The signal recognition particle (SRP) is a universally conserved protein targeting machine that in eukaryotes comprises six proteins and a 7S RNA (aka 7SL RNA). SRP-mediated protein targeting is a strictly cotranslational process that begins when a nascent polypeptide destined for the endoplasmic reticulum emerges from the ribosome. The N-terminal signal sequence on the nascent polypeptide serves as the 'signal' that allows the ribosome-nascent chain (RNC) complex to engage the SRP. Through interaction with the SRP receptor, the RNC is transferred to the Sec61 translocon, which then either integrates the protein into the lipid bilayer or translocates it across the membrane to enter the secretory pathway. (Adapted from PMID:23414305.)