Question

What guides free ribosomes to the endoplasmic reticulum?

a. A signal sequence and a TOM protein.

b.A signal sequence alone.

c.A signal sequence and a signal recognition particle.

d. A signal recognition particle alone.

Answer 1

c) option is correct. Synthesis of all proteins starts in cytoplasm by free ribosomes and then completed in either within cytoplasm or on rough endoplasmic reticulum (RER) membrane. All proteins of endoplasmic reticulum (ER), golgi complex, lysosome, plasma membrane (collectively called endomembranous system) are synthesized on RER membrane. All proteins of endomembranous system contain a signal sequence present at N terminal that appear in early steps of translation. As signal sequence emerge out of the ribosomes, signal recognition particle binds to the ribosomes. Signal recognition particle (SRP) is a ribonucleoprotein complex that consists of six different proteins and one RNA, called 7-SL-RNA. Out of 6 one subunit of SRP binds directly and blocks entry of aminoacyl tRNA and inhibits peptidyl transferase activity. GTP bound SRP catalyse translation of whole complex (ribosome, mRNA and nascent polypeptide) to the RER membrane. On RER membrane ribosome interact with translocase and then SRP hydrolyse their GTP and dissociate. Now, ribosome restarts translation and growing polypeptide enter in ER lumen. In lumen, signal sequence is removed by signal peptidase enzyme and then expose another targetting signal.