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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.


Working model of PMF-driven protein translocation by SecDF. (a), F form, preprotein-capturing state. (b), F form to I form, preprotein-holding state (c), I form, preprotein-releasing state, (d) I form returned to F form.
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f5-7_129: Working model of PMF-driven protein translocation by SecDF. (a), F form, preprotein-capturing state. (b), F form to I form, preprotein-holding state (c), I form, preprotein-releasing state, (d) I form returned to F form.

Mentions: Based on its structural features and subsequent functional analyses, we verified the following SecDF functions: (1) The driving force for the achievement of ATP-independent protein translocation by SecDF is the PMF. (2) The TM region of SecDF participates in proton transport and contains several essential hydrophilic residues. (3) The P1 domain of SecDF, which interacts with an unfolded protein, undergoes functionally important structural alterations. Taken together, we propose a working model of SecDF-mediated enhancement of protein translocation (Fig. 5). SecYEG could reside just beneath the protruding head domain of SecDF (Fig. 5a), enabling it to capture a translocating preprotein via the translocon channel. The protein-capturing F form could then change to the I state, preventing the backward movement26 of the substrate and driving the forward movement (Fig. 5b). Thus, SecDF can actively assist with protein translocation. The conversion from the I to F form could occur after the bound protein is released from the I form (Fig. 5c, d). The reformed F form can bind to the more C-terminal part of the substrate. By repeating this conformational transition cycle coupled with proton flow, SecDF can enhance protein transport.


The mechanism of protein export enhancement by the SecDF membrane component
Working model of PMF-driven protein translocation by SecDF. (a), F form, preprotein-capturing state. (b), F form to I form, preprotein-holding state (c), I form, preprotein-releasing state, (d) I form returned to F form.
© Copyright Policy
Related In: Results  -  Collection

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

f5-7_129: Working model of PMF-driven protein translocation by SecDF. (a), F form, preprotein-capturing state. (b), F form to I form, preprotein-holding state (c), I form, preprotein-releasing state, (d) I form returned to F form.
Mentions: Based on its structural features and subsequent functional analyses, we verified the following SecDF functions: (1) The driving force for the achievement of ATP-independent protein translocation by SecDF is the PMF. (2) The TM region of SecDF participates in proton transport and contains several essential hydrophilic residues. (3) The P1 domain of SecDF, which interacts with an unfolded protein, undergoes functionally important structural alterations. Taken together, we propose a working model of SecDF-mediated enhancement of protein translocation (Fig. 5). SecYEG could reside just beneath the protruding head domain of SecDF (Fig. 5a), enabling it to capture a translocating preprotein via the translocon channel. The protein-capturing F form could then change to the I state, preventing the backward movement26 of the substrate and driving the forward movement (Fig. 5b). Thus, SecDF can actively assist with protein translocation. The conversion from the I to F form could occur after the bound protein is released from the I form (Fig. 5c, d). The reformed F form can bind to the more C-terminal part of the substrate. By repeating this conformational transition cycle coupled with proton flow, SecDF can enhance protein transport.

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.