Another type of molecular interaction that has been exploited to generate detectable activities in recombinant gene expression includes those involving specific, high-affinity ligand binding. In this manner, the recombinant product possesses the ability to interact with a specific ligand, which ideally is not a property of the host cell proteins. Using a labeled ligand, the corresponding recombinant product may be clearly identified. Alternatively, another reagent, either a protein or a chemical (which is itself labeled), may be used to detect the bound ligand, and thus the recombinant protein. In many instances, the ligand may be immobilized onto a solid support, such as chromatography resins and gels, to develop affinity fractionation methods for isolation and purification of the desired recombinant product. Examples of these protocols may be found in a number of chapters in this book, dealing with specific ligand-binding entities such as: maltose-binding protein, which allows purification of the chimeric protein on amylose columns; Protein A, which recognizes the Fc domain of immunoglobulin G; streptavidm, which binds with extremely high affinity and specificity to biotin, and hexahistidme peptide se quence, which has high metal affinity, i.e., applicable for affinity purification on nickel-nitrilotriacetate column. These topics are covered m detail in Chapters 9, 10, 11,and 12
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