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The mechanism of protein export enhancement by the SecDF membrane component

View Article: PubMed Central - PubMed

ABSTRACT

Protein transport across membranes is a fundamental and essential cellular activity in all organisms. In bacteria, protein export across the cytoplasmic membrane, driven by dynamic interplays between the protein-conducting SecYEG channel (Sec translocon) and the SecA ATPase, is enhanced by the proton motive force (PMF) and a membrane-integrated Sec component, SecDF. However, the structure and function of SecDF have remained unclear. We solved the first crystal structure of SecDF, consisting of a pseudo-symmetrical 12-helix transmembrane domain and two protruding periplasmic domains. Based on the structural features, we proposed that SecDF functions as a membrane-integrated chaperone, which drives protein movement without using the major energetic currency, ATP, but with remarkable cycles of conformational changes, powered by the proton gradient across the membrane. By a series of biochemical and biophysical approaches, several functionally important residues in the transmembrane region have been identified and our model of the SecDF function has been verified.

No MeSH data available.


Conserved residues of SecDF. The conserved regions of SecDF are shown in red. Functionally important, conserved residues are colored orange in a sphere representation.
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f4-7_129: Conserved residues of SecDF. The conserved regions of SecDF are shown in red. Functionally important, conserved residues are colored orange in a sphere representation.

Mentions: The TM arrangement of SecDF is similar to that of a member of the RND superfamily22, Multi-Drug Efflux Transporter AcrB23. Homotrimeric AcrB extrudes a variety of drugs by utilizing the proton gradient across the membrane, whereas SecDF functions as a monomer associated with SecYEG. We propose that the conserved Asp and Arg residues at the transmembrane interface between the TM1-6 and TM7-12 bundles (Fig. 4), in a similar manner to the conserved charged residues of AcrB24, play essential roles in the movements of protons and preproteins. The mutants in which the charged residues in E. coli SecDF were replaced by uncharged residues lacked the SecDF activity. This observation is consistent with the hypothesis that the charged residues in the TM region of SecDF participate in proton transport. Furthermore, function analysis of Vibrio alginolyticus SecDF demonstrated that a cation gradient across the membrane was required for the SecDF activity.


The mechanism of protein export enhancement by the SecDF membrane component
Conserved residues of SecDF. The conserved regions of SecDF are shown in red. Functionally important, conserved residues are colored orange in a sphere representation.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC5036778&req=5

f4-7_129: Conserved residues of SecDF. The conserved regions of SecDF are shown in red. Functionally important, conserved residues are colored orange in a sphere representation.
Mentions: The TM arrangement of SecDF is similar to that of a member of the RND superfamily22, Multi-Drug Efflux Transporter AcrB23. Homotrimeric AcrB extrudes a variety of drugs by utilizing the proton gradient across the membrane, whereas SecDF functions as a monomer associated with SecYEG. We propose that the conserved Asp and Arg residues at the transmembrane interface between the TM1-6 and TM7-12 bundles (Fig. 4), in a similar manner to the conserved charged residues of AcrB24, play essential roles in the movements of protons and preproteins. The mutants in which the charged residues in E. coli SecDF were replaced by uncharged residues lacked the SecDF activity. This observation is consistent with the hypothesis that the charged residues in the TM region of SecDF participate in proton transport. Furthermore, function analysis of Vibrio alginolyticus SecDF demonstrated that a cation gradient across the membrane was required for the SecDF activity.

View Article: PubMed Central - PubMed

ABSTRACT

Protein transport across membranes is a fundamental and essential cellular activity in all organisms. In bacteria, protein export across the cytoplasmic membrane, driven by dynamic interplays between the protein-conducting SecYEG channel (Sec translocon) and the SecA ATPase, is enhanced by the proton motive force (PMF) and a membrane-integrated Sec component, SecDF. However, the structure and function of SecDF have remained unclear. We solved the first crystal structure of SecDF, consisting of a pseudo-symmetrical 12-helix transmembrane domain and two protruding periplasmic domains. Based on the structural features, we proposed that SecDF functions as a membrane-integrated chaperone, which drives protein movement without using the major energetic currency, ATP, but with remarkable cycles of conformational changes, powered by the proton gradient across the membrane. By a series of biochemical and biophysical approaches, several functionally important residues in the transmembrane region have been identified and our model of the SecDF function has been verified.

No MeSH data available.