confirmed in vivo. FtsZ from the Alphaproteobacterium Agrobacterium tumefaciens (FtsZAt) localizes to distinct polar foci during growth and contains an intrinsically disordered region much longer than FtsZEc, suggesting a greater tendency to form condensates. Here, we show that purified FtsZAt readily formed dynamic condensates in vitro under molecular crowding conditions, and the addition of GTP induced the formation of filaments radiating from the FtsZAt droplets. Notably, FtsZAt formed similar foci and filaments in vivo that dynamically assembled and coalesced, supported by fluorescence recovery after photobleaching analysis of FtsZAt and another Alphaproteobacterial FtsZ. These results suggest that FtsZAt forms polar condensates in A. tumefaciens cells, perhaps to sequester FtsZAt before cell division as an adaptation to slow generation times.IMPORTANCEAlphaproteobacteria are highly diverse; however, most orthologs of the essential cell division protein FtsZ from this class feature an unusually long intrinsically disordered internal domain compared with other bacterial FtsZs. As proteins harboring such domains have an increased tendency to assemble into phase-separated biomolecular condensates, we investigated the condensate-forming properties of Alphaproteobacterial FtsZ proteins in vivo and in vitro. We found that Agrobacterium tumefaciens FtsZ has a strong tendency to form condensates in vitro and in vivo as part of its cell cycle, as does FtsZ of Caulobacter crescentus, a distantly related Alphaproteobacterium. This behavior contrasts with FtsZ from Escherichia coli, which has a much shorter internal disordered domain and forms condensates in combination with partner proteins under more stringent conditions. Our results suggest that Alphaproteobacterial FtsZs form condensates during normal growth, perhaps serving as part of a conserved mechanism of cell cycle control in this class of bacteria.
Cameron et al. (Wed,) studied this question.