A Novel β-Galactoside-Binding Lectin in Adult Rat Kidney

Abstract

Two carbohydrate-binding proteins with subunit molecular weight of about 17, 500 and 16, 500, respectively, were isolated from Triton X-100 extracts of rat kidney using a lactose affinity column. They did not require Ca²⁺ for the carbohydrate-binding nor reducing agents for maintaining their activity. The partial amino acid sequence of the 17.5-kDa protein (rkCBP-17.5), the main component, revealed that this protein is a novel member of a superfamily of β-galactoside-binding animal lectins. The N-terminal amino acid sequence of the 16.5 kDa component (rkCBP-16.5) indicated that it is a fragment derived from the IgE-binding protein (IgEBP). Monoclonal antibodies to rkCBP-17.5 were prepared and used to examine the distribution of the lectin in various organs of adult rats. Immunoreactive protein with the same molecular weight was found in lung, spleen and liver, in lesser amounts in heart, and in trace amounts in brain and skeletal muscle. rkCBP-17.5 exhibits binding activity to various saccharides with the following order of affinity: N-acetyllactosamine>lactose>D-galactose>methyl α-D-galactopyranoside>N-acetyl-D-galactosamine>methyl β-D-galactopyranoside. It binds to Engelbreth-Holm-Swarm(EHS) tumor laminin and rat plasma fibronectin, but does not bind to human plasma fibronectin.