Which of the following is true about targeting of a secretor

Which of the following is true about targeting of a secretory proteins?

Solution

Answer: D. The signal recognition particle (SRP) binds to the signal peptide soon after it appears outside the ribosome.

Reason:

Signal peptidase normally localized in the “endoplasmic reticulum” of eukaryotic cell. These serine proteases digest the “nascent secretory polypeptides” after translation & post & co-translational modifications. These signal peptidase enzymes are going to cleave signal peptides of membrane proteins at their N-terminals.

Signal sequence of \"6 to 10 amino acid length\" normally cleaved by signal peptidases during or after completion of translocation from ribosome & before entering into the endoplasmic reticulum

Cotranslational translocation is mediated by signal recognition particle (SRP) & it is a 11S ribonucleoprotien essential for translocation of nascent polypeptide to the “ER lumen”. This SRP is formed of a nucleoprotein with 7S RNA, 11S & combines with several proteins to form the eukaryotic signal recognition particle. The nascent polypeptide is emerged from the ribosomes, followed by “recognition of signal sequence & bound by signal recognition particle (SRP), a hydrophobic residue with 6 polypeptides 7SL RNA.

Therefore, when this SRP sequence exits from the ribosome, the nascent polypeptide stops being translocated into the ER lumen. Signal recognition particle has bound by a signal sequence and flowed by slow protein synthesis in the translation. This SRP further is going to bind to the location near by endoplasmic reticulum followed by release of signal recognition particle but the growing polypeptide is in the form of protein-ribosome complex that is at the exact location for further protein translocation through Golgi-vesicle translocation channel.