Sec61, termed SecYEG in prokaryotes, is a membrane
protein complex 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 transport ...
, it transports proteins to the
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 p ...
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
archaeal 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 ...
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.
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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