CHARACTERIZATION AND PURIFICATION OF HUMAN RETINOIC ACID RECEPTOR-GAMMA-1 OVEREXPRESSED IN THE BACULOVIRUS INSECT CELL SYSTEM

The full-length cDNA for the human retinoic acid receptor-gamma1 (RAR-gamma1) has been expressed to high levels in Spodoptera frugiferda (Sf9) cells using the baculovirus expression system. Western blot analysis revealed that RAR-gamma1 expression increased between 32 and 60 h post-infection. The recombinant receptor was expressed primarily as a nuclear protein and displayed a molecular mass of 50 kDa as determined by SDS/PAGE and gel-filtration chromatography, consistent with its cDNA-deduced size. Based on ligand binding, 2 x 10(6) RAR-gamma1 molecules were expressed per Sf9 cell, a level approx. 2000 times greater than in mammalian cells. The receptor was partially purified 300-fold by sequential anion-exchange, gel-filtration and DNA affinity chromatographies. The overexpressed receptor specifically bound all-trans-retinoic acid (RA) and the synthetic retinoid CD367 with high affinity (K(d) 0.15 nM and 0.23 nM respectively). The RA metabolites 4-hydroxy-RA and 4-oxo-RA were poor competitors for [H-3]CD367 binding to recombinant RAR-gamma1 (K(i) > 1 muM), indicating that 4-oxidation of RA greatly reduces its affinity for RAR-gamma1. Gel-retardation analysis demonstrated that RAR-gamma1 specifically bound the RA response element of the mouse RAR-beta gene. RAR-gamma1 species expressed from recombinant baculovirus (in Sf9 cells) and vaccinia virus (in HeLa cells) exhibited similar affinities for RA and CD367 and had comparable DNA-binding properties in gel-retardation experiments. Moreover, a similar requirement for additional DNA-binding stimulatory factor(s) was observed in both cases. These results provide a basis-for the use of baculovirus-expressed RAR-gamma1 in further functional and structural studies.