Guanine nucleotide-
binding proteins,
G proteins, propagate incoming messages from receptors to effector
proteins. They switch from an inactive to active state by exchanging a
GDP molecule for
GTP, and they return to the inactive form by hydrolyzing
GTP to
GDP. Small
monomeric G proteins, such as Ras, are involved in controlling cell proliferation, differentiation and apoptosis, and they interact with membranes through isoprenyl moieties, fatty acyl moieties, and electrostatic interactions. This
protein-
lipid binding facilitates productive encounters of Ras and Raf
proteins in defined membrane regions, so that signals can subsequently proceed through
MEK and ERK
kinases, which constitute the canonical MAP
kinase signaling cassette. On the other hand,
heterotrimeric G proteins undergo co/post-translational modifications in the alpha (myristic and/or
palmitic acid) and the gamma (
farnesol or
geranylgeraniol) subunits. These modifications not only assist the
G protein to localize to the membrane but they also help distribute the heterotrimer (Galphabetagamma) and the subunits generated upon activation (Galpha and Gbetagamma) to appropriate membrane microdomains. These
proteins transduce messages from ubiquitous
serpentine receptors, which control important functions such as taste, vision, blood pressure,
body weight, cell proliferation, mood, etc. Moreover, the exchange of
GDP by
GTP is triggered by
nucleotide exchange factors. Membrane receptors that activate
G proteins can be considered as such, but other cytosolic, membranal or amphitropic
proteins can accelerate the rate of
G protein exchange or even activate this process in the absence of receptor-mediated activation. These and other
protein-
protein interactions of
G proteins with other signaling
proteins are regulated by their
lipid preferences. Thus,
G protein-
lipid interactions control the features of messages and cell physiology.