Phosphodiesterase 6 (PDE6), a multisubunit (abg 2d) enzyme, plays a major role in visual function by hydrolysing cGMP in response to a light stimulus. Solubilized bovine rod PDE6 molecules depleted of their g subunits were puri®ed to homogeneity from bovine retinal rods and their molecular organization was investigated by electron microscopy. Image analysis of single particles revealed the three-dimensional dimeric arrangement of the puri®ed abd complex, and the internal organization of each catalytic subunit into three distinct domains at a resolution of 2.8 nm. The relative volume of each domain is consistent with sequence analysis and functional data, which suggest that these domains correspond to the catalytic and two GAF domains. This hypothesis was con-®rmed by immunolabelling experiments, which located the N-terminal part of the catalytic subunit where the major interaction between the two ab subunits was found to occur. The 3D molecular organization of human platelet PDE5 appears highly homologous to that of bovine rod PDE6, as predicted by similarities in their primary sequences. These observations describe the quaternary organization of the catalytic PDE6 ab complex, and place the catalytic and regulatory domains on a structural model. # 2001 Academic Pres
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