Directed evolution of novel binding proteins
In order to obtain a novel binding protein against a chosen target, DNA molecules, each encoding a protein comprising one of a family of similar potential binding domains and a structural signal calling for the display of the protein on the outer surface of a chosen bacterial cell, bacterial spore or phage (genetic package) are introduced into a genetic package. The protein is expressed and the potential binding domain is displayed on the outer surface of the package. The cells or viruses bearing the binding domains which recognize the target molecule are isolated and amplified. The successful binding domains are then characterized. One or more of these successful binding domains is used as a model for the design of a new family of potential binding domains, and the process is repeated until a novel binding domain having a desired affinity for the target molecule is obtained. In one embodiment, the first family of potential binding domains is related to bovine pancreatic trypsin inhibitor, the genetic package is M13 phage, and the protein includes the outer surface transport signal of the M13 gene III protein.
1. A method of obtaining a nucleic acid encoding a proteinaceous binding domain that binds a target material comprising:
a) providing a variegated population of eukaryotic cells,
each cell including a nucleic acid that encodes a potential binding protein, wherein the nucleic acid comprises DNA encoding (i) a potential binding domain having one or more variable residue positions and one or more non-variable residue positions, and (ii) an outer surface transport signal for obtaining the display of the potential binding domain on the outer surface of the cell, wherein said variegated population of cells collectively display a plurality of different potential binding domains, the differentiation among said potential binding domains, occurring as a result of amino acid variation at said one or more variable residue positions,
(b) allowing the expression of said potential proteins and the display of said potential binding domains on the outer surface of said cells;
(c) contacting said cells with the target material such that said potential binding domains and the target material may interact;
(d) separating cells displaying a potential binding domain that binds the target material from cells that do not so bind, wherein a potential binding domain that binds the target material is a successful binding domain (SBD) and
(e) recovering at least one cell displaying on its outer surface a binding protein comprising an SBD, said cell enclosing SBD-encoding nucleic acid, and amplifying said SBD-encoding nucleic acid in vivo or in vitro.
2. The method of claim 1 where one or more of said potential binding domains comprise an amino acid sequence which is at least 88% identical with the variable region of an immunoglobulin heavy chain.
3. The method of claim 1 where one or more of said potential binding domains comprise an amino acid sequence which is at least 88% identical with the variable region of an immunoglobulin light chain.
4. The method of claim 1 where said potential binding protein comprises a single chain antibody.
5. The method of claim 1 where said potential binding comprises an Fab fragment of a naturally occurring antibody.
6. The method of claim 2 in which one or more variable residues correspond to residues in a hypervariable region of an immunoglobulin heavy chain.
7. The method of claim 3 in which one or more 30 variable residues correspond to residues in a hypervariable region of an immunoglobulin light chain.
8. The method of claim 2 in which all of the variable residues correspond to residues in a hypervariable region of an immunoglobulin heavy chain.
9. The method of claim 3 in which all, of the variable residues correspond to residues in a hypervariable region an immunoglobulin light chain.
10. The method of claim 1 wherein, in said step (a), the differentiation among said potential binding domains is limited to no more than 20 amino acid residue positions of said domains.
11. The method of claim 1 wherein said potential binding domains are all at least 30% identical in amino acid sequence to each other.
12. The method of claim 1 in which said population of eukaryotic cells is obtained by subcloning a mixture of DNA encoding a plurality of different potential binding proteins, comprising different potential binding domains.
13. The method of claim 1 in which the outer surface transport signal is an outer surface protein or a truncated outer surface protein of said cell functional to direct said display.