Question

Explain in detail how the SRP ( signal recognition particle) works to deliver a protein to the ER lumen and then how that protein gets delivered to the exterior of the cell. You may wish to use a specific example if so, cite the source.

Answer 1

Proteins to be secreted outside the cell are cotranslationally translocated. Intially translation of the protein begins in the ribosome, as a short sequence of hydrophobic aminoacids present in the N terminal of the protein, known as the signal sequence comes out of the ribosome, it is bound by the signal recognition particle (SRP) , which targets the ribosome to the endoplasmic reticulum. The SRP binds to the SRP receptor on the ER, which opens up a transmembrane pore called the translocon through which the polypeptide enters the ER lumen and SRP is recycled in the cytoplasm upon hydrolysis of GTP from the GTP binding domain present in the SRP and its receptor. Translation of the polypeptide continues and it is fed into the ER lumen.The signal peptide is cleaved by a signal peptidase, translation continues and finally the synthesised polypeptide is released into the ER lumen. The peptide is then packaged in vesicles and transported into the cis golgi folloed by the trans golgi where it undergoes certain post translational modifications and finally it is packaged in vesicles and sent towards the cell membrane where the vesicles fuse and release the contents to the exterior of the cell through exocytosis.