Transport of opsin between cellular compartments of cone photoreceptors

Opsin is a G-protein that detects light in retinal rods and cones. It localizes to the continuously renewed outer segment (OS) of these photoreceptors and is densely packed into membrane layers that form discs in rods and lamellae in cones. Mechanisms of opsin transport in photoreceptor OS are insufficiently studied – in particular, the relative contribution of passive and active transport remains unclear. We investigate opsin transport in live cones of genetically modified African clawed frog (Xenopus laevis) using high-resolution confocal laser scanning FRAP microscopy combined with mathematical modeling. The modelling allows for quantitative interpretation of the experimental signal, study the system on different time- and length-scales and its response to parameter variations as well as to external perturbations. Our results show that opsin transport between cone OS lamellae may represent a purely diffusive process, that is it can occur without involving an active cellular transport.