Question

Explain the different steps leading a 7 pass transmembrane receptor protein being exposed on the plasma membrane. You will start as the mRNA encoding for this protein was just exported outside of the nucleus.

Answer 1

1. Movement of proteins to be express on plasma membrane occurs translationally that is the proteins are synthesized by the bound ribosomes.
2. Synthesis of transmembrane proteins occurs in ER lumen.
3. Translation of targeted proteins is initiated in free ribosomes in cytosol.
4. Growing peptide or nascent peptide contains N-terminal ER target signal which is recognised by signal recognition particle (SRP).
5. SRP binding to the peptide induces conformational change in ribosome and translation is blocked for a small period of time this period is also known as elongation arrest.
6. SRP receptor present on ER membrane is made up of alpha and beta subunits where alpha subunit is peripheral which recognises SRP and beta subunit is transmembrane.
7. SRP receptor is associated with the translocon channel known as 661 complex which is responsible for movement of translated protein into ER lumen.
8. an interaction of SRP with SRP receptor translation is re-initiated on the surface of endoplasmic reticulum and movement of protein is is directed by Sec61 Complex into the lumen.
9. From ER lumen, target protein movement occurs by vesicular transport process in which a transport Vesicle is produced from the donor compartment which later fuses with the acceptor compartment.
10. Transport vesicles are COP II coated( transport from ER to Golgi, anterograde transport) or COP I coated ( transport from Golgi to ER, retrograde transport).
11. Transmembrane proteins of plasma membrane are first expressed on membrane of ER then transported to plasma membrane by secretory pathway(ER--->golgi--> plasma membrane).
12. Multipass transmembrane proteins contain multiple stop and start transfer signals for spanning into membrane.