Sec61, termed SecYEG in prokaryotes, is a membrane

protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multienzyme complexes, in which multiple catalytic domains are found in a single polypeptide chain. Protein ...

found in all domains of life. As the core component of the

translocon The translocon (also known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transpor ...

, it transports proteins to the

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

in eukaryotes and out of the cell in prokaryotes. It is a doughnut-shaped pore through the membrane with 3 different subunits (heterotrimeric), SecY (α), SecE (γ), and SecG (β). It has a region called the plug that blocks transport into or out of the ER. This plug is displaced when the hydrophobic region of a nascent

polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A ...

interacts with another region of Sec61 called the seam, allowing translocation of the polypeptide into the ER lumen.

Structure

Much of the knowledge on the structure of the SecY/Sec61α pore comes from an X-ray crystallography structure of its

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...

l version. The large SecY subunit consists of two halves, trans-membrane segments 1-5 and trans-membrane segments 6-10. They are linked at the extracellular side by a loop between trans-membrane segments 5 and 6. SecY can open laterally at the front (lateral gate). SecE is a single spanning membrane protein in most species. It sits at the back of SecY, wrapping around the two halves of SecY. Secβ (SecG) is not essential. Its sits on the side of SecY and makes only few contacts with it. In a side view, the channel has an hourglass shape, with a cytoplasmic funnel that is empty, and an extracellular funnel that is filled with a little helix, called the plug. In the middle of the membrane is a construction, formed from a pore ring of hydrophobic

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

s that project their side chains inwards. During protein translocation, the plug is moved out of the way, and a polypeptide chain is moved from the cytoplasmic funnel, through the pore ring, the extracellular funnel, into the extracellular space. Hydrophobic segments of membrane proteins exit sideways through the lateral gate into the lipid phase and become membrane-spanning segments.

By taxon

The bacterial SecYEG channel interacts with the signal sequences of secretory proteins as well as SecA, an ATPase which drives translocation. SecY is an integral

plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

membrane protein of 419 to 492 amino acid residues that typically contains 10 transmembrane (TM), 6 cytoplasmic and 5 periplasmic regions. Instead of SecA, eukaryotic translocon can use the ER-lumenal chaperone BiP to drive translocation. The archaeal translocon is less understood. It might use SecDF- YajC and YidC like bacteria, as homologs have been found. An ATPase is yet to be identified.

Species-specifics

Human proteins: * SecY: SEC61A1, SEC61A2 * SecG: SEC61B * SecE: SEC61G

Budding yeast ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been o ...

have two such homologous complexes; the essential one is named Sec61, and the non-essential one is called Ssh1. Like Sec61, Ssh1 does dock to the ribosome.

References

*Alberts, Bruce et al. ''Molecular Biology of the Cell.'' Garland, 2002.

External links

* {{MeshName, SEC61+protein Integral membrane proteins Secretion

Structure

By taxon

Species-specifics

See also

References

External links