In a small bowl, dissolve yeast in warm water. Let stand 5 minutes. Stir in warm buttermilk; set aside. , In a large bowl, combine the flour, sugar, salt, baking powder and baking soda. Cut in shortening with a pastry blender until mixture resembles coarse crumbs. Stir in yeast mixture. , Turn onto a lightly floured surface; knead lightly 3-4 times. Roll out to 1/2-in. thickness; cut with a 2-1/2-in. biscuit cutter. Place 2 in. apart on lightly greased baking sheets. Cover with kitchen towels and let rise in a warm place until almost doubled, about 1 hour., Bake at 450degrees for 8-10 minutes or until golden brown. Lightly brush tops with melted butter. Serve warm.