Surface glycoconjugates on rat photoreceptor cilium. Effect of neuraminidase digestion

In retinal photoreceptors the connecting cilium constitutes a boundary between the inner and outer segments. In previous studies we demonstrated that, while opsin could be localized in abundance in the distal ciliary membrane, very little opsin was detected in the proximal ciliary plasma membrane. In the present study we extended our view of molecular specialization on the ciliary membrane with respect to glycoconjugates. Saccharide moieties of ciliary glycoconjugates were studied in immature and mature rat photoreceptors. Surface saccharides were detected and localized by means of ferritin-labeled lectins and electron microscopy. Dense labeling of the ciliary membrane surface with wheat germ agglutinin (WGA) was observed. In immature photoreceptors the labeling was restricted to the proximal ciliary membrane, in a region where opsin molecules could not be detected. Neuraminidase digestion abolished WGA binding to the proximal ciliary membrane surface, indicating that sialic acids mediate WGA binding to this domain. Peanut agglutinin (PNA) did not label the ciliary surface, nor did it bind to the surface of other photoreceptor domains. Neuraminidase digestion exposed numerous PNA binding sites on the ciliary membrane surface. In view of the carbohydrate specificity of PNA, we suggest that a terminal trisaccharide sequence, sialic acid-galactose-(beta 1----3)-N-acetyl galactosamine, is present in high density on the proximal ciliary membrane surface.