Membrane proteins have not yielded as well as other proteins to biochemical techniques, mainly because of the problems involved in isolating and purifying hydrophobic proteins in physiologically active form.
Procedures such as SDS-polyacrylamide electrophoresis and hydropathy analysis have certainly been useful, as have labeling techniques involving radioisotopes or fluorescent antibodies. In the last three decades, however, the study of membrane proteins has been revolutionised by the techniques of molecular biology, especially DNA sequencing and recombinant DNA technology. DNA sequencing makes it possible to deduce the amino acid sequence of a protein, a prerequisite for hydropathy analysis to identify trans-membrane segments.
Moreover, sequence comparisons between proteins often reveal evolutionary and functional relationships that might not otherwise have been appreciated. DNA pieces can also be used as probes to identify and isolate sequences that encode related proteins. In addition, the DNA sequence for a particular protein can be altered at specific nucleotide positions to determine the effects of selected changes in the sequence on the activity of the mutant protein for which it codes. What functions do membrane proteins perform?
Some of the proteins in membranes are enzymes, which accounts for the localisation of specific functions to specific membranes. Each of the organelles in a eukaryotic cell is in fact characterised by its own distinctive set of membrane-bound enzymes. Closely related to enzymes in their function are electron transport proteins such as the cytochromes and iron-sulphur proteins that are involved in oxidative processes in mitochondria, chloroplasts, and the plasma membranes of prokaryotic cells. Other membrane proteins function in solute transport across membranes.
These include transport proteins, which facilitate the movement of nutrients such as sugars and amino acids across membranes, and channel proteins, which provide hydrophilic passageways through otherwise hydrophobic membranes. Also in this category are transport ATPases, which use the energy of ATP to pump ions across membranes.
Still other membrane proteins are receptors involved in recognising and mediating the effects of specific chemical signals that impinge on the surface of the cell. Hormones, neurotransmitters, and growth-promoting substances are examples of chemical signals that interact with specific protein receptors on the plasma membrane of target cells. A final group of membrane-associated proteins are those with structural roles in stabilising and shaping the cell membrane.
Examples include spectrin, ankyrin, and band 4.1 protein, the erythrocyte peripheral membrane proteins. A spectrin-based network gives the red blood cell its distinctive biconcave shape and enables the cell to withstand the stress on its membrane as it is forced through narrow capillaries in the circulatory system. Proteins that are structurally homologous to spectrin and spectrinassociated proteins are found just beneath the plasma membrane in many other cell types also, indicating that a cytoskeletal meshwork of peripheral membrane proteins underlies the plasma membrane of many different kinds of cells.
The function of a membrane protein is usually reflected in the way in which the protein is associated with the lipid bilayer. For example, a protein that functions on only one side of a membrane is likely to be a peripheral protein or a lipidanchored protein.
Membrane-bound enzymes that catalyse reactions on only one side of a membrane, such as the ER enzyme glucose phosphatase, are in this category. In contrast, the tasks of transporting solutes or transmitting signals across a membrane clearly require trans-membrane proteins. Signal transduction is mediated by transmembrane receptors that bind signalling molecules such as hormones on the outside of the plasma membrane and generate signals inside the cell.
(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])