Membrane-bound and soluble proteins synthesised by ribosomes attached to the rough Endoplasmic reticulum must be directed to a variety of intracellular locations, including the ER itself, the Golgi complex, endosomes, and lysosomes.
Moreover, once a protein reaches an organelle where it is supposed to remain, there must be a mechanism for preventing it from leaving. Other groups of proteins synthesised in the rough ER are destined for incorporation into the plasma membrane or for release to the outside of the cell.
Each protein contains a specific “tag” that targets the protein for inclusion in transport vesicles that will convey material from one specific cellular location to another. Depending on the protein and its destination, the tag may be a specific amino acid sequence, an oligosaccharide side chain, a hydrophobic domain, or another structural feature.
Tags may also be involved in excluding material from certain vesicles. It has recently been shown that membrane lipids can also be tagged to help vesicles reach their proper destinations.
This tag can be one or more phosphate groups attached to positions 3, 4, and/or 5 of a phosphatidyl-inositol molecule in the membrane. For example, it has been shown that a functional PI-3-kinase is required for proper sorting of vesicles to the vacuole in yeast.
Inhibition of inositol kinases in mammalian cells perturbs vesicle trafficking to the lysosome.
Aside from tags, the length and degree of saturation of certain membrane lipids have also been shown to be important in vesicle trafficking.
Sorting of proteins begins in the ER and early compartments of the Golgi stack, which contain mechanisms for retrieving or retaining compartment-specific proteins.
This is an important step in the sorting process, for it preserves the compartment-specific functions needed to maintain the integrity of glycosylation and processing pathways.
The final sorting of material that will leave the Golgi complex occurs in the Trans Golgi Network, where lipids and proteins are selectively packaged into distinct populations of transport vesicles, each destined for a different location in the cell.
In some cells, the Golgi complex is also involved in the processing of proteins that enter the cell by endocytosis.
The writer is associate professor, head, department of botany, Ananda Mohan College, Kolkata, and also fellow, botanical society of bengal, and can be contacted at [email protected]