nel activity on cGMP concentration all the way to 
saturation. From these experiments, they con- 
cluded that each channel molecule has perhaps five 
binding sites for cGME When fully liganded, the 
channel switches rapidly and repeatedly between 
the closed state and an open state of conductance 
near 50 pS. The opening and closing rate constants 
are of the order of 10"* s"^ and 10^ s"\ respectively. 
In addition, at least one of the partially liganded 
states (most probably the next-to-full liganded 
state) likely can transit into a smaller open state of 
conductance 15-20 pS. Direct transitions between 
the large and the small open states may also occur, 
through gaining or losing of liganded cGMP. Com- 
puter simulation based on a simple kinetic scheme 
incorporating the above features broadly repro- 
duces the temporal features exhibited by the re- 
corded channel openings. 
PUBLICATIONS 
A prominent feature of the channel that comes 
out of this study is that the liganded channel opens 
and closes extremely rapidly (with both time con- 
stants <1 ms). Moreover, the liganded cGMP also 
unbinds from the channel very quickly (with a time 
constant also <1 ms). These properties enable the 
channel to act as a rapid sensor for the decrease in 
free cGMP in rods and cones during illumination. 
Finally, this channel, unlike many other ligand- 
gated channels, does not show desensitization to 
ligand. This unusual property, however, is impor- 
tant for phototransduction, by allowing a continu- 
ous dark current to be sustained by a steady basal 
level of free cGMP in the cells, and stopped only by 
light. 
Dr. Yau is also Professor of Neuroscience at The 
Johns Hopkins University School of Medicine. 
Books and Chapters of Books 
Nakatani, K., and Yau, K.-W 1988. Phototransduction mechanism in cone receptors. In Molecular Physiology 
of Retinal Proteins: Proceedings ofYamada Conference XXI (Hara, T, Ed.). Osaka: Yamada Science Foun- 
dation, pp 233-240. 
Articles 
Nakatani, K., and Yau, K.-W 1989. Sodium-dependent calcium extrusion and sensitivity regulation in retinal 
cones of the salamander./ P/tj^^o/ 409:525-548. 
Tamura, T, Nakatani, K., and Yau, K.-W 1989. Light adaptation in cat retinal rods. Science 245:755-758. 
Yau, K.-W, and Baylor, D.A. 1989. Cyclic GMP-activated conductance of retinal photoreceptor cells. Annu Rev 
Neurosci 12:289-327. 
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