Question

The role of srp in the process of protein translocation into the endoplasmic reticulum lumen.a short note

Answer 1

Proteins destined to enter the secretory pathway typically possess N-terminal hydrophobic signal sequences, which direct them to the endoplasmic reticulum (ER) membrane, where they are translocated into the lumen of ER. Once inside the ER lumen they can traverse the secretory pathway by vesicular transport, hence transport across the ER membrane is the only membrane translocation event these proteins require and represents the commitment step in entry to the secretory pathway. There are several distinct pathways, which target secretory protein to the ER. In this review, we focus on the universally conserved co-translational translocation pathway, which also integrates membrane proteins into lipid bilayer .

The co-translational pathway utilizes the signal recognition particle (SRP) to deliver secretory proteins to the ER membrane while they are still being synthesized by ribosome . SRP delivers the nascent secretory protein together with the associated ribosome to the ER protein translocon (Sec61 complex) via the interaction with its cognate receptor (SR) an ER resident membrane protein. When the ribosome nascent chain complex (RNC) engages the Sec61 complex protein synthesis continues, enabling the nascent chain to be directly conveyed into the lumen of the ER where it can then fold to its final conformation. During translocation enzymes such as oligosaccharyl transferase (OST) and signal peptidase (SP-ase) can associate with the translocon and either N-glycosylate or cleave the signal peptide from the trans-locating nascent chain, respectively.