Secretion of the baby hamster kidney 30-kDa galactose-binding lectin from polarized and nonpolarized cells: a pathway independent of the endoplasmic reticulum-Golgi complex.

A carbohydrate-binding protein of molecular weight 30 kDa (CBP30) isolated from baby hamster kidney (BHK) cells binds polylactosamine glycans present prominently on extracellular matrix glycoproteins of oncofetal origin, such as Engelbroth-Holm-Swarm (EHS) tumor laminin and amniotic fluid fibronectin, and inhibits attachment and spreading of BHK cells to EHS laminin substrata mediated by integrin(s) suggesting an extracellular function for the lectin (S. Sato and R. C. Hughes (1992) J. Biol. Chem. 267, 6983-6990). Here we show that CBP30 shares amino acid sequence homologies with other lectins of similar size, e.g., murine CBP35, Mac2 antigens, and rat IgE-binding protein. Unlike most secreted proteins these lectins contain no signal sequence and we report that drugs, such as brefeldin A and monensin, which inhibit the intracellular transport of classical secretory (glyco)proteins do not block secretion of CBP30 from BHK cells. Secretion is inhibited by methylamine and serum starvation and is increased by heat shock and calcium ionophore A23187, treatments known to block or stimulate exocytosis, respectively. Immunofluorescence and biochemical analysis shows that CBP30 is distributed throughout the cytoplasm of subconfluent BHK cells where it turns over with a half-life of about 30 h, and small amounts are also deposited on the cell surface and substratum. At confluency, the CBP30 assembles into patches that eventually appear to underlie the plasma membrane and extracellular deposits become more numerous. In filter-grown confluent monolayers of Madin-Darby canine kidney cells the lectin is secreted from and expressed at the apical domain of the polarized cells whereas laminin is secreted from the basal domain and becomes incorporated into the matrix between cells and substratum.