Biological membranes fulfil vital functions as interfaces to the outside world, as interfaces between cells, and as boundaries of intracellular compartments. Membrane proteins share a common property: part of their structure is embedded in a lipid bilayer. Therefore, being located at an interface, it is almost inevitable that they mediate communication between both sides of the membrane; receptors, pores and channels are all signal transducers. Membrane proteins are abundant; they are estimated to constitute a third of the complement of proteomes. An analysis of the genomes of eubacterial, Archaean and eukaryotic organisms predicted that 20-30% of the open reading frames encode integral membrane proteins (Wallin and von Heijne 1998). The number of lipid-interacting proteins should be increased if we consider the proteins presenting two conformational states, one of them stable in solution and the other one attached to a membrane or a lipid interface. This is the case for the great family of pore-forming toxins, for lipases and also for lipid-carriers. Therefore their scientific importance cannot be overstated, as they are involved in almost every process in the cell. Besides, membrane proteins represent over 50% of the targets of all prescription drugs. This reflects the physiological importance of these proteins, which play key roles in such physiological functions as controlling the homeostasy of the cells and organs, ensuring communication within and across tissues, detoxification, control of cell growth, etc. Describing their structure and function at the molecular level is a determinant asset in understanding their interactions with ligands and devising new and better drugs.
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