title: Critical clamp loader processing by an essential AAA+ protease in Caulobacter crescentus.
abstract: Chromosome replication relies on sliding clamps that are loaded by energy-dependent complexes. In Escherichia coli, the ATP-binding clamp loader subunit DnaX exists as both long (τ) and short (γ) forms generated through programmed translational frameshifting, but the need for both forms is unclear. Here, we show that in Caulobacter crescentus, DnaX isoforms are unexpectedly generated through partial proteolysis by the AAA+ protease casein lytic proteinase (Clp) XP. We find that the normally processive ClpXP protease partially degrades DnaX to produce stable fragments upon encountering a glycine-rich region adjacent to a structured domain. Increasing the sequence complexity of this region prevents partial proteolysis and generates a τ-only form of DnaX in vivo that is unable to support viability on its own. Growth is restored when γ is provided in trans, but these strains are more sensitive to DNA damage compared with strains that can generate γ through proteolysis. Our work reveals an unexpected mode of partial processing by the ClpXP protease to generate DnaX isoforms, demonstrates that both τ and γ forms of DnaX are required for Caulobacter viability, and identifies a role for clamp loader diversity in responding to DNA damage. The conservation of distinct DnaX isoforms throughout bacteria despite fundamentally different mechanisms for producing them suggests there may be a conserved need for alternate clamp loader complexes during DNA damaging conditions.
authors.affiliation: Molecular and Cellular Biology Graduate Program, Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, MA 01003.
authors.forename: Robert H
authors.initials: RH
authors.lastname: Vass
authors.forename: Peter
authors.initials: P
authors.lastname: Chien
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D015153:Blotting, Western; D016935:Caulobacter crescentus / Q000201:enzymology*; D002876:Chromosomes, Bacterial / Q000378:metabolism* / Q000502:physiology; D004258:DNA Polymerase III / Q000378:metabolism*; D004261:DNA Replication / Q000502:physiology*; D049071:Endopeptidase Clp / Q000378:metabolism*; D008853:Microscopy; D021122:Protein Subunits / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; C103076:DnaX protein, Bacteria; D021122:Protein Subunits; D004258:DNA Polymerase III; D049071:Endopeptidase Clp
keywords: ClpP; ClpX internal recognition; protease stalling