Light-activated GTPase in vertebrate photoreceptors

Abstract

IN studies of the regulation of cyclic GMP levels by light we found that rod outer segment disk membranes contain a cyclic GMP phosphodiesterase (PDE) with a molecular weight of 240,000, whose activation depends on the photoisomerisation of rhodopsin the presence of a nucleoside triphosphate^1–3. We report here an additional light-dependent enzyme activity, the hydrolysis of GTP to form GDP and inorganic phosphate, which functions as another enzymatic component in the regulation of photoreceptor guanosine nucleotide levels by light. This GTPase has the action spectrum of rhodopsin and is half-maximally activated by bleaching only one in 2,000 rhodopsins. The enzyme has an apparent K_m for GTP of 0.5 µM. GTPase activity can be quantitatively removed from the disk membranes and the activity can be fully restored to the disk membranes by a partially purified heat labile protein with an apparent molecular weight of 32,000.