Exchange factors, effectors, GAPs and motor proteins: common thermodynamic and kinetic principles for different functions

In this article, we review the properties of several classes of proteins that interact with ATPases and GTPases involved in energy and signal transduction. We show that certain common basic thermodynamic principles apply to the manner in which the nucleotide hydrolases interact with their partner molecules, and that the principles involved in signal transduction can be quantitatively modified to give systems with the known properties of energy transducing systems. As an example, actin can be described as an exchange factor for myosin, with its exchange activity being specific for ATP or inorganic phosphate in the myosin.ADP.P-i complex, in contrast to the unspecific exchange activity of guanosine nucleotide exchange factors operating on GTPases involved in signal transduction and regulatory processes. These common aspects are reflected in shared structural features, suggesting an evolutionary relationship between such systems.