Hypothetical steps in the SRP-mediated targeting of archaea proteins. Step : A ribosome (gray, with A, P and E tRNA binding sites) in the cytoplasm translates a mRNA molecule (black, and ends are indicated) which encodes a N-terminal signal or membrane-anchor sequence (black dots). Step : As the signal emerges from the large ribosomal subunit, it is recognized by the elongated SRP and further translation may be halted. Step : The SRP-bound ribosome nascent chain complex (RNC) binds to free or membrane-associated FtsY (arrow). Step : After GTP hydrolysis, SRP has been released, translation resumes, and the targeted protein is threaded through the protein-conducting channel (PCC). The surface (S) layer, present in most archaea, is anchored to a glycerol-ether lipids-containing cell membrane.
