L3 Protein Sorting and Secretion

Question 1

Identify the main intracellular compartments describing their functions

Answer 1

Proteins are sorted depending on their final destination depending on whether the sitting of the ribosome is free or fixed.

All ribosomes start free and then become fixed.

Proteins from free ribosomes remain in the cytosol or are translated via a translocator into other compartments ie a perioxsome or nucleus.

All proteins from the fixed ribosomes enter the endoplasmic reticulum via co translational translator. They can span the membrane or enter the ER lumen. Most proteins in the ER lumen are transported to the golgi via vesicles.

Question 2

Explain how ribosomes can become fixed and how this helps to determine the intracellular fate of synthesised proteins

Answer 2

Ribosomes become fixed if the mRNA contains a ER localisation signal. After the protein is translated it becomes free again.

Question 3

Describe the secretory endocytic pathway, both constitutive and regulated and its role in import, export and processing`

Answer 3

There are two different types of secretory pathways; the regulated pathway and the constitutive pathway. In the regulated pathway proteins are consolidated into vesicles that are stored in the cell until they are secreted in response to a signal. In the constitutive pathway vesicles continuously form and carry proteins from the golgi to the cell surface.

Eg
Constitutive proteins and lipids constantly made for the plasma membrane

Regulated via the insulin release from beta cells in the pancreas