Question

Explain where proteins are synthesized and then trace different routes taken by various types of proteins from the site of initial manufacture to the ultimate destination in the cell.

Answer 1

1.Proteins are synthesized in cytosol specifically in ribosomes except for some proteins that are synthesized in mitochondria and chloroplast.

2. After synthesis, protein passes through step by step manner and checked for molecular tags. This tags decide the route of protein and destination.

(A) 1. As the translation starts, first branch appear in a short while. At this point, protein will remain in the cytosol for the rest of translation or fed to Endoplasmic reticulum(ER).

2. Proteins are fed to ER only if they possess an amino acid sequence called signal peptide. Proteins having target in endomembrane system must enter ER at this stage. Signal peptide is a series of hydrophobic amino acids found at N-terminal of protein. As sequence is synthesized, it is recognized by Signal recognition particle and transported to the lumen of ER.

3.In ER, proteins folds in to their structure and may attach to some sugar groups. Then proteins are transported to golgi apparatus in membrane vesiicles. Once protein is in golgi apparatus, it undergoes modification that are required to reach to its final destination which include lysosomes, plasma membrane, cell exterior.

4. If proteins does not have specific tags, they are sent to cell surface and secreted into cell exterior or delivered to plasma membrane.

(B)1. Proteins that do not enter ER, may remain in cyotosol permanently and take a route to nonmembrane destinations in cell. Proteins that are bound for mitochondria, peroxisomes, chloroplast are synthesized in cytosol and transported after complete translation.

2. To be delivered to the target, protein must contain address lable that is recognised by other proteins, which transport the protein to it's destination.