The molecular evolutionary history of a winged bean alpha-chymotrypsin inhibitor and modeling of its mutations through structural analyses.
A serine protease inhibitor of the Kunitz-STI (soybean trypsin inhibitor) family, isolated from the legume seeds of winged bean, was found to inhibit chymotrypsin at a 1:2 stoichiometric ratio. When the structure was determined in our laboratory, it was found to form a characteristic beta-trefoil fold, which is also seen in other proteins from distant families and sources. The folding organization divides the protein into three approximately equal subdomains related by a pseudo-threefold axis of symmetry passing parallel to the barrel axis of the trefoil. Following the now established idea that the present-day genes originated from ancestral minigenes through evolution, the origin of the proteins having this beta-trefoil organization is scrutinized using its subdomain motif as the search probe. The results, based mainly on structural analyses, indicate the independent existence of such a motif, mimicking the unknown ancestral protein(s) that might have been distributed in nature, not only by gene duplication, but also by insertion and permutation in other folds. The understanding led to a hypothesis for the possible origin of the Kunitz-STI family. On the basis of this model of evolution, structurally hypervariable regions were located on the protein where mutations could be designed and a broad range of engineering of the protein's activity could be conceived.
Mukhopadhyay
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Crystallography and Molecular Biology Division, Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Calcutta 700 064, India. deb@mb2.saha.ernet.in
D000595:Amino Acid Sequence; D002918:Chymotrypsin / Q000037:antagonists & inhibitors*; D019143:Evolution, Molecular; D007887:Fabaceae / Q000378:metabolism*; D008957:Models, Genetic; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010802:Phylogeny; D010946:Plants, Medicinal; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D014361:Trypsin Inhibitors / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism
D014361:Trypsin Inhibitors; D002918:Chymotrypsin