IMAGING OF PHOTOTRANSDUCTION PROCESS IN RENTINAL CELLS

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The objective in this international cooperative research with Dr Akio Yamazaki, Kresge Eye Institute, Wayne State University, was to reveal morphological counterpart of the photo-transduction process in the vertebrate photoreceptor cells. In paticular, we aim to explore regulation of phosphodiesterase (PDE), one of the key components in phototransduction, by characterizing both functional and structural changes of PDE subunits and to elucidate new roles of PDE in visual signal transduction mechanism. In contrast to significant progress in understanding the flow of signals in phototransduction and the roles of each component in the flow of signals, the relationship between mechanism and structure of these components still remain unclear. Especially, the relationship in PDE subunits is unknown because of diffeculty of purification and expression of intact Pαβand lack of information about regulation of Pαβ-Pγ interactions. However, in this study, Pαβ-Pγ interactions were partially elucidated with new findings on the machanism of Pγ modifications and unexpected finding on the PDE role in the increase of cytoplasmic cGMP level in rod outer segments. Our recent development in the isolation and purification of bovine Pαβ with or without Pγ also made possible to observe the molecules under electron microscope. All types of PDE (αβγγ,αβγ,αβ) consisted basically of two crooked strands with characteristic shape that were arranged as making a ring. Theses two components faced each other relative to bending or twisting at about 30 - 60℃. Unfortunately, γ subunit was too small to be dected accurately by this technique. Nevertheless, suggestible structural changes induced by dislocation of Pγ from complete PDE were arrested as three dimentional shift of molecules in position.At any rate, understanding molecular basis of Pαβ functional structure provided new insight into phototransduction mechanism.