Three-Dimensional Membrane Crystals in Amphibian Cone Outer Segments: 1. Light-Dependent Crystal Formation in Frog Retinas

Abstract

When frog retinas are exposed to light, a series of three-dimensional crystals develop within the outer segment disk system of cones but not rods. The crystals involve components that span cytoplasmic, disk membrane, and intradisk domains of the outer segment. The crystalline membrane domains are directly continuous with adjacent, noncrystalline lamellar regions. In axial extent, the crystals may involve as few as 1 or 2 disks or as many as 30 disks. However, within each disk, only one crystalline domain typically is observed. Within a crystal, the membranes are more planar in shape and more uniform in axial spacing then adjacent, noncrystalline lamellar regions. Furthermore, as crystalline domains expand laterally, one observes increased axial spacing disorder in noncrystalline lamellar regions, along with an increase in the width of the intradisk compartment. Thus, crystals appear to grow laterally by depleting adjacent lamellar regions of components that influence the normal membrane pair separation and axial spacing of cone outer segment disks. In isolated retinas, the crystalline domains appear to be randomly distributed along the length of the outer segment and show no preference for association with either the closed or open margins of the disk. After 45 min in the light, the crystals occupy ∼10% of the cone outer segment volume. On the basis of comparative structural, biochemical, and physiological data, cone outer segment crystals may represent a cocrystal between bleached, phosphorylated opsin (providing transmembrane and intradisk elements) and the cytoplasmic protein, arrestin (providing trans-cytoplasmic elements). Thus, crystal formation may provide one mechanism of light adaptation within the cone outer segment. The spontaneous, bleaching-induced formation of these crystals in situ offers the possibility that cocrystals of cone outer segment components can be prepared in vitro for higher resolution crystallographic analyses.