Signal recognition particle
|signal recognition particle 9kDa|
|Locus||Chr. 1 q42.12|
|signal recognition particle 19kDa|
|Locus||Chr. 5 q21-q22|
|signal recognition particle 54kDa|
|Locus||Chr. 14 q13.2|
|signal recognition particle 68kDa|
|Locus||Chr. 17 q25.1|
|signal recognition particle 72kDa|
|Locus||Chr. 4 q11|
The signal recognition particle (SRP) is an abundant, cytosolic, universally conserved ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes.
The function of SRP was discovered by the study of processed and unprocessed immunoglobulin light chains, newly synthesized proteins in eukaryotes carry N-terminal hydrophobic signal sequences, which are bound by SRP when they emerge from the ribosome.
In eukaryotes, SRP binds to the signal sequence of a newly synthesized peptide as it emerges from the ribosome. This binding leads to the slowing of protein synthesis known as "elongation arrest," a conserved function of SRP that facilitates the coupling of the protein translation and the protein translocation processes. SRP then targets this entire complex (the ribosome-nascent chain complex) to the protein-conducting channel, also known as the translocon, in the ER (Endoplasmic reticulum) membrane. This occurs via the interaction and docking of SRP with its cognate SRP receptor that is located in close proximity to the translocon.
In eukaryotes there are three domains between SRP and its receptor that function in guanosine triphosphate (GTP) binding and hydrolysis. These are located in two related subunits in the SRP receptor (SRα and SRβ) and the SRP protein SRP54 (known as Ffh in bacteria). The coordinated binding of GTP by SRP and the SRP receptor has been shown to be a prerequisite for the successful targeting of SRP to the SRP receptor.
Upon docking, the nascent peptide chain is inserted into the translocon channel where it enters into the ER. Protein synthesis resumes as SRP is released from the ribosome. The SRP-SRP receptor complex dissociates via GTP hydrolysis and the cycle of SRP-mediated protein translocation continues.
Once inside the ER, the signal sequence is cleaved from the core protein by signal peptidase. Signal sequences are therefore not a part of mature proteins.
The composition of SRP
Despite SRP function being analogous in all organisms, its composition varies greatly. 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:
- SRP RNA
- 4.5S RNA
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.
|Crystallographic structures of representative SRPs|
Anti-signal recognition particle antibodies are mainly associated with, but are not very specific for, polymyositis. For individuals with polymyositis, the presence of anti-SRP antibodies are associated with more prominent muscle weakness and atrophy.
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- Grudnik P, Bange G, Sinning I (August 2009). "Protein targeting by the signal recognition particle". Biological Chemistry 390 (8): 775–82. doi:10.1515/BC.2009.102. PMID 19558326.
- Lütcke H (March 1995). "Signal recognition particle (SRP), a ubiquitous initiator of protein translocation". European Journal of Biochemistry / FEBS 228 (3): 531–50. doi:10.1111/j.1432-1033.1995.0531m.x. PMID 7737147.
- Luirink J, Sinning I (November 2004). "SRP-mediated protein targeting: structure and function revisited". Biochimica et Biophysica Acta 1694 (1-3): 17–35. doi:10.1016/j.bbamcr.2004.03.013. PMID 15546655.
- Shan SO, Walter P (February 2005). "Co-translational protein targeting by the signal recognition particle". FEBS Letters 579 (4): 921–6. doi:10.1016/j.febslet.2004.11.049. PMID 15680975.
- Kao, A. H.; Lacomis, D.; Lucas, M.; Fertig, N.; Oddis, C. V. (2004). "Anti-signal recognition particle autoantibody in patients with and patients without idiopathic inflammatory myopathy". Arthritis & Rheumatism 50 (1): 209–215. doi:10.1002/art.11484. PMID 14730618.
- Signal Recognition Particle at the US National Library of Medicine Medical Subject Headings (MeSH)
- Signal Recognition Particle Database
- www.dnaTube.com video showing an SRP in action
- Another SRP video at www.dnaTube.com
- The Nobel Prize in Physiology or Medicine 1999, "for the discovery that proteins have intrinsic signals that govern their transport and localization in the cell" to Günter Blobel, USA. Press Release, Illustrated Presentation, Presentation Speech