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Functional Activation of the Flagellar Type III Secretion Export Apparatus.

Phillips AM, Calvo RA, Kearns DB - PLoS Genet. (2015)

Bottom Line: Flagella are assembled sequentially from the inside-out with morphogenetic checkpoints that enforce the temporal order of subunit addition.Genetic suppressor analysis indicates that SwrB activates the flagellar type III secretion export apparatus by the membrane protein FliP.We conclude that SwrB enhances the probability that the flagellar basal body adopts a conformation proficient for secretion to ensure that rod and hook subunits are not secreted in the absence of a suitable platform on which to polymerize.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, Indiana University, Bloomington, Indiana, United States of America.

ABSTRACT
Flagella are assembled sequentially from the inside-out with morphogenetic checkpoints that enforce the temporal order of subunit addition. Here we show that flagellar basal bodies fail to proceed to hook assembly at high frequency in the absence of the monotopic protein SwrB of Bacillus subtilis. Genetic suppressor analysis indicates that SwrB activates the flagellar type III secretion export apparatus by the membrane protein FliP. Furthermore, mutants defective in the flagellar C-ring phenocopy the absence of SwrB for reduced hook frequency and C-ring defects may be bypassed either by SwrB overexpression or by a gain-of-function allele in the polymerization domain of FliG. We conclude that SwrB enhances the probability that the flagellar basal body adopts a conformation proficient for secretion to ensure that rod and hook subunits are not secreted in the absence of a suitable platform on which to polymerize.

No MeSH data available.


Related in: MedlinePlus

SwrB activates flagellar type III secretion.A model depicting the early formation of a secretion inactive proto-basal body (left). Basal bodies spontaneously mature to an active state at a low frequency in the absence of SwrB and the flagella synthesized are sufficient to swim through liquid but insufficient to swarm over surfaces (middle). SwrB is required to accelerate the basal body maturation process, increase the number of secretion active basal bodies, increase the number of flagellar hooks and therefore increase flagellar number to levels needed for swarming. We hypothesize that a conformational change in FliF is mediating SwrB activation, because SwrB requires FliF, FliF is required for flagellar secretion, FliF occupies a space between FliG, FliP and SwrB, and FliF complexes have been reported to adopt open and closed conformations. Dark gray bar indicates membrane, light gray bar indicates peptidoglycan, purple shapes indicate FliF, magenta shapes indicate FliG, pink shapes indicate FliM and FliN/Y, orange indicates the secretion complex, and light blue indicates SwrB. FliP is colored red to indicate an inactive secretion complex or light green to indicate an active secretion complex. Protein secretion is indicated by a dark green dotted line.
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pgen.1005443.g011: SwrB activates flagellar type III secretion.A model depicting the early formation of a secretion inactive proto-basal body (left). Basal bodies spontaneously mature to an active state at a low frequency in the absence of SwrB and the flagella synthesized are sufficient to swim through liquid but insufficient to swarm over surfaces (middle). SwrB is required to accelerate the basal body maturation process, increase the number of secretion active basal bodies, increase the number of flagellar hooks and therefore increase flagellar number to levels needed for swarming. We hypothesize that a conformational change in FliF is mediating SwrB activation, because SwrB requires FliF, FliF is required for flagellar secretion, FliF occupies a space between FliG, FliP and SwrB, and FliF complexes have been reported to adopt open and closed conformations. Dark gray bar indicates membrane, light gray bar indicates peptidoglycan, purple shapes indicate FliF, magenta shapes indicate FliG, pink shapes indicate FliM and FliN/Y, orange indicates the secretion complex, and light blue indicates SwrB. FliP is colored red to indicate an inactive secretion complex or light green to indicate an active secretion complex. Protein secretion is indicated by a dark green dotted line.

Mentions: Based on cytological and genetic suppressor data, we conclude that SwrB functions as an assembly chaperone to enhance the probability that the flagellar basal body adopts a conformation proficient for secretion (Fig 11). We propose that the flagellar type III secretion apparatus and FliF form first in the membrane as a “proto-basal body” that is inactive for export (Fig 11A) [7]. The proto-basal body is able to spontaneously mature to become proficient for hook secretion at a low frequency (Fig 11B). Under normal conditions, the frequency of proto-basal body maturation is increased by both SwrB and assembly of the C-ring as the absence of either share a low hook-to-basal body ratio (Fig 11C). Indeed, SwrB and FliG appear to be required at the same step as FliG gain-of-function alleles that enhance rotor stability bypass the requirement of SwrB for hook assembly, and artificial overexpression of SwrB bypasses the need for FliG. The most likely convergence point for SwrB and FliG is the membrane-basal body protein FliF, as FliG is a known interactor and SwrB, itself a membrane protein, could be adjacent. We hypothesize that the previously documented changes in FliF conformation are responsible for activating the export apparatus and do so by activating the membrane protein FliP. In sum, maturation of the flagellar basal body to a secretion proficient state is a morphogenetic checkpoint that must be passed prior to proceeding to the export and assembly of more distal components.


