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Volume 271, Number 33,
Issue of August 16, 1996
pp. 19826-19830
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
Phosphorylation of Non-bleached Rhodopsin in Intact Retinas and
Living Frogs
(Received for publication, March 6, 1996, and in revised form, May 9, 1996)
Brad M.
Binder
,
Teresia M.
O'Connor
,
M. Deric
Bownds
§
and
Vadim Y.
Arshavsky
¶
From the Laboratory of Molecular Biology and
§ Department of Zoology, University of Wisconsin, Madison,
Wisconsin 53706 and the ¶ Harvard Medical School/Massachusetts Eye
and Ear Infirmary, Boston, Massachusetts 02114
The photoresponse in retinal photoreceptors
begins when a molecule of rhodopsin is excited by a photon of light.
Photoexcited rhodopsin activates an enzymatic cascade including the
G-protein transducin and cyclic GMP phosphodiesterase. As a result,
cytoplasmic cyclic GMP concentration is decreased and the photoresponse
is initiated. This process is terminated when rhodopsin is
phosphorylated by rhodopsin kinase and subsequently blocked by a
protein called arrestin. It has been noted by several investigators
that light can cause phosphorylation of not only photoexcited but also
non-excited rhodopsin in rod photoreceptors. A goal of this study was
to determine how much non-bleached rhodopsin is phosphorylated. To
determine how the structural integrity of the photoreceptor influences
the extent of non-bleached rhodopsin phosphorylation, we studied the
reaction in electropermeabilized rod outer segments, in rod outer
segments still attached to isolated retinas and in living frogs. In the
first two preparations, we found that the maximum extent of
non-bleached rhodopsin phosphorylation was approximately 1%
of the total rhodopsin pool. In living frogs, the maximal amount of
non-bleached rhodopsin phosphorylation was ~2% of the total
rhodopsin pool and occurred after prolonged illumination by the
relatively dim light intensity of 20 lux. These data appear to exclude
models for light adaptation that postulate high levels of
phosphorylation of non-bleached rhodopsin in rod photoreceptors.

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Copyright © 1996 by the American Society for Biochemistry and Molecular Biology.
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