One cycle for all signals G-proteins

Proteins are the cell s action molecules. They come in an enormous variety of shapes and sizes and fulfill an equally wide range of tasks. All this is achieved by combination of just 20 building blocks, the amino acids. During protein biosynthesis on the ribosome, these are combined to a chain molecule by a reaction merging the carbonic acid group (-COOH) of one amino acid to the amino group ( NH2) of the next, which leads to what is known as a peptide bond (-CO-NH-). The chain molecule then folds up to a complex but well-defined three-dimensional structure stabilized by a large number of weak interactions between different parts of the chain (see Box 5). Only this correctly folded structure, known as the native state is functionally active. 

For instance, if the hormone adrenalin arrives at a liver cell and binds to the adrenalin-specific receptor embedded in its membrane, this leads to a whole cascade of reactions involving a G-protein, an effector catalysing the synthesis of cyclic adenosin monophosphate (AMP) and eventually results in the export of glucose from the cell (Fig. 2.4). Another GTP binding protein, Ras, is important for the regulation of cell growth and differentiation. (Mutations of this protein are notorious for causing a variety of cancers.) 

When the first crystal structures of complete G proteins came out in December 1995 and January 1996, the complexity of the structures revealed prompted Nature to use the headline The G protein nanomachine . Indeed, the structure, which 

More generally, the flood of high resolution structures of complex protein systems solved in the years 1993-95 should give us hope that within a few years from now we should be able to describe the cell s communication technology not only in general concepts, but also in molecular and mechanistic detail. 

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