D. Photoreceptor PDE is the central regulatory enzyme

The rod PDE6 holoenzyme is a tetramer consisting of homologous α and β catalytic subunits which form a dimer and to which two inhibitory γ subunits bind. Cone PDE6 differs from rod PDE6 in having a catalytic dimer of two identical α subunits to which two cone-specific inhibitory β subunits bind. The functional consequence of rod and cone subunits having somewhat different amino acid sequences is currently not well understood.

Electron microscopic analysis of purified rod PDE6 catalytic dimers at 2.8 nm resolution reveals a 3-D structure comprised of three distinct globular domains (Kameni Tcheudji et al., 2001). The largest is the catalytic domain, while the two smaller domains correspond to the tandem GAFa-GAFb domains. The primary site of dimerization is between the GAFa domains of two catalytic subunits.

Each inhibitory γ subunit interacts with at least two different regions of the catalytic subunit, and the affinity of γ binding is regulated by cGMP binding to the GAF domains of the catalytic subunit (Norton et al., 2000; Mou and Cote, 2001). The C-terminal residues of the 87 amino acid γ subunit bind directly to the active site within the catalytic domain (Granovsky et al., 1997). The N-terminal half of γ binds to the catalytic dimer with 50-fold higher affinity than its C-terminal half, and is responsible for the cGMP-dependent modulation of γ affinity by the GAF domain (Mou and Cote, 2001).

Upon light activation of rhodopsin during visual excitation, activated transducin (αt*-GTP) binds to the PDE6 holoenzyme. PDE6 activation results from physical displacement of its inhibitory γ subunit from the active site. Because transducin can activate rod PDE6 to only 50% of the activity of purified catalytic dimers, it is likely that only one of the two catalytic sites is subject to activation by transducin during phototransduction. Furthermore, only under conditions where cGMP dissociates from the GAF domains of PDE6 will the γ subunit completely dissociate from the PDE6 catalytic dimer (D'Amours and Cote, 1999; Norton et al., 2000).

Reference List

D'Amours,M.R. and Cote,R.H. (1999). Regulation of photoreceptor phosphodiesterase catalysis by its noncatalytic cGMP binding sites. Biochem. J. 340, 863-869.

Granovsky,A.E., Natochin,M., and Artemyev,N.O. (1997). The γ subunit of rod cGMP-phosphodiesterase blocks the enzyme catalytic site. J. Biol. Chem. 272, 11686-11689.

Kameni Tcheudji,J.F., Lebeau,L., Virmaux,N., Maftei,C.G., Cote,R.H., Lugnier,C., and Schultz,P. (2001). Molecular organization of bovine rod cGMP-phosphodiesterase 6. J. Mol. Biol. 310, 781-791.

Mou,H. and Cote,R.H. (2001). The catalytic and GAF domains of the rod cGMP phosphodiesterase (PDE6) heterodimer are regulated by distinct regions of its inhibitory γ subunit. J. Biol. Chem. 276, 27527-27534.

Norton,A.W., D'Amours,M.R., Grazio,H.J., Hebert,T.L., and Cote,R.H. (2000). Mechanism of transducin activation of frog rod photoreceptor phosphodiesterase: allosteric interactions between the inhibitory γ subunit and the noncatalytic cGMP binding sites. J. Biol. Chem. 275, 38611-38619.