Jm. Corless et al., 3-DIMENSIONAL MEMBRANE CRYSTALS IN AMPHIBIAN CONE OUTER SEGMENTS .1. LIGHT-DEPENDENT CRYSTAL-FORMATION IN FROG RETINAS, Journal of structural biology, 113(1), 1994, pp. 64-86
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 mem
brane, and intradisk domains of the outer segment. The crystalline mem
brane domains are directly continuous with adjacent, noncrystalline la
mellar 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 membr
anes are more planar in shape and more uniform in axial spacing then a
djacent, noncrystalline lamellar regions. Furthermore, as crystalline
domains expand laterally, one observes increased axial spacing disorde
r in noncrystalline lamellar regions, along with an increase in the wi
dth of the intradisk compartment. Thus, crystals appear to grow latera
lly by depleting adjacent lamellar regions of components that influenc
e 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 similar to
10% of the cone outer segment volume. On the basis of comparative str
uctural, biochemical, and physiological data, cone outer segment cryst
als may represent a cocrystal between bleached, phosphorylated opsin (
providing transmembrane and intradisk elements) and the cytoplasmic pr
otein, arrestin (providing trans-cytoplasmic elements). Thus, crystal
formation may provide one mechanism of light adaptation within the con
e 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 cry
stallographic analyses. (C) 1994 Academic Press, Inc.