Asymmetric Interaction between Rod Cyclic GMP Phosphodiesterase γ Subunits and αβ Subunits.

Rod phosphodiesterase (PDE6) is the central effector enzyme in vertebrate visual transduction. Holo-PDE6 consists of two similar catalytic subunits (Pαβ) and two identical inhibitory subunits (Pγ). Pαβ is the only heterodimer in the PDE superfamily, yet its significance for the function of PDE6 is poorly understood. An unequal interaction of Pγ with Pβ as compared with Pα in the PDE6 complex has not been reported. We investigated the interaction interface between full-length Pγ and Pαβ, by differentiating Pγ interaction with each individual Pαβ subunit through radiolabel transfer from various positions throughout the entire Pγ molecule. The efficiency of radiolabel transfer indicates that the close vicinity of serine 40 on Pγ makes a major contribution to the interaction with Pαβ. In addition, a striking asymmetry of interaction between the Pγ polycationic region and the Pαβ subunits was observed when the stoichiometry of Pγ versus the Pαβ dimer was below 2. Preferential photolabeling on Pβ from Pγ position 40 and on Pγ from position 30 increased while lowering the Pγ/Pαβ ratio, but diminished when the Pγ/Pαβ ratio was over 2. Our finding leads to the conclusion that two classes of Pγ binding sites exist on Pαβ, each composed of GAF domains in both Pα and Pβ, differing from the conventional models suggesting that each Pγ binds only one of the Pαβ catalytic subunits. This new model leads to insight into how the unique Pαβ heterodimer contributes to the sophisticated regulation in visual transduction through interaction with Pγ. [ABSTRACT FROM AUTHOR]