Question

Describe how a polypeptide to be secreted reaches the endomembrane system.

Short answer

A signal peptide is present at the N-terminus end of the polypeptide chain. This signal is received by a signal-recognition particle (SRP), which escorts the ribosome to the membrane of ER (endoplasmic reticulum).

The ribosome attached to the membrane continues polypeptide synthesis and releases the polypeptide into the ER lumen.

Step-by-step solution

Signal peptide

The signal peptide is a sequence of 20 amino acids present at the polypeptide chain's N-terminus end (also called the leading end).The signal peptide is recognized by an SRP which is a protein-RNA complex.

The function of SRP is to guide the ribosome to the receptor protein of the ER membrane. The receptor is a multi-protein translocation complex that transfers the polypeptide into the ER lumen.

Polypeptide synthesis on free ribosome and recognition by SRP

The synthesis of the polypeptide chain begins in the cytosol. It occurs on the free ribosome, where the mRNA (messenger ribonucleic acid) sequence is translated to form a polypeptide chain.The polypeptide chains that are destined for secretion carry a signal peptide.

The SRP recognizes and binds to the signal peptide, which temporarily stops polypeptide synthesis. The SRP then brings the ribosome to the receptor of the ER membrane.

Release of the polypeptide into the ER lumen

On binding to the receptor, SRP leaves, and polypeptide synthesis continues.The synthesized polypeptide simultaneously translocate across the membrane into the ER lumen.

Once the polypeptide synthesis is complete, the signal peptide is cleaved by an enzyme. The polypeptide is then completely released into the ER lumen.