the "e. coli" scs heterotetramer has been crystallized and characterized in great detail. as can be seen in image 2, the two α subunits (pink and green) reside on opposite sides of the structure and the two β subunits (yellow and blue) interact in the middle region of the protein. the two α subunits only interact with a single β unit, whereas the β units interact with a single α unit (to form the αβ dimer) and the β subunit of the other αβ dimer. a short amino acid chain links the two β subunits which gives rise to the tetrameric structure.