Question

PLEASE ANSWER ALL THE QUESTIONS IN EACH PART. THANK YOU!

1. (14pts) A. What are the functions of the signal recognition particle (SRP) and translocon in protein transport to the ER? SRP Translocon B. How does the translocon determine which orientation to insert membrane proteins? C. How do budding & fusion affect the orientation of integral membrane proteins during vesicle transport? D. Consider a ligand-gated ion channel at the plasma membrane that binds to ligands in the cytoplasm. From a hydropathy plot, you determine the protein has the following general structural elements: 5 Trans-membrane domains (boxes 1-5) - Flanking charges around the first transmembrane domain (+ and-) - 6 soluble domains (wavy lines A-F) N-term 1 2 3 4 C-term Your colleague states that either region B or E contains the ligand binding domain, but is unsure which. Which region is more likely to contain the ligand-binding domain? Please explain your reasoning

Answer 1

Part A,

SRP is a ribonucleoprotein particle which helps in targeting protein to prokarotic plasma memebrane or eukaryotic endoplasmic reticulum memebrane. it binds to secretory proteins that comes from ribosome or signal peptide present in the membrane and forms a Ribosome nascent chain (RNC) and SRP complex (RNC-SRP complex), which gets associated with the SRP receptor on membrane.

Translocon, which is also called translocation channel is protein complex which helps in polypeptides translocation across membrane of eukarotic or prokaryotic cell. In eukaryotic cell, translocon help to move nascent polypeptides alongwith a targeting signal sequence into ER, while in prokaryotes, it transports polypeptides across plasma membrane and helps integrating membrane proteins.

Part B,

Translocon have signal sequence alongwith nascent polypeptide, this signal sequence anchored in phospholipid bilayer of membrane and remain there. Ths sequence act as both stop-transfer and membrane-anchor sequence. The nascent polypeptide does not transferred to ER, its C-terminus remains in the cytosol.