Surfaces of rod photoreceptor disk membranes: light-activated enzymes.

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Abstract
The light-activated GTP-binding protein (GBP) in toad [Bufo marinus] rod outer segments was located on the cytoplasmic surface (CS) of rod disk membranes by correlating biochemical results with images of quick-frozen, freeze-fractured, and deep-etched rod outer segments. This was accomplished by selectively removing and replacing the 8-12 nm particles found on the CS of disk membranes, exactly in parallel with the GBP. In contrast, the large particles are not correlated with another major disk enzyme, the light-activated cGMP phosphodiesterase could not be visualized. The surface density of large particles, 1 particle/11 rhodopsins in isolated rod outer segments and 1 particle/9 rhodopsins in intact retina, correlates well with previous biochemical estimates of GBP number based on enzyme activity. After the identification of the large particles the effects of light on the density of particles on the surface of disk membranes were tested in intact retinas. Retinas quick-frozen at various intervals after a bright flash of light show a modest increase (.apprx. 30%) in particle density by 10 s after the flash but no increase before 1 s. The number of particles on the disk membrane returns to dark levels between 1-10 min after the flash. The 1-s latency in the change of particle binding would appear to rule out this process as a mechanism for initiating phototransduction in the rod.

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