SOLUBLE-PROTEINS OF INTACT BOVINE ROD CELL OUTER SEGMENTS

Centrifugation of a particulate fraction from homogenates of bovine retinas to isopycnic equilibrium on sucrose density gradients yielded two discrete bands of particles containing rhodopsin (rod outer segments). The band having the lower buoyant density appeared on microscopic examination to be isolated discs and fragments of outer segments and contained little protein other than rhodopsin. The band having a higher buoyant density appeared to be outer segments with intact plasma membranes and exhibited osmotic properties different from those of the disrupted outer segments. The intact outer segments were shown by gel electrophoresis to contain five major polypeptide species other than rhodopsin, amounting to about 30% of the total protein. These polypeptides did not separate from the rhodopsin of the intact outer segments during isopycnic or zone sedimentation in isotonic or hypertonic media, but could be extracted by hypotonic solutions, in which they remained soluble under conditions where all the rhodopsin-containing membranes were sedimented. The extracted proteins all sedimented at less than 30 S, and are regarded as soluble proteins contained within an osmotic compartment of intact outer segments. Three of the five polypeptides of these soluble proteins were found in only trace quantities among the soluble proteins prepared from the retina as a whole or from other particulate fractions; these proteins may be peculiar to the rod cell or its outer segment. The remaining two species were more widely distributed. © 1979.