Signal recognition particle

Template:Short description Template:Missing information Template:Infobox protein Template:Infobox protein Template:Infobox protein Template:Infobox protein Template:Infobox protein Template:Infobox protein Template:Infobox nonhuman protein 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.<ref name=":0">Template:Cite book</ref>

History

The function of SRP was discovered by the study of processed and unprocessed secretory proteins, particularly immunoglobulin light chains;<ref>Template:Cite journal</ref> and bovine preprolactin. Newly synthesized proteins in eukaryotes carry N-terminal hydrophobic signal sequences, which are bound by SRP when they emerge from the ribosome.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Mechanism

In eukaryotes, SRP binds to the signal sequence of a newly synthesized peptide as it emerges from the ribosome.<ref name=":0" /> 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.<ref>Template:Cite journal</ref> SRP then targets this entire complex (the ribosome-nascent chain complex) to the protein-conducting channel, also known as the translocon, in the endoplasmic reticulum (ER) membrane. This occurs via the interaction and docking of SRP with its cognate SRP receptor<ref>Template:Cite journal</ref> 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)<ref>Template:Cite journal</ref> and the SRP protein SRP54 (known as Ffh in bacteria).<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The SRP-SRP receptor complex dissociates via GTP hydrolysis and the cycle of SRP-mediated protein translocation continues.<ref>Template:Cite journal</ref>

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.

Composition and evolution

Despite SRP function being analogous in all organisms, its composition varies greatly. The SRP54-SRP RNA core with GTPase activity is shared in all cellular life, but some subunit polypeptides are specific to eukaryotes.

SRP Subunits in three domains of life

Eukaryote	Archaea	Bacteria
SRP9 SRP14	Template:No	Template:No
SRP19	Template:Yes	Template:No
SRP54	Template:Yes	Template:Yes
SRP68 SRP72	Template:No	Template:No
7SL RNA	Template:Yes	Template:Yes

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Autoantibodies and disease

Anti-signal recognition particle antibodies are mainly associated with, but are not very specific for, polymyositis.<ref name=Kao2004/> For individuals with polymyositis, the presence of anti-SRP antibodies are associated with more prominent muscle weakness and atrophy.<ref name=Kao2004>Template:Cite journal</ref>

See also

• Signal recognition particle RNA

Further reading

• Template:Cite journal
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External links

• Template:MeshName
• Signal Recognition Particle DatabaseTemplate:Dead link
• 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