Functional Activation of the Flagellar Type III Secretion Export Apparatus.

Phillips AM, Calvo RA, Kearns DB - PLoS Genet. (2015)

SwrB activates flagellar type III secretion.A model depicting the early formation of a secretion inactive proto-basal body (left). Basal bodies spontaneously mature to an active state at a low frequency in the absence of SwrB and the flagella synthesized are sufficient to swim through liquid but insufficient to swarm over surfaces (middle). SwrB is required to accelerate the basal body maturation process, increase the number of secretion active basal bodies, increase the number of flagellar hooks and therefore increase flagellar number to levels needed for swarming. We hypothesize that a conformational change in FliF is mediating SwrB activation, because SwrB requires FliF, FliF is required for flagellar secretion, FliF occupies a space between FliG, FliP and SwrB, and FliF complexes have been reported to adopt open and closed conformations. Dark gray bar indicates membrane, light gray bar indicates peptidoglycan, purple shapes indicate FliF, magenta shapes indicate FliG, pink shapes indicate FliM and FliN/Y, orange indicates the secretion complex, and light blue indicates SwrB. FliP is colored red to indicate an inactive secretion complex or light green to indicate an active secretion complex. Protein secretion is indicated by a dark green dotted line.
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pgen.1005443.g011: SwrB activates flagellar type III secretion.A model depicting the early formation of a secretion inactive proto-basal body (left). Basal bodies spontaneously mature to an active state at a low frequency in the absence of SwrB and the flagella synthesized are sufficient to swim through liquid but insufficient to swarm over surfaces (middle). SwrB is required to accelerate the basal body maturation process, increase the number of secretion active basal bodies, increase the number of flagellar hooks and therefore increase flagellar number to levels needed for swarming. We hypothesize that a conformational change in FliF is mediating SwrB activation, because SwrB requires FliF, FliF is required for flagellar secretion, FliF occupies a space between FliG, FliP and SwrB, and FliF complexes have been reported to adopt open and closed conformations. Dark gray bar indicates membrane, light gray bar indicates peptidoglycan, purple shapes indicate FliF, magenta shapes indicate FliG, pink shapes indicate FliM and FliN/Y, orange indicates the secretion complex, and light blue indicates SwrB. FliP is colored red to indicate an inactive secretion complex or light green to indicate an active secretion complex. Protein secretion is indicated by a dark green dotted line.
Mentions: Based on cytological and genetic suppressor data, we conclude that SwrB functions as an assembly chaperone to enhance the probability that the flagellar basal body adopts a conformation proficient for secretion (Fig 11). We propose that the flagellar type III secretion apparatus and FliF form first in the membrane as a “proto-basal body” that is inactive for export (Fig 11A) [7]. The proto-basal body is able to spontaneously mature to become proficient for hook secretion at a low frequency (Fig 11B). Under normal conditions, the frequency of proto-basal body maturation is increased by both SwrB and assembly of the C-ring as the absence of either share a low hook-to-basal body ratio (Fig 11C). Indeed, SwrB and FliG appear to be required at the same step as FliG gain-of-function alleles that enhance rotor stability bypass the requirement of SwrB for hook assembly, and artificial overexpression of SwrB bypasses the need for FliG. The most likely convergence point for SwrB and FliG is the membrane-basal body protein FliF, as FliG is a known interactor and SwrB, itself a membrane protein, could be adjacent. We hypothesize that the previously documented changes in FliF conformation are responsible for activating the export apparatus and do so by activating the membrane protein FliP. In sum, maturation of the flagellar basal body to a secretion proficient state is a morphogenetic checkpoint that must be passed prior to proceeding to the export and assembly of more distal components.

Bottom Line: Flagella are assembled sequentially from the inside-out with morphogenetic checkpoints that enforce the temporal order of subunit addition.Genetic suppressor analysis indicates that SwrB activates the flagellar type III secretion export apparatus by the membrane protein FliP.We conclude that SwrB enhances the probability that the flagellar basal body adopts a conformation proficient for secretion to ensure that rod and hook subunits are not secreted in the absence of a suitable platform on which to polymerize.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, Indiana University, Bloomington, Indiana, United States of America.

ABSTRACT
Flagella are assembled sequentially from the inside-out with morphogenetic checkpoints that enforce the temporal order of subunit addition. Here we show that flagellar basal bodies fail to proceed to hook assembly at high frequency in the absence of the monotopic protein SwrB of Bacillus subtilis. Genetic suppressor analysis indicates that SwrB activates the flagellar type III secretion export apparatus by the membrane protein FliP. Furthermore, mutants defective in the flagellar C-ring phenocopy the absence of SwrB for reduced hook frequency and C-ring defects may be bypassed either by SwrB overexpression or by a gain-of-function allele in the polymerization domain of FliG. We conclude that SwrB enhances the probability that the flagellar basal body adopts a conformation proficient for secretion to ensure that rod and hook subunits are not secreted in the absence of a suitable platform on which to polymerize.

No MeSH data available.


Related in: MedlinePlus