Overproduction of rat 1,25-dihydroxyvitamin D3 receptor in insect cells using the baculovirus expression system

The rat 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptor has been expressed at elevated levels in Spodoptera frugiperda cells using the baculovirus expression vector system. The recombinant 1,25-(OH)2D3 receptor is full-length, binds 1,25-(OH)2D3, and is recognized by a monoclonal antibody specific for 1,25-(OH)2D3 receptor. Densitometric scanning of Coomassie brilliant blue-stained SDS/polyacrylamide gels indicated a recombinant receptor protein level comprising 5% of the total soluble protein from the insect cells. The hydroxylapatite binding assay revealed average levels of 2 nmol of unoccupied 1,25-(OH)2D3 receptor per mg of protein in insect cells at 72 hr after infection with recombinant baculovirus. A measure of total 1,25-(OH)2D3 receptor using a ligand-independent, immunoradiometric assay disclosed average levels of 2.3 nmol of receptor per mg of protein produced by these same cells. A monoclonal antibody directed against the 1,25-(OH)2D3 receptor, and reported to cross-react with this receptor derived from several species, recognized the recombinant rat 1,25-(OH)2D3 receptor upon Western analysis. A monoclonal antibody directed specifically against the porcine receptor failed to recognize the recombinant rat 1,25-(OH)2D3 receptor protein. The cytosolic preparation of insect cells infected with recombinant baculovirus exhibited an equilibrium dissociation constant of 1 x 10(-11) M as determined by a 1,25-(OH)2D3 saturation analysis plotted by the method of Scatchard. This expression system provides an adequate source from which abundant quantities of 1,25-(OH)2D3 receptor can be purified for subsequent x-ray crystallographic analyses.