Photoreceptor cGMP Phosphodiesterase δ Subunit (PDEδ) Functions as a Prenyl-binding Protein^*

Abstract

Bovine PDEδ was originally copurified with rod cGMP phosphodiesterase (PDE) and shown to interact with prenylated, carboxymethylated C-terminal Cys residues. Other studies showed that PDEδ can interact with several small GTPases including Rab13, Ras, Rap, and Rho6, all of which are prenylated, as well as the N-terminal portion of retinitis pigmentosa GTPase regulator and Arl2/Arl3, which are not prenylated. We show by immunocytochemistry with a PDEδ-specific antibody that PDEδ is present in rods and cones. We find by yeast two-hybrid screening with a PDEδ bait that it can interact with farnesylated rhodopsin kinase (GRK1) and that prenylation is essential for this interaction. In vitro binding assays indicate that both recombinant farnesylated GRK1 and geranylgeranylated GRK7 co-precipitate with a glutathione S-transferase-PDEδ fusion protein. Using fluorescence resonance energy transfer techniques exploiting the intrinsic tryptophan fluorescence of PDEδ and dansylated prenyl cysteines as fluorescent ligands, we show that PDEδ specifically binds geranylgeranyl and farnesyl moieties with a K_d of 19.06 and 0.70 μm, respectively. Our experiments establish that PDEδ functions as a prenyl-binding protein interacting with multiple prenylated proteins.