1. Preheat oven to 350°.
2. Weigh or lightly spoon potato starch and flours into dry measuring cups; level with a knife. Combine potato starch, flours, sugar, and next 4 ingredients (through salt) in a large bowl, stirring with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal.
3. Combine buttermilk, orange rind, and egg in a medium bowl, stirring with a whisk. Add egg mixture to flour mixture; stir just until moist. Gently fold in currants.
4. Pour batter into a 9 x 5-inch loaf pan coated with cooking spray. Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.