In a large bowl, combine yeast, 1/8 cup sugar & lukewarm water; allow to dissolve. Stir in remaining 1/8 cup of sugar, salt, milk, sour cream, lemon juice & vanilla; mix well. Add egg & blend.
With fingertips, rapidly work the butter into 2 1/2 cups of the flour until coarse, meal-like consistency. Add to the yeast mixture & knead in bowl, adding more flour if necessary to make a smooth, elastic dough. Shape into a ball & place in a lightly buttered bowl. Cover tightly and let rise in a draft-free place until doubled in bulk.
Apple Filling
Peel & slice apples. In a small saucepan, combine all filling ingredients except pecans. Cook over medium heat, stirring constantly until apples are tender, & juice has evaporated. Stir in pecans; set aside to cool.
Streusel Topping
In a small bowl, combine flour, sugar, salt, cinnamon & lemon zest. With fingertips, rub in butter until mixture is coarse & crumbly. Set aside.
When dough has doubled in size, turn out on a lightly floured piece of wax paper. Press out gently into a rectangle about 10 x 14-inches in size. Spread apple filling to within 1/4-inch of edges & very gently press into dough. Roll up from the wide end, jelly-roll fashion.
Preheat oven to 375 F. Butter a 9-inch tube or bundt pan. Sprinkle half of the streusel in pan. Carefully, (dough will be very soft) with the help of the wax paper, fit the roll into the pan so that the ends of the dough join. Pinch ends of together. Sprinkle cake with remaining streusel. Bake for 30 minutes. Remove from oven & allow cake to cool before slicing.