title: Lon protease quality control of presecretory proteins in Escherichia coli and its dependence on the SecB and DnaJ (Hsp40) chaperones.
abstract: Various environmental insults result in irreversible damage to proteins and protein complexes. To cope, cells have evolved dedicated protein quality control mechanisms involving molecular chaperones and proteases. Here, we provide both genetic and biochemical evidence that the Lon protease and the SecB and DnaJ/Hsp40 chaperones are involved in the quality control of presecretory proteins in Escherichia coli. We showed that mutations in the lon gene alleviate the cold-sensitive phenotype of a secB mutant. Such suppression was not observed with either clpP or clpQ protease mutants. In comparison to the respective single mutants, the double secB lon mutant strongly accumulates aggregates of SecB substrates at physiological temperatures, suggesting that the chaperone and the protease share substrates. These observations were extended in vitro by showing that the main substrates identified in secB lon aggregates, namely proOmpF and proOmpC, are highly sensitive to specific degradation by Lon. In contrast, both substrates are significantly protected from Lon degradation by SecB. Interestingly, the chaperone DnaJ by itself protects substrates better from Lon degradation than SecB or the complete DnaK/DnaJ/GrpE chaperone machinery. In agreement with this finding, a DnaJ mutant protein that does not functionally interact in vivo with DnaK efficiently suppresses the SecB cold-sensitive phenotype, highlighting the role of DnaJ in assisting presecretory proteins. Taken together, our data suggest that when the Sec secretion pathway is compromised, a pool of presecretory proteins is transiently maintained in a translocation-competent state and, thus, protected from Lon degradation by either the SecB or DnaJ chaperones.
authors.affiliation: Laboratoire de Microbiologie et Génétique Moléculaire, CNRS and the Université Paul Sabatier, 31062 Toulouse Cedex 9, France.
authors.forename: Samer
authors.initials: S
authors.lastname: Sakr
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Cirinesi
authors.forename: Ronald S
authors.initials: RS
authors.lastname: Ullers
authors.forename: Françoise
authors.initials: F
authors.lastname: Schwager
authors.forename: Costa
authors.initials: C
authors.lastname: Georgopoulos
authors.forename: Pierre
authors.initials: P
authors.lastname: Genevaux
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D003080:Cold Temperature; D004926:Escherichia coli / Q000201:enzymology* / Q000378:metabolism* / Q000502:physiology; D029968:Escherichia coli Proteins / Q000378:metabolism*; D017353:Gene Deletion; D050956:HSP40 Heat-Shock Proteins / Q000378:metabolism*; D009154:Mutation; D049070:Protease La / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D011786:Quality Control
chemical_list: D001426:Bacterial Proteins; C494635:DnaJ protein, E coli; D029968:Escherichia coli Proteins; D050956:HSP40 Heat-Shock Proteins; C057842:SecB protein, Bacteria; D049070:Protease La