In a eukaryotic cell proteins are synthesized and pass throu

In a eukaryotic cell, proteins are synthesized, and pass through a series of specialized organelles before reaching their target. Describe how a trans-membrane protein would be synthesized, processed and ultimately become a part of the plasma membrane of the cell.

Solution

The transmembrane proteins and secretory proteins are manufactured by ribosomes bound to Endoplasmic reticulum. The secreted and transmembrane proteins pass through the Endoplasmic reticulum (ER) membrane. As these are large and polar, the proteins pass through rough ER membrane. This process occurs through secretory pathway. It was first described by Gunter Blobel, Cesar Milstein and David Sabatini in 1975.

The different proteins are synthesized by cell, out of which 25% proteins are integral proteins and many other proteins are secretory proteins. Many proteins as many as 40% are synthesized by a cell are processed by secretory pathway or some other protein targeting pathway.

The secretory pathway is summarized as follows:

The vehicles in which proteins are transported between Rough Endoplasmic Reticulum (RER), different compartments of Golgi complex and their final destinations are known as ‘coated vesicles’.

A 60-150 nm diameter membranous sacs enclose these proteins on their outer surface with specific proteins that act as flexible scaffolding in promotion of vesicle formation process.

A vesicle is formed by budding from its membrane and later on fuses with the target membrane.

This process maintains the orientation of transmembrane proteins, because the lumen of ER and cisternae of Golgi complex are topologically similar to external surface of the cell. Due to this reason it is understandable that the carbohydrate residues of integral glycoproteins and GPI linked proteins occurs only on external surfaces of plasma membrane.

Three types of coated vesicles are known. These are characterized by their protein coats.

All these vesicles bear receptors for proteins to be transported and for proteins that mediate the fusion of these vesicles with target membranes.