The structure of transducin G αt: More to view than just Ras

Abstract

Alfred Wittinghofer Max-Planck-lnstitut ftir Molekulare Physiologie Abteilung Strukturelle Biologie 44139 Dortmund Federal Republic of Germany In the age of molecular biology, structural studies can be done on any protein whose cDNA is available, provided that the protein can be expressed in large enough quanti- ties and can be crystalized. Therefore, it is refreshing that the three-dimensional structure of a protein whose prepa- ration begins in the slaughterhouse with the purchase of hundreds of cow eye retinas has finally, after many years of blood and tears, been solved recently by Noel et al. (1993). As in many other cases, the trick used to obtain crystals of transducin a (G,,) complexed with GTPyS was proteolysis to remove the first 25 amino acid residues from the N-terminus (Mazzoni et al., 1991). Following the bacterial protein synthesis elongation fac- tor Tu (EF-Tu) (Jurnak, 1985; LaCour et al., 1985; Berch- told et al., 1993; Kjeldgaard et al., 1993) and ~21”” (Pai et al., 1989, 1990; Milburn et al., 1990), G,t is the third member of the GTP-binding class of proteins to have its three-dimensional structure determined. The crystal struc- ture of transducin a, however, is the first representative of the family of G, proteins. As expected, its G domain, the structure that binds guanine nucleotides and hydrolyzes GTP, is the same as that of EF-TU and p21”“, making it very likely that any protein with the conserved sequence elements of GTP-binding proteins has this topology (Fig- ure 1). inserted into this conserved G domain (amino acid residues 59-172 of G,,) is a region unique to transducin a. that may hold the key to understanding not only its func- tion as a molecular switch but also give us insights into the function of the other G. family members. Transducin is a heterotrimeric G protein (aPr subunits) that transmits the signal received by the photon receptor rhodopsin to activate the intracellular effector, cGMP phosphodiesterase, by binding to one of the two inhibitory y subunits. The resulting decrease in intracellular cGMP reduces the flow of cations through a cGMP-gated channel and hyperpolarizes the retinal rod cell, creating a signal that is detected at the level of a synaptic connection (Cha- bre and Deterre, 1989). Transducin functions as a switch (Bourne et al., 1990, 1991; Gilman, 1987); the heterotri- merit GDP-bound form is in the off state. Photoactivated rIhodopsin, called metarhodopsin II, binds to the trimer, causes GTP to exchange for GDP, and leads to the separa- tion of the active GTP G,, from the BIr heterodimer. Hydro- lysis of GTP by G,, returns the switch to the off state. Comparison of G. and p2F: Functional Differences What do we learn about G,t and about heterotrimeric G proteins in general, and how does this compare with what we know about other signal switch molecules such as ~21”” (Ras) or the protein synthesis factor EF-Tu? This r’ eview will focus on the comparison of the functional and underlying structural distinctions between two important intracellular signal transducers, Gat and ~21”“.