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Hybrid Rop-pIII proteins for the display of constrained peptides on filamentous phage capsids.

In order to increase the versatility of phage display technology, it is desirable to be able to impose some structural constraints on the peptides that are presented by the phage particles. This is currently not feasible since the conformation of the capsid proteins, used to link the foreign peptide to the phage, are either unknown (pIII) or too simple (pVIII) to permit the engineering of peptide inserts into a constrained context. To reach this scope we have modified the amino-terminus of gene III by appending a well-characterized protein motif, the four-helix bundle of the bacterial protein Rop. Phage particles displaying Rop can be separated from wild-type (wt) particles by affinity purification with an antibody. Rop can be extensively modified by substituting its solvent-exposed residues and/or by inserting peptides either into the carboxy-terminal tail or into the bend region that connects the two alpha-helices of the monomer. These results open the possibility to construct peptide libraries where the peptides are constrained either into an omega-loop type conformation or an alpha-helix. Libraries formed by peptides inserted into the carboxy-terminus can also be constructed. Furthermore, the system that we have developed permits to produce large quantities of the elements of the libraries in the cytoplasm or to display them on the capsid of filamentous phages.