The signal recognition particle (SRP) is a ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes.
Mechanism
The SRP is found in the cytosol. In eukaryotes, it binds to the endoplasmic reticulum signal sequence found in an emerging secretory protein and subsequently delivers the protein along with the ribosome to the membrane of the endoplasmic reticulum. When SRP binds the endoplasmic reticulum signal sequence, it also binds the ribosome and thereby pauses translation until the entire SRP-bound translation complex is transported to the cytoplasmic surface of the endoplasmic reticulum, where translation resumes.
In the GTP-bound state, it recognizes an endoplasmic reticulum signal sequence of eight or more nonpolar amino acid residues at its center.[1] It transiently binds to the endoplasmic reticulum signal sequence in a nascent protein, to the large ribosomal unit, and to the SRP receptor.
SRP and its receptor initiate the transfer of the nascent chain across the endoplasmic reticulum membrane. Once the chain reaches the translocon, SRP and its receptor both hydrolyze their bound GTP to GDP, then dislocate allowing translation to restart.
Structure
The core of the SRP is universal, being conserved in all three kingdoms. The eukaryotic SRP is composed of six distinct polypeptides bound to an RNA molecule (the 7SL RNA), with GTPase activity. The components of the complex are:
- SRP9
- SRP14
- SRP19
- SRP54
- SRP68
- SRP72
- SRP RNA
The prokaryotic SRP is composed of one polypeptide bound to an RNA molecule (the 4.5S RNA), with GTPase activity. The components of the complex are:
Ffh is the structural and functional homolog of the SRP54 protein in eukaryotes. The 4.5S RNA shares sequence and structural homology with one domain of the larger 7S RNA.
History
SRP was identified and characterized by Peter Walter when he was a graduate student in the laboratory of Günter Blobel.[2]
References
External links
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
