A protein has a molecular mass of 400 kDa when measured by size exclusion. When subjected to gel electrophoresis in the presence of SDS (but no BME), the protein gives three bands of 180, 160, and 60 kDa. When electrophoresis is carried out in the presence of SDS and BME (or DTT), three bands are again observed, this time with masses of 160, 90, and 60 kDa. Determine the subunit composition of the protein.