We present an asymmetric isotopic labeling strategy for probing unambiguously membrane protein binding interfaces in homo-oligomers. This technique is highly sensitive, rapid, and requires only one C-13 methyl-edited nuclear Overhauser effect spectroscopy (NOESY) spectrum. As a proof of principle, we apply this technique to the homo-pentameric membrane protein phospholamban, defining the leucine-isoleucine zipper interface between its monomers with atomistic resolution.