| 摘要 |
Circularly permuted proteins are described wherein the natural termini of th e polypeptide are joined and the resulting circular protein is opened at anoth er point to create new C- and N-termini. The resulting protein exhibits some altered characteristic such as reduced substrate binding, for example. Fusio n proteins can be made from the circularly permuted protein by attaching the second polypeptide to these newly created termini. These fusion proteins wil l have altered properties from a fusion protein made by attaching the second polypeptide to the natural termini. For example, the second peptide or prote in can be attached at a position where it is more accessible to its substrate o r intended target. In the preferred embodiment, the base polypeptide is streptavidin. Circular permutation of streptavidin results in a circularly permuted biotin binding protein. In one embodiment, a flexible polypeptide loop important for the binding of biotin was opened by creation of the circularly permuted protein. The original termini (residues 13 and 139 of Sequence ID NO:1) were joined by a linker. The biotin association constant w as reduced approximately six orders of magnitude below that of wild type streptavidin to 107 M-1. Fusion proteins of the circularly permuted streptavidin can be made with secondary peptides/proteins such as IgG bindin g protein A or single-chain antibodies.
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