Figure 4. Effects of guanine nucleotides on PDE localization. Rod outer segment (ROS) from dark-adapted rats were isolated and permeabilized as described in Methods and subsequently mixed with either 0.5 mM GTP, 0.5 mM GTPγS, or no nucleotide. ROS suspensions were kept either in the dark or were exposed to room light for 30 min, followed by fixation and preparation for immunogold electron microscopy with PDEα antibody. The distances of each gold particle from the nearest ROS plasma membrane were measured, and the distribution profile of phosphodiesterase (PDE) was plotted. Panels A and B represent PDE distribution in both light- and dark-adapted conditions with guanosine triphosphates (GTP) and GTPγS, respectively. Panel C represents the distribution of PDE without additional nucleotides.