Expression of an Integral Membrane Protein, the 5HT5A Receptor

Research in the field of membrane proteins has undergone explosive growth during the last decade, primarily owing to the influence of the powerful techniques of modern molecular biology. Membrane proteins fulfill essential functions, such as communication, selective transport of metabolites and ions, and energy transformation. It is estimated that one-third of the genes of an organism encode integral membrane proteins (1 ). We are just now beginning to understand the molecular structures of this group of proteins and how they function within the confines of the cellular membranes. Among the different families of membrane proteins, the so-called G protein-coupled receptors (GPCRs) comprise the largest family. From the viewpoint of pharmacology, this family is of great importance, since about 60% of all pharmaceuticals known today mediate their effects via interaction with GPCRs. Therefore, much progress has been made in the characterization of the pharmacological and biochemical properties, as well as the signal transduction mechanisms of the GPCRs. Nevertheless, in order to understand the function and molecular dynamics of these receptors, detailed structural information will be needed. Despite the steady progress in understanding of GPCRs, solid three-dimensional (3D) structural data are still missing. To date, the crystallization and 3D determination have been successfully performed on only a handful of membrane proteins. All these structural determinations were performed on membrane proteins that are naturally highly expressed and can be purified in large quantities from their natural sources.