Protein conducting channel secY/secG/secE complex
Methanococcus jannaschii (archaea)
A conserved heterotrimeric membrane protein complex, called the Sec61 complex in eukaryotes and SecY complex in eubacteria and archaea, forms a protein-conducting channel, which allows polypeptides to be transferred across or integrated into membranes. The channel itself is a passive conduit for polypeptides and must therefore associate with other components that provide a driving force. In co-translational translocation, the major partner is the ribosome. The elongating polypeptide chain moves directly from the ribosome into the associated membrane channel. The energy for translocation comes from GTP hydrolysis during translation. Many (or perhaps all) cells also have post-translational translocation, in which polypeptides are completed in the cytosol and then transported across the membrane. In yeast, the post-translational translation partners are another membrane protein complex (the tetrameric Sec62/63p complex) and the lumenal protein BiP, member of the Hsp70 family of ATPases. BiP promotes translocation by acting as a molecular ratchet, preventing the polypeptide chain from sliding back into the cytosol. In the eubacterial post-translocational pathway, the cytosolic ATPase SecA pushes polypeptides through the channel.
The crystal structure of the complex from Methanococcus jannaschii was determined at a resolution of 3.2 angstrom. The structure suggests that one copy of the heterotrimer serves as a functional translocation channel. The alpha subunits (secYs) and gamma subunits (secEs) show significant sequence conservation and are required for cell viability, while the beta subunits (secGs) are not essential for cell viability. The alpha-subunit forms the channel pore and has two linked halves, transmembrane segments 1-5 and 6-10, that are clamped together by the gamma-subunit. A cytoplasmic funnel leading into the channel is plugged by a short helix. Plug displacement can open the channel into an 'hourglass' shape with a ring of hydrophobic residues at its constriction. This ring forms a seal around the translocating polypeptide, which hinders the permeation of other molecules. The structure also suggests mechanisms for signal-sequence recognition and for the lateral exit of transmembrane segments of nascent membrane proteins into lipid, and indicates binding sites for partners that provide the driving force for translocation.
Protein Data Bank (PDB)
author: Naoyuki Miyazaki