Question

4. Protein trafficking has been one of the biggest challenges to explore and elucidate in the past few decades. Yet we now know much about this multi-step process.

a. What is the fundamental difference between proteins that end up in the lysosomes, plasma membranes, or are secreted from cells versus the group that are exported to, and are incorporated in, the mitochondria and chloroplasts?

b. What is the role of the signal sequence and the signal recognition particle (SRP) in RER linked protein synthesis?

c. Challenge: You are using a cell free system and studying RER linked protein synthesis. You find that it works perfectly fine (secreted proteins synthesized successfully) when you use GTP, but when you substitute GTP with GMP-PNP the system fails. Why would this most likely be the case?

Answer 1

a. The proteins that gets secreted are the ones which move out of the cell are exported and and the ones that gets incorporated to mitochondria. The proteins when secretted are sent to lumen of ER while they are being translated. When proteins are secreted outside, into plasma membrane or lysosomes, they enters the secretory pathway and the ribosomes produces protein when they directlt enter ER when they fold. Afterwards they goes to golgi and move to cellular locations through secretory pathway via their vesicles. But in mitochondria and chloroplasts, the proteins have a signal sequence that moves them to the places where they enter the organelle and gets folded and is not through secretory pathway.

b. signal sequence is a short peptidase that are linked to the N-terminus and it signals for the transport mechanisms and the signal recognition protein when gets binded to signal sequence it stops protein synthesis. The secretory proteins enter ER by cotranslational translocation while in case of yeast some secretory proteins enter ER lumen in post translational translocation. SRP and its receptor which is involved in cotranslational translocation but are not involved in the post translational method. The chaperone protein Bip keeps the protein unfolded and slipping is prevented.

c. when GMP-PNP is substituted by GTP, the system will fail, because GMP-PNP does not produce GNP.