An ordered membrane-cytoskeleton network in squid photoreceptor microvilli

Abstract

To study the organization of microvilli in the photoreceptor cells of an invertebrate. X-ray diffraction patterns were obtained from aldehyde-fixed squid retinas to a resolution of (40 Å) −1 and correlated with results from electron microscopy and sodium dodecyl sulphate/polyacrylamide gel electrophoresis. Squid photoreceptor microvilli are packed in extensive hexagonal arrays; in addition each microvillus has a hexagonal substructure. Image reconstruction from thin section electron micrographs shows that the microvilli are linked together with specialized membrane junctions at their neighbour contacts, and phosphotungstic acid-stained sections show a central cytoskeleton connected to the membrane by side-arms. The X-ray patterns also reveal two axial periodicities in the microvilli. A weak and diffuse (50 Å) −1 band is tentatively assigned to rhodopsin molecules ordered in the plane of the membrane. In addition, an arc at (85 Å) −1 is attributed to a cytoplasmic or extracellular structure. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis of the isolated microvilli shows that the major component, rhodopsin, comprises about 50% of the total protein. There are two major detergent-insoluble polypeptides with molecular weights of 145,000 and 42,000. The 42,000 component is identified as actin by papain digestion fragment mapping. Cephalopod photoreceptors are highly sensitive to the polarization vector of linearly polarized light. In consequence, the linear rhodopsin chromophores must be aligned relative to the microvillar axes. The membrane junctions and cytoskeleton described here may provide a mechanism for maintaining this rhodopsin alignment.