Question

Explain how the vesicles are carried from the Golgi apparatus to the plasma membrane

Answer 1

Secretory pathway of protein:

The protein secretion pathway has the movement of newly synthesized protein from Endoplasmic reticulum-Golgi apparatus- secretory vesicles-plasma membrane-exocytosis.

• The ribosomes which is synthesising the protein bind to the endoplasmic reticulum.
• The proteins synthesized enter into the lumen of the ER which is determined by the presence of the ER signal sequence at the N-terminal of the protein.
• This newly formed protein is incorporated into the transport vesicles which fuse to the cis- Golgi apparatus.
• Proteins from cis- Golgi it moves to the trans- Golgi apparatus.
• From trans- Golgi apparatus the protein moves to the secretory vesicles. These secretory vesicles move further to the plasma membrane, it fuses and release the protein through exocytosis.

This process of protein secretion takes place in 2 ways

1. Continuous secretion: for some protein which is synthesized continuously. Example collagen.
2. Regulated secretion: some proteins are not needed all time. They wait in the Golgi apparatus for the proper signal, when the signal is received the protein is moved to the transport vesicles or secretory vesicle and the protein is released by fusion to the plasma membrane through exocytosis.

Example: pancreatic acinar cells which secrete digestive enzymes.

Transport from Golgi apparatus to plasma membrane:

• The cells secreting the proteins on demand are stored in the secretory vesicles.
• These secretory vesicles are formed in the trans Golgi apparatus, they are released into the exterior by exocytosis in response to the extracellular signal.
• Proteins for secretory vesicles are packed into the vesicles in the trans Golgi apparatus, the process of aggregation of secretory vesicles is not known.
• It is said that secretory vesicles have unique protein in its membrane, which might serve as the receptors for aggregated protein in the trans Golgi apparatus.
• The secretory vesicle that leave the trans Golgi apparatus is loosely packed, as the vesicles mature the contents of the pack becomes concentrated.

Processing of the proteins in the secretory vesicles:

• Proteins are secreted and it is in inactive form in the secretory vesicles, proteolysis is the process by which the proteins are processed to active.
• This proteolysis begin in the trans golgi network and continue in the secretory vesicles and some times in the extracellular fluid after secretion.

Exocytosis:

• In the continuous secretion, the vesicles fuse to the plasma membrane once it reaches plama membrane.
• In the regulatory secretion, the vesicles wait for the signal to secrete and fuse with plasma membrane.
• The signal is usually a hormone that binds to the cell surface receptors, which results in the intracellular signalling which increases the concentration of free Ca2+ in the cytoplasm.
• Ca2+ ions bind to the sensors which results in the fusion of secretory vesicles to the plasma membrane, this process is called as exocytosis.

After the process of exocytosis the components of secretory vesicles become the part of plasma membrane. These components are removed from the plasma membrane by the process of endocytosis into the cell.

The components of secretory vesicles are thought to be degraded in the lysosomes.