Caulobacter PopZ forms a polar subdomain dictating sequential changes in pole composition and function

Abstract

SummaryThe bacterium Caulobacter crescentus has morphologically and functionally distinct cell poles that undergo sequential changes during the cell cycle. We show that the PopZ oligomeric network forms polar ribosome exclusion zones that change function during cell cycle progression. The parS/ParB chromosomal centromere is tethered to PopZ at one pole prior to the initiation of DNA replication. During polar maturation, the PopZ-centromere tether is broken, and the PopZ zone at that pole then switches function to act as a recruitment factor for the ordered addition of multiple proteins that promote the transformation of the flagellated pole into a stalked pole. Stalked pole assembly, in turn, triggers the initiation of chromosome replication, which signals the formation of a new PopZ zone at the opposite cell pole, where it functions to anchor the newly duplicated centromere that has traversed the long axis of the cell. We propose that pole-specific control of PopZ function co-ordinates polar development and cell cycle progression by enabling independent assembly and tethering activities at the two cell poles.

Bacterial scaffold directs pole-specific centromere segregation

“…4A), as previously reported (29,31). Throughout extended PopZ polar zones, ParA G16V -eYFP was robustly recruited, creating large regions of colocalization with ParA fluorescence (Fig.…”

“…Together, these results are consistent with a model in which robust interactions between PopZ and ParA are required for proper functioning of the ParA centromere segregation machine. displace large objects such as ribosomes and nucleoid DNA but allow smaller molecules to enter (18,19,29,31). This suggests that the filamentous PopZ structure forms a 3D microenvironment near the pole that is distinct from the general cytoplasm.…”

“…PopZ scaffolds, therefore, function not only as polar docking stations for cell cycle regulatory complexes (18,19,31), but also as local activation centers that direct the centromere-positioning machine. PopZ thus comprises a polar nexus that enables the spatial and temporal coupling of chromosome replication and segregation to ensure proper cell cycle progression.…”

Replication initiator DnaA binds at the Caulobacter centromere and enables chromosome segregation

“…4A, referred here as parS + ). This chromosomal region serves as a protein hub where the segregation machinery and additional interacting factors contribute to the temporal and spatial control of chromosome segregation (8,9,19,20,22,27). Because DnaA is known to bind ori to enable replication initiation (28), we considered the possibility that DnaA binds directly to the parS centromere region to activate ParB/parS translocation.…”

“…One such protein is the polar scaffold PopZ, which interacts with the ParB/parS complex to enable directional chromosome segregation (18,19). PopZ is positioned at the stalked cell pole early in the cell cycle and forms a second focus at the opposite pole upon the initiation of replication and segregation of the chromosome to anchor the translocated centromere to the new cell pole (18)(19)(20)(21). To determine whether replication-independent translocation of ParB/parS is accompanied by the assembly of a second PopZ focus at the new cell pole, we constructed a popZ merodiploid derived from the P vanA -dnaA depletion strain containing CFP-ParB and expressing a xylose-inducible allele of popZ fused to mCherry.…”