A. Eleven families of PDE regulate cyclic nucleotide signaling

The central effector enzyme of visual transduction in rods and cones, PDE6, is a member of a large superfamily of phosphodiesterases. Eleven families of class I phosphodiesterases have been identified in the human genome, all of which share the ability to catalyze the breakdown of cGMP and/or cAMP.

All eleven PDE families contain a homologous C-terminal domain (purple box in figure) that represents the catalytic pocket of the enzyme. The determination of the 3-D structure of the catalytic domain for PDE4 by X-ray crystallography (Xu et al., 2000) is providing important information that links the structure of the active site to its biochemical and pharmacological properties, not only for PDE4 but also for the other PDE families.

The N-terminal, regulatory domain of PDE is what best distinguishes the eleven PDE families. Although not yet demonstrated for each PDE family, it is generally assumed that changes in the N-terminal region of PDE induced by post-translational modifications or ligand binding result in a conformational change that induces a change in the catalytic domain, thus altering enzyme activity.

Five of the eleven PDE families (including the photoreceptor PDE^ family) contain regulatory modules that have been termed GAF domains (blue ovals in figure). Several of the GAF-containing PDEs have been shown to bind cGMP with high affinity. It is intriguing that this enzyme is capable of hydrolyzing cGMP at its active site, while also binding cGMP to a distinct noncatalytic site. Typically, two GAF domains are arranged in tandem, but it seems most likely that only one of the two GAF binds cGMP. The structure of the tandem GAF domains has been visualized by X-ray crystallography of the PDE2 regulatory region (Martinez et al., 2002).

Using a low resolution image of the PDE6 catalytic dimer obtained by electron microscopy and image analysis [left image in figure; (Kameni Tcheudji et al., 2001), along with structural homology modeling of the catalytic and GAF domain amino acid sequences of PDE6 (structures on right), progress is being made in defining the structural basis for the unique catalytic and regulatory properties of PDE6 that make it well suited as the central control point of the visual signaling pathway.

Reference List

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.

Martinez,S.E., Wu,A.Y., Glavas,N.A., Tang,X.B., Turley,S., Hol,W.G.J., and Beavo,J.A. (2002). The two GAF domains in phosphodiesterase 2A have distinct roles in dimerization and in cGMP binding. Proc. Natl. Acad. Sci. U. S. A. 99, 13260-13265.

Xu,R.X., Hassell,A.M., Vanderwall,D., Lambert,M.H., Holmes,W.D., Luther,M.A., Rocque,W.J., Milburn,M.V., Zhao,Y., Ke,H., and Nolte,R.T. (2000). Atomic structure of PDE4: insights into phosphodiesterase mechanism and specificity. Science 288, 1822-1825.