The structure of peptides displayed on the filamentous bacteriophage virus particles can be studied as epitopes and independent folding units of proteins. Peptides function as hormones or other types of effectors when they bind soluble or membrane bound receptors. They also act as epitopes when they bind to antibodies. There are basically two sources of peptide sequences. Peptides can be selected from libraries, which can be prepared by biological methods, in particular phage display , or by a variety of synthetic strategies. The purpose of the library is to generate all of the possible peptides of interest, and then to select the one or few of interest based on their binding affinity for a target molecule such as an antibody or receptor protein. The N-terminal region of the coat protein provides an environment conducive to stabilizing peptide conformation, as shown by the increased biological activity of amino acid sequences inserted into the coat protein versus the free peptide. Solid-state NMR spectra of several epitopes displayed on the phage gave direct evidence of the structural fold of the peptides. 15N chemical shift anisotropy (CSA) and 15N-1H dipolar coupling measurements of the amide backbone sites of these peptides indicate that the peptides are immobilized on the virus particle and provide structural details of their fold. Epitopes of a 12 residue peptide from the main antigenic determinant of the human malaria sporozite were found to fold into three turns. The 6 residue epitope of HIV-1 V3 loop was found to be mobile on the phage particle. Binding of an HIV-1 antibody to this epitope displayed on phage resulted in an immobile folded structure.
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