Date of Award

Fall 2007

Project Type

Thesis

Program or Major

Biochemistry

Degree Name

Master of Science

Abstract

Photoreceptor phosphodiesterase (PDE6) amplifies visual signals in rod and cone photoreceptors. We developed an assay to quantify binding of radiolabeled PDE inhibitors to the catalytic sites of PDE6 to explore the activation and regulation of PDE6. We determined that both catalytic domains on the PDE6 heterodimer are catalytically active and equivalent. Only one catalytic subunit on PDE6 readily binds to and becomes activated by the G-protein, transducin. The other PDE6 catalytic site requires a large excess of activated transducin to fully activate PDE6. We conclude that transducin activates the two catalytic subunits of rod PDE6 differently. We also demonstrate direct allosteric communication between the regulatory GAF domain and catalytic domain of PDE6: binding of a fragment of the inhibitory gamma subunit to the GAF domain stabilizes radiolabeled PDE inhibitor binding to the catalytic sites; conversely, occupancy of the catalytic site by PDE inhibitors alters cGMP binding to the GAF domain.

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