Complex glycoproteins associated with the detergent-resistant membrane matrix of the rhabdomeral microvilli of crayfish photoreceptors

Abstract

The lectin-binding properties of crayfish photoreceptor membrane were examined using isolated rhabdoms and frozen sections of whole retinas after permeabilization. Fluorescently-labelled concanavalin A (Con A) and wheat germ agglutinin (WGA) bind specifically to the rhabdoms, the accessory pigment cells, and the basement membrane of the retina in permeabilized frozen sections. Con A also binds to the clear zone in sectioned material. Peanut agglutinin (PA) binds strongly to domains associated with the periphery of rhabdoms. Intact isolated rhabdoms also bind Con A and WGA with high affinity. The major lectin-binding proteins of the photoreceptor membrane were characterized on nitrocellulose blots after SDS electrophoresis of purified photoreceptor membrane. Two major glycoproteins were found to bind a number of lectins. A 130 000 molecular weight peptide (GPII) specifically bound Con A, WGA, Dolichos biflorus agglutinin, (DBA) Ulex europaeus agglutinin (UEA) and PA, whereas the major 142 000 d protein (GPI) of the photoreceptor membrane bound only Con A and WGA implying a rather simple hexose chain consisting mainly of mannose and N-acetylglucosamine. Photoreceptor membrane vesicles, extracted with a high concentration of non-ionic detergent and centrifuged, show a specific enrichment of both proteins as well as actin and a 40 000 d component. Significant amounts of both glycoproteins are removed from the membrane residue by 0·6 m KI, even under conditions known to stabilize actin-based cytoskeletons. Both the stoichiometric amounts of these peptides in relation to the actin complement and their physicochemical properties are consonant with the hypothesis that one or both may be involved in linking a submembranous, actin-based cytoskeleton to the lipid bilayer. Rhodopsin, the major component of photoreceptor membrane, appears to be a glycoprotein by periodic acid-Schiff (PAS) staining methods but fails to bind significant amounts of lectins on Western blots. These findings are discussed in the light of post-translational processing and intracellular pathways for photoreceptor membrane components in the compound eye.