ALTERNATE PROTEIN FRAMEWORKS FOR MOLECULAR RECOGNITION

In an effort to determine whether proteins with structures other than the immunoglobulin fold can be used to mimic the ligand binding proper ties of antibodies, we generated a library from the four-helix bundle protein cytochrome b(562) in which the two loops were randomized. Fann ing of this library against the bovine serum albumin (BSA) conjugate o f N-methyl-p-nitrobenzylamine derivative 1 by phage display methods yi elded cytochromes in which residues Trp-20, Arg-21, and Ser-22 in loop A and Arg-83 and Trp-84 in loop B were conserved. The individual muta nts, which fold into native-like structure, bind selectively to the BS A-1 conjugate with micromolar dissociation constants (K-d), in compari son to a monoclonal antibody that binds selectively to 1 with a K-d Of 290 nM. These and other antibody-like receptors may prove useful as t herapeutic agents or as reagents for both intra- and extracellular stu dies.