SRPribosoomcomplex

SRPribosoomcomplex is a transient assembly formed during the early stages of protein synthesis when the signal recognition particle binds to a ribosome translating a polypeptide that contains a signal sequence. The complex serves to pause translation momentarily and to target the ribosome-nascent chain to the appropriate membrane system for protein translocation.

Composition and conservation: In eukaryotes, SRP is a ribonucleoprotein comprising a 7SL RNA molecule and multiple protein subunits, including SRP9/14 and SRP54, with the latter recognizing the hydrophobic signal at the N-terminus of the nascent chain. In bacteria, the SRP consists of a small RNA component and the protein Ffh, with the receptor FtsY fulfilling a similar role. The SRPribosoomcomplex thus links the ribosome, the emerging signal peptide, and the membrane targeting machinery.

Mechanism: The signal peptide emerges from the ribosome exit tunnel and is recognized by SRP54 (or Ffh). Binding of SRP to the ribosome slows translation and simultaneously docks the ribosome on the endoplasmic reticulum membrane via the SRP receptor (SRα/SRβ in eukaryotes; FtsY in bacteria). This interaction aligns the ribosome with the Sec61 translocon, preparing the nascent chain for translocation. GTP binding and hydrolysis by SRP and the receptor drive the handover of the nascent chain to Sec61, enabling translocation into the ER lumen or membrane insertion, and resumption of translation.

Disassembly and function: Following transfer, SRP and its receptor are released, the SRPribosoomcomplex dissociates, and translation continues with the growing polypeptide threaded into the translocation channel. The complex is essential for co-translational targeting of many secretory and membrane proteins and is highly conserved across domains of life.