G protein deactivation mechanisms in vertebrate phototransduction

Heterotrimeric G proteins are widely used in nature to facilitate cellular responses to extracellular stimuli. In humans, these G proteins mediate vision and other senses, modulate neurotransmission, and are required for hormonal actions. The signaling system involves three groups of molecules, namely, the receptors with heptahelical transmembrane motifs, the trimeric G proteins themselves, and the effectors through which G proteins alter cellular homeostasis. Advances in genome sciences have revealed the full complement of this system in multiple species, and the current challenges are to elucidate which, when, where, and how each component is used. The field of phototransduction has historically provided unrivaled details in describing general principles of G-protein signaling. This chapter intends to cover the reactions that dominate the rate of phototransduction recovery in rod and cone photoreceptors.