| Research Abstract |
The role of visual pigments in the phototransduction process of visual cells is to receive a photon from the outer environment and transfer the signal to a retinal G protein transducin. We have investigated the molecular mechanism of high photosensitivity of visual pigment and its efficient ability in activating transducin by various techniques, and following results were obtained :
1.A fluorescent state of rhodopsin was detected by frmtosecond up-conversion spectroscopy and found that it decayd with a time constant of 60 fs. Combined with the analysis of the excited state of rhodopsin from a Fourier transform of optical absorption spectra, the ultrafast process was interpreted in terms of chromophore isomerization occuring as fast as vibrational motions of the chromophore.
2.The replacement of Glu-122 of rhodopsin by the residue containing cone pigment converted rhodopsin's rates of reguneration and meta II decay into those of the cone pigment. The characteristics of cone pigment changed to those of rhodopsin by introduction of Glu-122 into cone pigment. Thus the amino acid residue at position 122 is a functional determinant of rod and cone visual pigments.
3.Using time-resolved low temperature spectroscopy, we have found that two rhodopsin intermediates, meta Ib and meta II,interacted with transducin, but the manner of interaction with transducin is different : Meta Ib only binds to transducin, while meta II catalyzes a GDP-GTP exchange in transducin.
4.From the sequencing of cDNAs, in situ hybridization, and immunohistochemical experiments, we have found that scallop ciliary photoreceptor cells express a novel subtype of visual pigment which coupled with Go type of G protein. Thus a novel, Go-mediated, phototransduction cascade is present in the scallop ciliary cells.
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