Protein translocation across the rough endoplasmic reticulum

Abstract

The rough endoplasmic reticulum is a major site of protein biosynthesis in all eukaryotic cells, serving as the entry point for the secretory pathway and as the initial integration site for the majority of cellular integral membrane proteins. The core components of the protein translocation machinery have been identified, and high-resolution structures of the targeting components and the transport channel have been obtained. Research in this area is now focused on obtaining a better understanding of the molecular mechanism of protein translocation and membrane protein integration.

Figure 1.

Targeting of RNCs to the Sec61 complex. The mRNAs encoding proteins with ER signal sequences may be targeted to the vicinity of the RER by a translation-independent mechanism and bind to a currently unidentified mRNA-binding protein (mRNA-BP). The SRP particle binds to the 80S ribosome and mediates targeting to the ER via interaction with SRα. Cooperative GTP binding to SRP54 and SRα leads to dissociation of SRP from the RNC and attachment of the RNC to the Sec61 complex. Signal sequence insertion into the SSB site gates the translocation channel.

Figure 2.

SecYEβ and SecYEG translocation channels. TM spans of SecY are color coded as follows: TMs 1, 4–6, 9–10 (green), TM2 (blue), TM3 (cyan), TM7 (red), and TM8 (magenta). (Yellow spheres) The plug domain. Cytosolic loops 6 and 8 are pink and chocolate, respectively, in panels B and D. (A) The cytosolic face of the Methanocaldococcus jannaschii SecYEβ complex in the closed conformation. (Black sticks) Pore ring residues. (B) Lateral gate of M. jannaschii SecYEβ viewed from the plane of the membrane. (Spheres) Lateral gate contact residues (LGCRs). (C) The partially open conformation of the Thermotoga maritima SecYEG complex. (D) The hinge domain of M. jannaschii SecYEβ. The HL-1 hinge loop is labeled. All structure views were generated using PyMOL and PDB files 1RHZ and 3DIN.

Figure 3.

Posttranslational translocation pathway in yeast. (A) A diagram of the yeast Sec62/Sec63 complex. (B) Posttranslational translocation through the SEC complex. Fungi-specific subunits (Sec66 and Sec71) are not shown for clarity. Substrate delivery to the Sec62/Sec63 complex by Hsc70 precedes signal sequence insertion into the SSB site. BiP is recruited to the SEC complex by the lumenal J-domain of Sec63. BiP binding to substrates promotes posttranslational translocation.