PopZ Interaction Hub Binding Determinants within ChpT and ParB

Bacterial cytoplasm is a complex and highly crowded space. However, it is also very organized both spatially and temporally. One mode of organization in the alphaproteobacteria Caulobacter crescentus (Cc) is through the intrinsically disordered interaction hub named polar organizing protein Z (PopZ). PopZ localizes at cell poles and interacts with at least ten binding proteins. These proteins are involved in various essential cellular activities including segregating chromosomes and regulating the cell cycle. Despite recent progress in understanding the binding site of PopZ, characterization of the interaction through the point of view of the binding partner remains incomplete. Here we investigate two of PopZ’s binding partners, ChpT and ParB. Previous studies have shown that Cc ChpT and ParB both colocalize to Cc PopZ at the cell pole. However, Agrobacterium tumefacien (At) homologs of ChpT and ParB both fail to colocalize to At PopZ. Because Cc and At PopZ have nearly identical binding sites, we hypothesized that the Cc homologs contain a unique sequence element that allows for PopZ recognition. Systematic creation and testing of chimeric binding protein derivatives has provided insight into the necessary and sufficient regions of both ChpT and ParB. We anticipate a detailed, molecular characterization of the PopZ-ChpT and PopZ-ParB interactions will aid in understanding hub function and cytoplasmic organization in bacteria. The simplicity of our bacterial model will additionally allow it to be used to better understand broader principles of hub-network connectivity that apply to all kingdoms of life.