Signal recognition particle (SRP) is a ribonucleoprotein particle. It is important for the targeting of proteins to the prokaryotic plasma membrane or to the eukaryotic endoplasmic reticulum membrane. Its job, or function, is to bind to the signal peptide of the membrane or secretory proteins coming from the ribosome at which time it forms a ribosome-nascent chain (RNC)-SRP complex. The RNC-SRP complex then associates with the SRP receptor that is attached to the endoplasmic membrane. SRP function is similar in all organisms although its composition varies. Eukaryotic SRP is made of six polypeptides bound to the RNA molecule whereas prokaryotic SRP is made of only one polypeptide bound to an RNA molecule. Scientists continue to study the complexities of SRP in order to deepen our understanding of this important particle.