Cyclic GMP phosphodiesterase (PDE6) in rod photoreceptors, a key enzyme in vertebrate phototransduction, consists of two homologous catalytic subunits (Pα and Pβ) and two identical regulatory subunits (Pγs). Pγ regulates the PDE activity through its direct interaction with transducin. Here, using electron microscopy and image analysis of single particles, we show the three-dimensional organization of the basic form of bovine PDE, Pαβγγ, and compare its average image with those of Pγ-released PDE. The structure of Pαβγγ appears to be a flattened bell-shape, with dimensions of 150 × 108 × 60 Å, and with a handle-like protrusion attached to the top of the structure. Except for the protrusion, the organization consists of two homologous structures arranged side by side, with each structure having three distinct regions, showing pseudo twofold symmetry. These characteristics are consistent with a model in which the overall structure of Pαβγγ is determined by hetero-dimerization of Pα and Pβ, with each subunit consisting of one catalytic and two GAF regions. A comparison of the average image of Pαβγγ with those of Pγ-released PDE suggests that Pγ release does not affect the overall structure of Pαβ, and that the Pαβ C-terminus, but not Pγ, is a determinant for the Pαβ orientation on carbon-coated grids. These observations suggest that the basic structure of PDE does not change during its regulation, which implies that Pαβ is regulated by its regional interaction with Pγ.
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