The mechanism of protein export enhancement by the SecDF membrane component

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.

Keywords: Sec translocon; SecDF; protein transport.

Figure 1

Protein translocation via the Sec translocon complex.

Figure 2

Crystal structure of SecDF. SecDF consists of a pseudosymmetrical 12-helix transmembrane domain (TM) and two protruding periplasmic domains (P1 and P4).

Figure 3

Conformational transition of SecDF. Stereo-views of the F form (a) and the I form (b).

Figure 4

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

Figure 5

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.