Signal recognition particle


The signal recognition particle is an abundant, cytosolic, universally conserved ribonucleoprotein that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes.

History

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.

Mechanism

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 to the protein-conducting channel, also known as the translocon, in the ER 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 binding and hydrolysis. These are located in two related subunits in the SRP receptor and the SRP protein SRP54. 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.

Composition

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.
EukaryoteArchaeaBacteria
SRP9
SRP14
SRP19
SRP54
SRP72
7SL RNA

Autoantibodies

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.