To date, the superfamily of GTP-binding proteins consists of several members including translational factors, tubulins and signal-transducing GTP-binding proteins. It is this last group that will be the subject of this review. As implied in the name, the GTP-binding proteins have the ability to efficiently bind and subsequently hydrolyse guanine nucleotides. In several cases it has been shown that GTP-binding proteins act as molecular signal transducers whose active/inactive state depends on the binding of GTP or GDP, respectively. The family of signal-transducing GTPbinding proteins includes two major subfamilies, the small GTP-binding proteins and the G-protein family [29], of which the members have been identified in different organisms. So far, there are more than 80 known proteins belonging to one of these two families and their number is still increasing.
Members of the so-called 'small' GTP-binding proteins are monomeric and have a molecular mass of 20 to 30 kDa. The ras-proto-oncogenes were the first members of this group to be discovered; therefore, small GTP-binding proteins have often been referred to as Ras-homologous proteins. Nowadays this name is only used to distinguish one specific subclass of the small GTP-binding protein family. Small GTP-binding proteins have been reviewed by Barbacid [4], Bourne [8], and Haubruck and McCormick [25]. The rapidly growing family of small GTP-binding proteins is now generally subdivided into three major groups: Ras, Rho and Ypt/Rab proteins.