Lateral diffusion of rhodopsin in photoreceptor cells measured by fluorescence photobleaching and recovery

Frog rod outer segments were labeled with the sulfhydryl-reactive label iodoacetamido tetramethylrhodamine. The bulk of the label reacted with the major disk membrane protein, rhodopsin. Fluorescence photobleaching and recovery (FPR) experiments on labeled rods showed that the labeled proteins diffused rapidly in the disk membranes. In these FPR experiments we observed both the recovery of fluorescence in the bleached spot and the loss of fluorescence from nearby, unbleached regions of the photoreceptor. These and previous experiments show that the redistribution of the fluorescent labeled proteins after bleaching was due to diffusion. The diffusion constant, D, was (3.0 +/- 10(-9) cm2 s-1 if estimated from the rate of recovery of fluorescence in the bleached spot, and (5.3 +/- 2.4) x 10(-9) cm2 s-1 if estimated from the rate of depletion of fluorescence from nearby regions. The temperature coefficient, Q10, for diffusion was 1.7 +/- 0.5 over the range 10 degrees--29 degrees C. These values obtained by FPR are in good agreement with those previously obtained by photobleaching rhodopsin in fresh, unlabeled rods. This agreement indicates that the labeling and bleaching procedures required by the FPR method did not significantly alter the diffusion rate of rhodopsin. Moreover, the magnitude of the diffusion constant for rhodopsin is that to be expected for an object of its diameter diffusing in a bilayer with the viscosity of the disk membrane. In contrast to the case of rhodopsin, FPR methods applied to other membrane proteins have yielded much smaller diffusion constants. The present results help indicate that these smaller diffusion constants are not artifacts of the method but may instead be due to interactions the diffusing proteins have with other components of the membrane in addition to the viscous drag imposed by the lipid bilayer.