Caulobacter PopZ forms an intrinsically disordered hub in organizing bacterial cell poles

Abstract

Despite their relative simplicity, bacteria have complex anatomy at the subcellular level. At the cell poles of Caulobacter crescentus, a 177-amino acid (aa) protein called PopZ self-assembles into 3D polymeric superstructures. Remarkably, we find that this assemblage interacts directly with at least eight different proteins, which are involved in cell cycle regulation and chromosome segregation. The binding determinants within PopZ include 24 aa at the N terminus, a 32-aa region near the C-terminal homo-oligomeric assembly domain, and portions of an intervening linker region. Together, the N-terminal 133 aa of PopZ are sufficient for interacting with all binding partners, even in the absence of homooligomeric assembly. Structural analysis of this region revealed that it is intrinsically disordered, similar to p53 and other hub proteins that organize complex signaling networks in eukaryotic cells. Through live-cell photobleaching, we find rapid binding kinetics between PopZ and its partners, suggesting many pole-localized proteins become concentrated at cell poles through rapid cycles of binding and unbinding within the PopZ scaffold. We conclude that some bacteria, similar to their eukaryotic counterparts, use intrinsically disordered hub proteins for network assembly and subcellular organization.PopZ | intrinsically disordered protein | bacteria | Caulobacter | hub protein

The SPOR Domain, a Widely Conserved Peptidoglycan Binding Domain That Targets Proteins to the Site of Cell Division

“…This region is often predicted to be intrinsically disordered. "Linker" is in quotes because this region in FtsN includes three short poorly formed helices (40) and because disordered domains often engage in protein-protein interactions (41,42). Exactly how SPOR domain proteins with this architecture contribute to cell division is not clear.…”

Functional dichotomy and distinct nanoscale assemblies of a cell cycle-controlled bipolar zinc-finger regulator

“…How ZitP promotes swarming motility from the stalked pole is unclear, but there is precedence of other regulators (SpmX/Y and CpdR) that localize exclusively to the stalked pole and affect Caulobacter motility indirectly by regulating cell cycle factors (Janakiraman et al, 2016; McGrath et al, 2006; Radhakrishnan et al, 2008). Moreover, SpmX and CpdR interact with PopZ directly and their localization is compromised in the absence of PopZ (Bowman et al, 2010; Holmes et al, 2016). It is therefore conceivable that ZitP also affects motility indirectly from the stalked pole, possibly via cell cycle regulation, flagellar performance and/or polarity.…”

“…PopZ is bipolar in the Caulobacter predivisional cell and it interacts directly with numerous cell cycle kinases, the ParAB chromosome segregation proteins and cell fate determinants (Holmes et al, 2016). Here, we dissect at the genetic and cytological level the polar localization and function of two poorly characterized trans-membrane proteins, the zinc-finger protein ZitP and the CpaM effector protein, that are polarly localized and that execute multiple regulatory functions.…”

Modularity and determinants of a (bi-)polarization control system from free-living and obligate intracellular bacteria

“…The cis -encoded ParB protein binds parS and the resulting ParB• parS complex is guided pole-ward by the ParA ATPase, likely reinforced by poorly understood biophysical constraints and properties of the chromosome (Lim et al, 2014; Mohl and Gober, 1997). The PopZ polar organizing protein is thought to assemble a porous homo-polymeric matrix at the cell poles that captures the segregated ParB• parS complex (Figure 1A) via a direct interaction with ParAB (Bowman et al, 2008, 2013; Ebersbach et al, 2008; Holmes et al, 2016; Laloux and Jacobs-Wagner, 2013).

10.7554/eLife.20640.002Figure 1.The Zinc finger (ZnR) of ZitP and orthologs is a polar localization signal.( A ) Schematics of PopZ and ParB localization and chromosome organization during the C. crescentus cell cycle.

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“…Thus, compared to the bipolar co-localization of PopZ and ParB observed in C. crescentus (Bowman et al, 2008; Ebersbach et al, 2008; Ptacin et al, 2014), PopZ seems to have undergone functional specialization in the Rhizobiales, presumably interacting with other (unknown) client proteins. The genomes of the obligate intracellular (rickettsial) lineage also encode PopZ and ParAB orthologs (Andersson et al, 1998), but not several other known client proteins of C. crescentus PopZ that depend on a short N-terminal stretch in PopZ to interact with it (Bowman et al, 2010; Holmes et al, 2016; Laloux and Jacobs-Wagner, 2013; Ptacin et al, 2014). …”