The Journal of General Physiology, Vol 103, 691-726, Copyright © 1994 by The Rockefeller University Press
Multi-step rhodopsin inactivation schemes can account for the size variability of single photon responses in Limulus ventral photoreceptors
MA Goldring and JE Lisman
Department of Biology, Brandeis University, Waltham, Massachusetts 02254.
Limulus ventral photoreceptors generate highly variable responses to the
absorption of single photons. We have obtained data on the size
distribution of these responses, derived the distribution predicted from
simple transduction cascade models and compared the theory and data. In the
simplest of models, the active state of the visual pigment (defined by its
ability to activate G protein) is turned off in a single reaction. The
output of such a cascade is predicted to be highly variable, largely
because of stochastic variation in the number of G proteins activated. The
exact distribution predicted is exponential, but we find that an
exponential does not adequately account for the data. The data agree much
better with the predictions of a cascade model in which the active state of
the visual pigment is turned off by a multi-step process.
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