The ribosome (combined with a nascent chain/SRP complex) attaches to a SRP recetpor (usually called (SR) on the membrane of the endoplasmic reticulum. SRP is the signal recognition particle. First the ribosome, freely floating in the cytosol, begins to synthesize a protein. Only then does the SRP recognize the N-terminal signal sequence of the protein; and the ribosome/nascent protein/SRP complex is targeted to the SR on the endoplasmic reticulum. The nascent protein directs this trio to the receptor and is inserted. The SRP binds simultaneously with the ribosome, nascent protein chain, and the SR (SRP receptor).
The SR is composed of two subunits: an alpha and a beta. The alpha faces the cytoplasm (outside of the ER) with an amino terminal spanning the membrane. There at the alpha (SR alpha), there is a site for GTP binding. GTP is Guanosine Triphosphate: a source of energy during protein synthesis. The SR beta has the same GTP binding capability, but its function is unknown. Both subunits act as anchor for the ribosome/nascent chain/SRP complex.
Once GTP is hydrolyzed, the SRP of the ribosome/nascent chain/SRP complex is free from that trio and also free from the SR located on the endoplasmic reticulum. We are left with the ribosome/nascent chain attached to a (TC) translocation channel on the endoplasmic reticulum membrane.
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