G protein

These molecules have been described as "biological traffic lights." Located inside the cell, G proteins are able respond to signals outside the cell — light, smell, hormones — and translate (transduce) these signals into action within the cell. Alfred G. Gilman and Martin Rodbell shared the 1994 Nobel Prize in Physiology or Medicine for "their discovery of G- proteins and the role of these proteins in signal transduction in cells."

* * *

G protein 'jē- n any of a class of cell membrane proteins that are coupled to cell surface receptors and upon stimulation of the receptor by an extracellular molecule (as a hormone or neurotransmitter) bind to GTP to form an active complex which mediates an intracellular event (as activation of adenylate cyclase)

* * *

any of a family of similar heterotrimeric proteins of the intracellular portion of the plasma membrane that bind activated receptor complexes and, through conformational changes and cyclic binding and hydrolysis of GTP, directly or indirectly effect alterations in channel gating and so couple cell surface receptors to intracellular responses. Some G proteins are named for their activities, e.g., G_s stimulates and G_I inhibits enzyme activity.