U3 AOS1 Topic 5: Protein Secretory Pathway

Proteins are synthesized in ribosomes. Ribosomes can either be attached to the rough endoplasmic reticulum or freely floating in the cytoplasm. The polypeptide chain which is synthesized in ribosome by means of translation is correctly folded into protein and transported to golgi apparatus to be processed and packaged. The packaged proteins are then exported outside the cell by the process of exocytosis.

Protein secretory pathway involves following steps:

1        Amino acids are assembled in ribosomes to form polypeptide chain.

2        The correct folding of polypeptide chain occurs in rough endoplasmic reticulum.

3        A vesicle called transport vesicle buds off from rough endoplasmic reticulum carrying the protein.

4        Transport vesicle takes the protein towards golgi apparatus where it fuses with golgi membrane and releases the protein into its lumen.

5        The protein is processed in golgi apparatus. Processing of protein involves addition or removal of certain chemical groups to the protein.

6        After processing, the protein is packaged to be exported outside the cell. The packaged protein buds off from golgi apparatus as secretory vesicle.

7        The secretory vesicle then travels through cytoplasm and reach plasma membrane where it fuses with plasma membrane to release the protein to the extracellular environment via the process of exocytosis.

The primary organelles involved in protein secretory pathway involves:

·         Ribosomes

·         Rough Endoplasmic Reticulum

·         Transport vesicles

·         Golgi apparatus

·         Secretory vesicle

 

Some other organelles like mitochondria and nucleus are indirectly involved in protein secretory pathway as mitochondria provides the energy in form of ATP for the movement of transport and secretory vesicles and nucleus provides the information for synthesis of proteins.