Detailed location of amino acid residues involved in intersubunit interactions. The amino acid residues can be grouped according to their protein domain and mediate interactions (indicated as dashed green lines) between the β-sandwich and core domain, between core domain and core domain, and between the activation peptide and core domain. The protein backbone of the first A-subunit is shown as gray ribbon, and the backbone of the second A-subunit is shown as orange ribbon. The underlined amino acid residues were mutated to disrupt the interactions. The 5 amino acid pairs assumed to form particularly strong interactions are written in bold italic font.