The GTP-binding protein (G protein) is a heterotrimeric protein composed of an alpha-, a beta- and a gamma-subunit. The G protein is functionally located between membrane receptors whose structures are characterized by seven membrane-spanning domains and effectors that are enzymes responsible for the generation of intracellular second messengers or ionic channels, thereby playing its essential role as a molecular switch for intracellular signal initiation. The switch turns on when GTP binds, in exchange for prebound GDP, to the alpha-subunit (G alpha), whereas it turns off upon the GTP hydrolysis due to the G alpha GTPase activity. The beta gamma-component plays a supporting role for the molecular switching and is also involved in signal transduction to certain effectors. One of the most exciting subjects to be currently studied as to the physiological roles of G proteins will be the mechanism by which the G protein-mediated second messenger system interacts (crosstalks) with the tyrosine kinase-mediated signaling system arising from other types of growth factor receptors.