Protein translocation first involves SecB. This protein binds to the nascent polypeptide and prevents folding of the protein (chaperone), transporting it the translocator protein, SecY. SecY is a transmembrane protein, forming a protein conductance channel through the membrane to allow the extracellular translocation of the protein, or its insertion into the membrane. SecA, a motor protein, uses ATP hydrolysis to push the protein through SecY.
The SecA protein contains a two helix finger, which is used to open the polypeptide conductance channel. This channel is very narrow, only being wide enough to allow a polypeptide through. This prevents the loss of concentration gradients etc…
Proteins that are being translocated will have a signal polypeptide at the N terminus. This is recognised by SecY, binding and being cleaved off by a signal peptidase. This ensures only extracellular proteins are translocated, providing specificity to this process. This signal peptide can also be used to instruct SecY to insert the protein into the membrane, with start and stop sequences controlling the opening of a lateral gate in the translocator protein.
Co-translational translocation occurs where the ribosome docks to the translocator, forming a translocon. This removes the need for the motor protein, SecA, as the driving force can come from protein synthesis.
This process is highly conserved in eukaryotes, with many of the proteins sharing similarities. These differences are reflected in the names, notable SecY → Sec61.