Transducin: The Molecular Switch in Visual Excitation and a Model for Biological Coupling Enzymes
Visual excitation in vertebrate rod photoreceptor cells involves a light-activated cGMP cascade (for a review see Liebman, et al., 1987). Photoexcitation of rhodopsin leads to the activation of a latent cGMP phosphodiesterase (PDE) in the rod outer segments and results in the rapid hydrolysis of cGMP to 5’-GMP. The transient decrease of cGMP concentration causes the closure of the cation channels within the plasma membrane and the subsequent hyperpolarization of the photoreceptor cell. Transducin, a GTP-binding protein which is composed of three polypeptides (Tα, Mr 40,000 and Tßγ, Mr 37,000 and 8,000), has been shown to mediate the light activation signal from photolyzed rhodopsin to the PDE. The excitation occurs in a two stage amplification cascade. In the dark-adapted state, transducin exists in its latent form where Tα-GDP is associated with Tßγ. Photolyzed rhodopsin catalyzes the exchange of bound GDP for GTP in hundreds of transducin molecules. The Tα-GTP and Tßγ subunits of the activated transducin then dissociate from the rod outer segment membrane. The Tα-GTP activates the latent PDE complex (Pαß, Mr 88,000 and 84,000 and Pγ, Mr 14,000) by removing the inhibitory constraints imposed by Pγ upon the Pαß catalytic sites.
KeywordsNucleotide Binding Coupling Function Nucleotide Binding Site Coupling Enzyme Visual Excitation
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