Sub-second turnover of transducin GTPase in bovine rod outer segments. A light scattering study.

A fast, regenerative light scattering signal from bovine ROS, the PA-signal, reflects the light-induced, transient activation of transducin. Its rate of recovery depends on the number of photolysed rhodopsin molecules, indicating that rhodopsin deactivation and not GTPase activity is rate limiting in our in vitro system. When rhodopsin deactivation is accelerated (in the presence of NH2OH), PA-signal recovery is also accelerated. A GTPase turnover number of more than 2 s-1 (at 37 degrees C) can be derived from these experiments. This is more than one order of magnitude faster than the GTPase rates so far described in the literature and is rapid enough for a physiological shut-off mechanism. The fast GTPase is attributed to a highly intact disk stack, which never releases transducin into the free aqueous space.