Protein Engineering vol. 16 no. 11 pp. 809-817, 2003
© 2003 Oxford University Press
Change in the crystal packing of soybean ß-amylase mutants substituted at a few surface amino acid residues
Laboratory of Food Quality Design and Development, Graduate School of Agriculture, Kyoto University, Uji, Kyoto 611-0011, Japan
1 To whom correspondence should be addressed. e-mail: mikami{at}kais.kyoto-u.ac.jp
In spite of the high similarity of amino acid sequence and three-dimensional structure between soybean ß-amylase (SBA) and sweet potato ß-amylase (SPB), their quaternary structure is quite different, being a monomer in SBA and a tetramer in SPB. Because most of the differences in amino acid sequences are found in the surface region, we tested the tetramerization of SBA by examining mutations of residues located at the surface. We designed the SBA tetramer using the SPB tetramer structure as a model and calculating the change of accessible surface area (
ASA) for each residue in order to select sites for the mutation. Two different mutant genes encoding SB3 (D374Y/L481R/P487D) and SB4 (K462S added to SB3), were constructed for expression in Escherichia coli and the recombinant proteins were purified. They existed as a monomer in solution, but gave completely different crystals from the native SBA. The asymmetric unit of the mutants contains four molecules, while that of native SBA contains one. The interactions of the created interfaces revealed that there were more intermolecular interactions in the SB3 than in the SB4 tetramer. The substituted charged residues on the surface are involved in interactions with adjacent molecules in a different way, forming a new crystal packing pattern.
Received June 9, 2003; revised September 11, 2003; accepted September 16, 2003.