A structural model for the α‐subunit of transducin Implications of its role as a molecular switch in the visual signal transduction mechanism

Abstract

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 T_α subunit of transducin is composed of three functional domains, one for rhodopsin/T_βγ interaction, another for guanine nucleotide binding, and a third for the activation of phosphodiesterase. The integration of the primary sequence of T_α 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 T_α subunit. A molecular mechanism which underlies the coupling action of T_α is suggested on the basis of this model.