A protein complex has a molecular mass of 400kDa when measured by size-exclusion chromatography. When subjected to electrophoresis in the presence of SDS only, the complex yields three bands with approximate molecular weights of 180kDa,160kDa, and 60kDa. When subjected to electrophoresis in the presence of SDS and a reductant, it yields three bands with approximate molecular weights of 160 kDa,90kDa, and 60kDa. What is the subunit composition of this protein complex? What type of oligomer is this? What can you say about the interactions that maintain the quaternary structure of the complex