Translocation of proteins through the Sec61 and SecYEG channels

Abstract

The Sec61 and SecYEG translocation channels mediate the selective transport of proteins across the endoplasmic reticulum and bacterial inner membrane, respectively. These channels are also responsible for the integration of membrane proteins. To accomplish these two critical events in protein expression, the transport channels undergo conformational changes to permit the export of lumenal domains and the integration of transmembrane spans. Novel insight into how these channels open during protein translocation has been provided by a combination of the analysis of new channel structures, biochemical characterization of translocation intermediates, molecular dynamics simulations, and in vivo and in vitro analysis of structure-based Sec61 and SecY mutants.

Figure 1

M. jannaschii SecYEβ (PDB 1RHZ). (a) Cytosolic face. TM spans are color coded as follows: TMs 1, 3–5, (cyan); TMs 6, 8–10 (magenta); TM2 (blue); TM7 (red). The plug domain (residues 52–74) is shown as yellow spheres. Pore ring residues (I75, V79, I170, I174, I260 and L406) are shown as black sticks. (b) Lateral gate of SecYEβ viewed from the plane of the membrane. TM spans are color coded as follows: TM2, blue; TM3, cyan; TM7, red and TM8, magenta. Loops 6 and 8 are pink and olive respectively. All other SecY segments are sky blue. Residues that make significant contacts across the lateral gate are shown as spheres. (c) The hinge domain of SecYEβ. The hinge helix is cyan. Hinge loops (HL-1 and HL-2) are red. Loops 6 and 8 are colored as in panel b. The arrow labeled M.B. is the estimated location of the membrane bilayer. All structure views were generated using Pymol.

Figure 2

Comparison of SecY lateral gate domains. Transmembrane spans 2, 3, 7 and 8 are colored blue, cyan, red and magenta, respectively. Loop 2 is slate and loop 7 is violet. The plug domain is shown as yellow spheres. (a) The M. jannaschii plug corresponds to residues 52–74. Lateral gate residues (shown as spheres) are as follows: TM2 (V79, T80, I83, L87 and S91); TM3 (E122, L125, F126 and A129); TM7 (L261, A264, L265, N268, L271, W272 and R278); TM8 (F330, F333, W334 and T337). (b) The T. thermophilus plug domain corresponds to residues 50–76 (PDB 2ZJS). Residues that align with those in panel a are shown as spheres. (c) The segment of the T. maritima plug domain shown here corresponds to residues 62–81 (PDB 3DIN). Residues that are shown as spheres include TM2 and TM3 residues that align with those shown in panel a, as well as the following: TM7 (F275, A278, I279, I282, A285, I286, I279); TM8 (F317, Y321). All structure views were generated using Pymol.