Three-dimensional structure of the bacterial protein-translocation complex SecYEG

Abstract

Transport and membrane integration of polypeptides is carried out by specific protein complexes in the membranes of all living cells. The Sec transport path provides an essential and ubiquitous route for protein translocation. In the bacterial cytoplasmic membrane, the channel is formed by oligomers of a heterotrimeric membrane protein complex consisting of subunits SecY, SecE and SecG. In the endoplasmic reticulum membrane, the channel is formed from the related Sec61 complex. Here we report the structure of the Escherichia coli SecYEG assembly at an in-plane resolution of 8 A. The three-dimensional map, calculated from two-dimensional SecYEG crystals, reveals a sandwich of two membranes interacting through the extensive cytoplasmic domains. Each membrane is composed of dimers of SecYEG. The monomeric complex contains 15 transmembrane helices. In the centre of the dimer we observe a 16 x 25 A cavity closed on the periplasmic side by two highly tilted transmembrane helices. This may represent the closed state of the protein-conducting channel.

The action of cardiolipin on the bacterial translocon

“…The dimer stabilized by CL also forms a cross-link between two SecE subunits (Fig. 3B, lane CL Ã ), consistent with the "back-to-back" arrangement of SecYEG complexes with adjacent SecEs at the dimer interface (12,18).…”

“…The membrane bound and active version of SecYEG is dimeric (11)(12)(13)(14), but only one copy is necessary to create the channel (15). We show that CL, and to a lesser degree PG, promotes the formation of SecYEG dimers and a high-affinity binding surface for SecA.…”

Direct Demonstration of ATP-dependent Release of SecA from a Translocating Preprotein by Surface Plasmon Resonance

“…Biochemical and electron microscopy studies have indicated that the translocation pore is formed by an oligomeric assembly of SecYEG complexes (7). In a three-dimensional structure reconstituted from two-dimensional SecYEG crystals the SecYEG complex is dimeric and displays a deep cavity at the dimer interface (38). Because this cavity is closed at the periplasmic side of the membrane, the structure was proposed to represent a "closed state" of the translocation channel (38).…”

“…In a three-dimensional structure reconstituted from two-dimensional SecYEG crystals the SecYEG complex is dimeric and displays a deep cavity at the dimer interface (38). Because this cavity is closed at the periplasmic side of the membrane, the structure was proposed to represent a "closed state" of the translocation channel (38). The structure would convert into an "open state" when the complex is actively involved in translocation, but it is not clear whether the open state reflects a different conformational or oligomeric state.…”

Move it on over: getting proteins across biological membranes

“…In both of these protein export machines, microscopic, biochemical and biophysical studies have shown the translocon complexes to be arranged around a central cavity estimated to range between 9 and 60 Å , depending on the translocation status of the channel. (8)(9)(10)(11) To maintain the permeability barrier provided by the membrane, passage through pores of such large diameter must be controlled. The mode of regulation is dependent on whether translocation is coupled to protein translation or rather occurs following the translation event.…”