Transducin is a GTP-binding protein which mediates the light activation signal from photolyzed rhodopsin to cGMP phosphodiesterase and is pivotal in the visual excitation process. Biochemical studies suggest that the Tα subunit of transducin is composed of three functional domains, one for rhodopsin/Tβγ interaction, another for guanine nucleotide binding, and a third for the activation of phosphodiesterase. The integration of the primary sequence of Tα along with secondary structure, hydropathy and folding topology predictions, and a comparison with homologous proteins have led to the construction of a three-dimensional model of the Tα subunit. A molecular mechanism which underlies the coupling action of Tα is suggested on the basis of this model.
