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.'