Governing the monomer-dimer ratio of human cystatin c by single amino acid substitution in the hinge region

Three dimensional domain swapping is one of the mechanisms involved in formation of insolu- ble aggregates of some amyloidogenic proteins. It has been proposed that proteins able to swap domains may share some common structural elements like conformationally constrained flexible turns/loops. We studied the role of loop L1 in the dimerization of human cystatin c using muta- tional analysis. Introduction of turn-favoring residues such as Asp or Asn into the loop sequence (in position 57) leads to a significant reduction of the dimer fraction in comparison with the wild type protein. On the other hand, introduction of a proline residue in position 57 leads to efficient dimer formation. Our results confirm the important role of the loop L1 in the dimerization proc - ess of human cystatin C and show that this process can be to some extent governed by single amino acid substitution.