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SecA, a remarkable nanomachine.

Kusters I, Driessen AJ - Cell. Mol. Life Sci. (2011)

Bottom Line: Recent crystal structures provide a detailed insight into the rearrangements required for channel opening.Here, we review our current understanding of the mode of operation of the SecA motor protein in concert with the dynamic SecYEG channel.We conclude with a new model for SecA-mediated protein translocation that unifies previous conflicting data.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 7, Groningen, The Netherlands. i.kusters@rug.nl

ABSTRACT
Biological cells harbor a variety of molecular machines that carry out mechanical work at the nanoscale. One of these nanomachines is the bacterial motor protein SecA which translocates secretory proteins through the protein-conducting membrane channel SecYEG. SecA converts chemically stored energy in the form of ATP into a mechanical force to drive polypeptide transport through SecYEG and across the cytoplasmic membrane. In order to accommodate a translocating polypeptide chain and to release transmembrane segments of membrane proteins into the lipid bilayer, SecYEG needs to open its central channel and the lateral gate. Recent crystal structures provide a detailed insight into the rearrangements required for channel opening. Here, we review our current understanding of the mode of operation of the SecA motor protein in concert with the dynamic SecYEG channel. We conclude with a new model for SecA-mediated protein translocation that unifies previous conflicting data.

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Crystal structures of SecYEG in top-view from the cytoplasm. SecY TMS 1–6 (red), TMS 7–10 (green), plug domain (blue), SecE (orange), SecG/β (yellow). a SecYEβ from M. jannaschii (PDB accession code: 1RH5). b SecYE from T. thermophilus co-crystallized with a Fab fragment (not shown) bound to the C5 loop of SecY (2ZJS). c SecYEG from T. maritima co-crystallized with SecA (not shown) (3DIN). d SecYE from P. furiosus. In the crystal, the C-terminal α-helix of a neighboring SecY molecule (not shown) inserts partially into the channel inducing opening of the lateral gate (3MP7)
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Fig1: Crystal structures of SecYEG in top-view from the cytoplasm. SecY TMS 1–6 (red), TMS 7–10 (green), plug domain (blue), SecE (orange), SecG/β (yellow). a SecYEβ from M. jannaschii (PDB accession code: 1RH5). b SecYE from T. thermophilus co-crystallized with a Fab fragment (not shown) bound to the C5 loop of SecY (2ZJS). c SecYEG from T. maritima co-crystallized with SecA (not shown) (3DIN). d SecYE from P. furiosus. In the crystal, the C-terminal α-helix of a neighboring SecY molecule (not shown) inserts partially into the channel inducing opening of the lateral gate (3MP7)

Mentions: Protein secretion in bacteria and import of proteins into the endoplasmic reticulum is mediated by a protein-conducting channel that is conserved throughout the three kingdoms of life. This hetero–trimeric integral membrane protein complex is termed SecYEG in prokaryotes and the Sec61 complex in eukaryotes. It consists of one major subunit with ten TMS, SecY or Sec61α, and smaller subunits that are located at the exterior of the channel. Several crystal structures of archaeal and bacterial Sec-complexes provide detailed structural insight into organization and dynamics of SecYEG and its homologues. The TMS of SecY assemble into a clamshell-like fold with TMS 1–5 and 6–10 forming the two half-shells that are connected by a flexible hinge between TMS 5 and 6 (Fig. 1) which contains a short loop called HL-1 that was suggested by molecular dynamic simulations to be the flexible link that allows opening of the clamshell [8]. SecE appears to stabilize the channel by embracing the clamshell as a clamp. SecG is a non-essential subunit that associates peripherally with the channel [9]. The number of TMS of SecE and SecG varies among the organisms but the functional relevance of the additional TMS is unclear. SecE of Escherichia coli consists of three TMS while the homologue Sec61γ of the archaeon Methanocaldococcus jannaschii has only one TMS. However, an E. coli SecE mutant with the two extra TMS truncated is fully functional [10]. The tilt of the SecY TMS creates an hourglass-shaped pore with a funnel-like entrance of 20–25 Å that narrows down to a central constriction of around 4 Å. This pore ring consists of six hydrophobic isoleucine residues and restricts access to the periplasmic side in the closed conformation. Opening of this constriction is necessary to accommodate a translocating polypeptide chain and will result in the formation of a water-filled pore if no translocating polypeptide would occupy the channel. The trans-side of SecYEG is occluded by an α-helical segment of SecY, called the plug domain which folds back into the periplasmic cavity of the channel (Fig. 1). Point mutations that reduce the dependence on the signal-sequence recognition map to both pore ring and plug domain [11–13], but the full contribution of these structural elements to channel function and quality control is poorly understood.Fig. 1


SecA, a remarkable nanomachine.

