<P> G proteins, also known as guanine nucleotide - binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior . Their activity is regulated by factors that control their ability to bind to and hydrolyze guanosine triphosphate (GTP) to guanosine diphosphate (GDP). When they are bound to GTP, they are' on', and, when they are bound to GDP, they are' off' . G proteins belong to the larger group of enzymes called GTPases . </P> <P> There are two classes of G proteins . The first function as monomeric small GTPases, while the second function as heterotrimeric G protein complexes . The latter class of complexes is made up of alpha (α), beta (β) and gamma (γ) subunits . In addition, the beta and gamma subunits can form a stable dimeric complex referred to as the beta - gamma complex . </P>

Name the two types of proteins that regulate the cell cycle. how do these proteins work