Surfaces of rod photoreceptor disk membranes: integral membrane components.

Abstract

The membrane surfaces within the rod outer segment of the toad, Burn marinus, were exposed by rapid-freezing followed by freeze-fracture and deep-etching. Platinum-carbon replicas of disk membranes prepared in this way demonstrate a distinct sidedness. The membrane surface that faces the lumen of the disk shows a fine granularity; particles of -6 nm are packed at a density of -30,000/gm 2. These dimensions suggest that the particles represent protrusions of the integral membrane protein, rhodopsin, into the intradisk space. In addition, when rhodopsin packing is intentionally perturbed by exhaustive digestion with phospholipase C, a concomitant change is observed in the appearance of the luminal surface granularity. The cytoplasmic surface of the disk rarely displays this rough texture; instead it exhibits a collection of much larger particles (8-12 nm) present at ~10% of the concentration of rhodopsin. This is about the size and concentration expected for certain light-regulated enzymes, cGMP phosphodiesterase and GTP-binding protein, which are currently thought to localize on or near the cytoplasmic surface of the disk. The molecular identity of the 8-12-nm particles will be identified in the following companion paper. A further differentiation of the cytoplasmic surface can be seen around the very edge, or rim, of each disk. This rim has relatively few 8-12-nm particles and instead displays short filamentlike structures connecting it to other membranes. These filaments extend between adjacent disks, across disk incisures, and from disk rims to the nearby plasma membrane.

Mg2+‐ATP induces filament growth from retinal rod outer segments with disrupted plasma membranes

“…The Mg'+-ATP-induced signal reported here has a half-saturation at about 4OOpM Mg'+-ATP compared to 1OrM for the AD signal of Uhl et al [6]. We attribute this difference to the much higher membrane concentration of our preparations (100 ,uM vs 1 ,uM rhodopsin used by Uhl et al), a contention supported by other work where Uhl [15] [17] show the presence of filament-like connections between adjacent disks and between the disks and the plasma membrane.…”

In vivo optophysiology reveals that G-protein activation triggers osmotic swelling and increased light scattering of rod photoreceptors

“…Freeze-fracture and cryoelectron tomographic investigations have revealed macromolecular structures that appear to link adjacent disc membranes (48,49). Because most proteins can act as elastic elements (50), it is reasonable to hypothesize that such disc-to-disc "linkers" serve as springs that resist swelling, affording some protection of the rod OS from stimulus-induced osmotic overload.…”

Fundamental Studies in the Molecular Basis of Laser Induced Retinal Damage

“…Fluoride ions (13)(14)(15)(16)(17)(18) vanadate (19) and molybdate (20,21) with brief centrifugation times to avoid increasing the dark PDE activity (2). Isolation procedures were carried out at 4 0 C or with reagents on ice using infrared illumination (.>750 nm) and an image convertor (NI-Tec, Inc., Niles, IL).…”

“…However, workers in two laboratories have recently demonstrated the presence of multiple filaments interconnecting the discs to one another and to the plasma membrane (16,17).…”

“…Although the composition of these filaments is not known, it is possible that they may play a significant role in visual transduction (16,17,23). If the filaments are composed of F-actin, we would expect labeling of these structures by rhodamine-phallicidin.…”