Kusters I, Driessen AJ - Cell. Mol. Life Sci. (2011)

Crystal structures of SecYEG in top-view from the cytoplasm. SecY TMS 1–6 (red), TMS 7–10 (green), plug domain (blue), SecE (orange), SecG/β (yellow). a SecYEβ from M. jannaschii (PDB accession code: 1RH5). b SecYE from T. thermophilus co-crystallized with a Fab fragment (not shown) bound to the C5 loop of SecY (2ZJS). c SecYEG from T. maritima co-crystallized with SecA (not shown) (3DIN). d SecYE from P. furiosus. In the crystal, the C-terminal α-helix of a neighboring SecY molecule (not shown) inserts partially into the channel inducing opening of the lateral gate (3MP7)
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Fig1: Crystal structures of SecYEG in top-view from the cytoplasm. SecY TMS 1–6 (red), TMS 7–10 (green), plug domain (blue), SecE (orange), SecG/β (yellow). a SecYEβ from M. jannaschii (PDB accession code: 1RH5). b SecYE from T. thermophilus co-crystallized with a Fab fragment (not shown) bound to the C5 loop of SecY (2ZJS). c SecYEG from T. maritima co-crystallized with SecA (not shown) (3DIN). d SecYE from P. furiosus. In the crystal, the C-terminal α-helix of a neighboring SecY molecule (not shown) inserts partially into the channel inducing opening of the lateral gate (3MP7)
Mentions: Protein secretion in bacteria and import of proteins into the endoplasmic reticulum is mediated by a protein-conducting channel that is conserved throughout the three kingdoms of life. This hetero–trimeric integral membrane protein complex is termed SecYEG in prokaryotes and the Sec61 complex in eukaryotes. It consists of one major subunit with ten TMS, SecY or Sec61α, and smaller subunits that are located at the exterior of the channel. Several crystal structures of archaeal and bacterial Sec-complexes provide detailed structural insight into organization and dynamics of SecYEG and its homologues. The TMS of SecY assemble into a clamshell-like fold with TMS 1–5 and 6–10 forming the two half-shells that are connected by a flexible hinge between TMS 5 and 6 (Fig. 1) which contains a short loop called HL-1 that was suggested by molecular dynamic simulations to be the flexible link that allows opening of the clamshell [8]. SecE appears to stabilize the channel by embracing the clamshell as a clamp. SecG is a non-essential subunit that associates peripherally with the channel [9]. The number of TMS of SecE and SecG varies among the organisms but the functional relevance of the additional TMS is unclear. SecE of Escherichia coli consists of three TMS while the homologue Sec61γ of the archaeon Methanocaldococcus jannaschii has only one TMS. However, an E. coli SecE mutant with the two extra TMS truncated is fully functional [10]. The tilt of the SecY TMS creates an hourglass-shaped pore with a funnel-like entrance of 20–25 Å that narrows down to a central constriction of around 4 Å. This pore ring consists of six hydrophobic isoleucine residues and restricts access to the periplasmic side in the closed conformation. Opening of this constriction is necessary to accommodate a translocating polypeptide chain and will result in the formation of a water-filled pore if no translocating polypeptide would occupy the channel. The trans-side of SecYEG is occluded by an α-helical segment of SecY, called the plug domain which folds back into the periplasmic cavity of the channel (Fig. 1). Point mutations that reduce the dependence on the signal-sequence recognition map to both pore ring and plug domain [11–13], but the full contribution of these structural elements to channel function and quality control is poorly understood.Fig. 1

Bottom Line: Recent crystal structures provide a detailed insight into the rearrangements required for channel opening.Here, we review our current understanding of the mode of operation of the SecA motor protein in concert with the dynamic SecYEG channel.We conclude with a new model for SecA-mediated protein translocation that unifies previous conflicting data.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 7, Groningen, The Netherlands. i.kusters@rug.nl

ABSTRACT
Biological cells harbor a variety of molecular machines that carry out mechanical work at the nanoscale. One of these nanomachines is the bacterial motor protein SecA which translocates secretory proteins through the protein-conducting membrane channel SecYEG. SecA converts chemically stored energy in the form of ATP into a mechanical force to drive polypeptide transport through SecYEG and across the cytoplasmic membrane. In order to accommodate a translocating polypeptide chain and to release transmembrane segments of membrane proteins into the lipid bilayer, SecYEG needs to open its central channel and the lateral gate. Recent crystal structures provide a detailed insight into the rearrangements required for channel opening. Here, we review our current understanding of the mode of operation of the SecA motor protein in concert with the dynamic SecYEG channel. We conclude with a new model for SecA-mediated protein translocation that unifies previous conflicting data.

Show MeSH