Cells receive much of their information through signaling pathways that use GTP binding proteins to convey signals from cell surface receptors to intracellular effector proteins(Figure 1). When these receptors bind agonists, they promote the binding of GTP to specific GTP binding proteins known as G proteins. GTP binding activates the G protein and thereby allows it to regulate the activities of specific effector proteins Effecters include enzymes that synthesize cytoplasmic second messengers, ion channels, and transporters. G proteins are thus involved in regulating the synthesis and release of neurotransmitters, the sensitivity of synaptic receptors, general cellular metabolism, cellular differentiation, and growth.

It is likely that more than 100 receptors communicate through G proteins. They include receptors for catecholamines and most other biogenic amines, muscarinic cholinergic and GABAs receptors, and receptors for most eicosanoids and for numerous peptide hormones and neuromodulators. The number of distinct G proteins is probably near 20, and there is probably a similar number of G protein-regulated effecters. Much of the effort in this field has been devoted simply to enumerating and identifying the components, and the list is likely to grow further over the next several years. G proteins are responsible for first amplifying signals from receptors and then directing them to the appropriate effecters. Thus, G proteins form the basis of a complex information processing network in the plasma membrdne. Their salient role is to organize the signals that they receive from multiple receptors and direct them to an appropriate array of effecters. Complex circuits are common. Signals from different receptors can be integrated through one or more G proteins to stimulate a single second messenger pathway. Stimulatory and inhibitory signals can be balanced at the G protein level to yield a damped messenger output. Information from a single receptor can also be directed to several different effector systems using one or more G proteins as transducers in the pathway. Much is known about the structures of G proteins and G protern-coupled receptors and about the mechanism of coupling. However, we have only begun to appreciate the remarkably complex organization of these sys-tems, This review will concentrate on the way in which G proteins coordinate a cell’s responses to multiple signals in its environment. Other recent reviews on G proteins are available(Gilman, 1987; Stryer and Bourne, 1986).