Bovine retinal rod guanyl cyclase represents a new N‐glycosylated subtype of membrane‐bound guanyl cyclases

The molecular properties of retinal rod guanyl cyclase were investigated. Peptides were derived from a 112‐kDa protein previously identified as the particulate bovine retinal rod guanyl cyclase. The peptides showed 100% identity to the deduced amino acid sequence of the cloned human retina‐specific membrane guanyl cyclase, whereas identity to the members of the natriuretic peptide receptor guanyl cyclases was 14–59%. The 112‐kDa protein was further purified by a new approach using wheat‐germ agglutinin chromatography. This indicated N‐linked glycosylation in retinal rod guanyl cyclase. N‐glycosylation was unexpected from the sequence of the human retina‐specific membrane guanyl cyclase, although it is a common property of natriuretic peptide receptors. Therefore, we further analyzed the carbohydrate composition of bovine retinal rod guanyl cyclase by lectin binding using the lectins Galanthus nivalis agglutinin, Sambucus nigra agglutinin, Maackia amurensis agglutinin, Ricinus communis agglutinin, Datura stramonium agglutinin, peanut agglutinin and by chromatography of the purified enzyme using concanavalin‐A‐Sepharose. The oligosaccharide side chains were of the high‐mannose type or hybrid type, probably with mannose, N ‐acetylglucosamine and sialic acid as terminal sugars. Enzymic deglycosylation by N ‐glycosidase F was achieved after proteolytic digestion with endoproteinase Glu‐C. Lectins neither influenced the basal nor the stimulated guanyl‐cyclase activity at low calcium concentrations. Our results indicate that the particulate rod guanyl cyclase represents an unusual new subtype of membrane‐bound guanyl cyclases.