Enzymatic characterization of peanut agglutinin-binding components in the retinal interphotoreceptor matrix

Previous histochemical and biochemical studies have documented the presence of carbohydrate-containing molecules in the retinal interphotoreceptor matrix (IPM). The lectin peanut agglutinin (PNA), which preferentially binds galactose-containing carbohydrates, especially galactose-galactosamine linkages, selectively labels cone photoreceptor-associated domains of the IPM ('cone matrix sheaths') in a variety of vertebrate retinas. In the studies described here, the nature of these PNA-binding components was investigated by monitoring the effects of proteolytic and glycosidic enzymes on binding of the lectin in the retina and IPM. All proteolytic enzymes tested cause a marked reduction in PNA-binding to cone matrix sheaths, suggesting that proteinaceous components are important to their organization. Exposure to O-glycanase, but not N-glycanase, markedly reduces binding of PNA to cone matrix sheaths indicating that O-linked oligosaccharides are probably responsible for its binding. Galactose oxidase treatment reduces PNA-binding throughout the retina and IPM, confirming that galactose moieties are involved in its binding. beta-Galactosidase (either before or after neuraminidase treatment) does not alter the pattern of PNA binding, suggesting that neither terminal nor penultimate beta-linked galactose residues constitute a major proportion of the lectin's binding sites in the retina. Neuraminidase treatment markedly increases the density and distribution of PNA binding throughout the retina and IPM, however, this effect appears to be, at least in part, the result of the binding of the lectin to neuraminidase molecules that become associated with tissue sections in addition to binding to carbohydrate groups unmasked by desialation. Exposure to chondroitinases causes disruption of the morphological integrity of cone matrix sheaths and slight diminution of PNA binding. Other enzymes acting on common constituents of extracellular matrices do not have similar effects. Taken together, these observations suggest that PNA-binding to cone matrix sheaths is due to the presence of glycoconjugates with galactose-containing, O-linked oligosaccharide chains.