SRPribosoomnascent

SRPribosoomnascent is a term used in cell biology to describe the ribosome-nascent chain complex bound by the signal recognition particle during the co-translational targeting of secretory and membrane proteins. It denotes the transient assembly formed when a nascent polypeptide bearing a signal sequence emerges from the ribosome and is captured by SRP to direct the translating ribosome to a membrane destination.

The signal recognition particle is a ribonucleoprotein that recognizes signal sequences and temporarily halts translation. In eukaryotes, SRP consists of an RNA core and multiple protein subunits, including SRP54 and SRP19, among others, while the SRP receptor (SR) on the endoplasmic reticulum membrane comprises SRα and SRβ. In bacteria, the system is represented by Ffh (SRP protein) with 4.5S RNA and the membrane receptor FtsY. The SRPribosoomnascent complex thus contains the ribosome, the growing nascent chain with its signal sequence, SRP, and, subsequently, the SRP receptor.

Mechanistically, SRP binds the emerging signal sequence and pauses elongation, guiding the ribosome to the SRP receptor at the target membrane. The ribosome-nascent chain is then transferred to the Sec translocon (Sec61 complex in eukaryotes, SecYEG in bacteria), SRP is released, and translation resumes with the nascent chain translocated into or across the membrane.

SRPribosoomnascent is central to co-translational protein sorting, ensuring correct targeting and reducing cytosolic misfolding. Disruption of this process can impair protein localization and cellular homeostasis; analogous components are conserved across bacteria, archaea, and eukaryotes.