In a large bowl, sprinkle the yeast over the water and let stand until foamy. Stir in the melted butter, egg and salt. Add the all-purpose and whole wheat flours and the walnuts, stirring until the dough becomes stiff. Scrape the dough onto a lightly floured surface and knead until a smooth dough forms. Transfer the dough to a lightly oiled large bowl, cover with plastic wrap and let stand in a warm place until doubled in bulk, about 1 1/2 hours.