A protein has a molecular mass of 600 kDa when measured by size-exclusion chromatography. When subjected to gel electrophoresis in the presence of SDS, the protein gives two bands with molecular masses of 200 and 100 kDa. When electrophoresis is carried out in the presence of SDS and dithiothreitol, three bands are formed, this time with molecular masses of 120, 100, and 80 kDa. Determine the likely subunit composition of the protein.