The role of prenylated small GTP-binding proteins in the regulation of osteoclast function

The Ras superfamily of small GTP-binding proteins (also known as small GTPases) comprises more than 80 highly conserved proteins of the Ras, Rho, and Rab subfamilies that are involved in multiple intracellular signalling pathways. These proteins are able to function as molecular switches in the transduction of signals from membrane receptors by cycling between an inactive, GDP-bound state and an active, GTP-bound state, which can then interact with a number of different effector molecules (Fig. 1). The activity of small GTPases is regulated by three classes of regulatory proteins: guanine nucleotide exchange factors (GEFs), which catalyse the exchange of GDP for GTP, thereby activating the small GTPase; GTPase-activating proteins (GAPs), which enhance the intrinsic ability of small GTPases to hydrolyse GTP, resulting in reversion to the inactive GDP-bound state; and guanine nucleotide dissociation inhibitors (GDIs), which preferentially bind to the GDP-bound GTPases in the cytoplasm, thereby inhibiting the release of GDP and maintaining the GTPase in the inactive state [1]. GDIs have not been identified for all small GTPases, but play an important role in the control of the Rho family GTPases.