The cadherin-related receptor of Manduca sexta, BT-R-1, for the Cry1A family of Bacillus thuringiensis insecticidal toxins, was expressed in cultured Spodoptera frugiperda (Sf21) insect cells utilizing the expression vector delta Op-gp64. Recombinant BT-R-1 was released by the Sf21 cells in soluble form into the culture medium and represents approximately 58% of total BT-R-1 produced by the cells. The soluble protein was purified by affinity chromatography using Cry1Ab toxin coupled to Sepharose 4B. The apparent molecular mass of purified soluble recombinant BT-R-1 is 195 kDa, Radiolabeled toxin bound to purified soluble BT-R-1 with a Kd value of 1.1 nM, which is similar to that of both membrane-bound BT-R-1 in Sf21 cells and natural BT-R-1 from M. sexta larval midgut tissue. Binding of radiolabeled toxin to soluble BT-R-1 was competitively inhibited by unlabeled Cry1Ab toxin but not by other Cry toxins as was observed also for membrane-bound BT-R-1. The recombinant soluble protein was stable in culture medium for at least 3 days at 27 degreesC and for 7 days at 4 degreesC and exhibited toxin-binding properties similar to the natural protein. Apparently, neither membrane association nor the extent of glycosylation influences the binding affinity and specificity of BT-R-1. Approximately 1 mg of purified BT-R-1 was obtained per liter of insect cell culture supernatant, representing -2 x 10(9) Sf21 cell.