Abstract
ABSTRACTSignaling hubs at bacterial cell poles establish cell polarity in the absence of membrane-bound compartments. In the asymmetrically dividing bacterium Caulobacter crescentus, cell polarity stems from the cell cycle-regulated localization and turnover of signaling protein complexes in these hubs, and yet the mechanisms that establish the identity of the two cell poles have not been established. Here, we recapitulate the tripartite assembly of a cell fate signaling complex that forms during the G1-S transition. Using in vivo and in vitro analyses of dynamic polar protein complex formation, we show that a polymeric cell polarity protein, SpmX, serves as a direct bridge between the PopZ polymeric network and the cell fate-directing DivJ histidine kinase. We demonstrate the direct binding between these three proteins and show that a polar microdomain spontaneously assembles when the three proteins are coexpressed heterologously in an Escherichia coli test system. The relative copy numbers of these proteins are essential for complex formation, as overexpression of SpmX in Caulobacter reorganizes the polarity of the cell, generating ectopic cell poles containing PopZ and DivJ. Hierarchical formation of higher-order SpmX oligomers nucleates new PopZ microdomain assemblies at the incipient lateral cell poles, driving localized outgrowth. By comparison to self-assembling protein networks and polar cell growth mechanisms in other bacterial species, we suggest that the cooligomeric PopZ-SpmX protein complex in Caulobacter illustrates a paradigm for coupling cell cycle progression to the controlled geometry of cell pole establishment.
Highlights
Signaling hubs at bacterial cell poles establish cell polarity in the absence of membrane-bound compartments
Synthesis of the DivJ histidine kinase marks the end of the G1-S transition, enabling the initiation of chromosome replication and driving the stalked cell genetic program
We found that when PopZ and SpmX lacking its lysozyme domain were coexpressed in E. coli, SpmX-ΔL-enhanced yellow fluorescent protein (eYFP) remained diffuse (Fig. 2F), while coexpression of PopZ and SpmX bearing only the lysozyme domain led to the polar localization of SpmX-L-eYFP (Fig. 2G)
Summary
Signaling hubs at bacterial cell poles establish cell polarity in the absence of membrane-bound compartments. In the asymmetrically dividing bacterium Caulobacter crescentus, cell polarity stems from the cell cycle-regulated localization and turnover of signaling protein complexes in these hubs, and yet the mechanisms that establish the identity of the two cell poles have not been established. By comparison to self-assembling protein networks and polar cell growth mechanisms in other bacterial species, we suggest that the cooligomeric PopZ-SpmX protein complex in Caulobacter illustrates a paradigm for coupling cell cycle progression to the controlled geometry of cell pole establishment. The swarmer cell undergoes a period of differentiation to become a stalked cell, culminating in the generation of further progeny (Fig. 1) During this G1-S transition, the developing swarmer cell releases the PleC phosphatase from the flagellated cell pole, sheds its polar flagellum, begins biogenesis of the polar stalk appendage, and initiates DNA replication. Synthesized DivJ is positioned at the nascent stalked pole, where it remains localized to propagate the stalked cell fate throughout future divisions [2,3,4]
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