Characterization of phage that bind plastic from phage-displayed random peptide libraries.

During routine screenings of random peptide libraries displayed at the N terminus of the pIII coat protein of M13 bacteriophage, clones were isolated that bound directly to the polystyrene (PS) surface used to immobilize the target protein. The plastic-binding phage (P-b phi) bind to both unblocked plastic (PS and polyvinyl chloride, PVC) and plastic blocked with bovine serum albumin (BSA) but require non-ionic detergent to bind to plastic blocked with milk. Comparison of the P-b phi to antibody-binding phage (Ab-b phi) indicates that similar numbers of phage particles are bound, but fewer P-b phi the recovered by acid elution. Sequence determination of the displayed peptides reveals they lack amino-acid sequence similarity yet are highly enriched for the Tyr and Trp residues. However, because not all phage that display peptides rich in Tyr and Trp residues bind to plastic, and other methods of screening random peptide libraries have identified different classes of plastic-binding peptides, the relative abundance of Tyr and Trp residues should not be considered diagnostic of plastic-binding. In summary, these results help characterize one of the most common methods used to screen random peptide libraries and suggest strategies to avoid isolating P-b phi. Furthermore, while it is generally believed that proteins bind to plastic by non-specific interactions, these results show that a bias in aa composition can exist.

An M13 phage library displaying random 38-amino-acid peptides as a source of novel sequences with affinity to selected targets.

We have examined the potential of isolating novel ligands from a library of M13 pIII-fusion phage displaying peptides composed of 38 random amino acids (aa). The library was panned with streptavidin (SA) and a polyclonal goat antimouse IgG Fc antibody (Ab) preparation coupled to paramagnetic beads. SA selected two classes of phage from the library. One class exhibited the aa motif, HP(Q/M) theta (where theta signifies a non-polar aa), similar to the motif identified by Devlin et al. [Science 249 (1990) 404-406] using a 15-aa random peptide library displayed on phage. The other class of phage had no discernible motif. In binding experiments, the non-HP(Q/M) theta phage had a slightly higher affinity for SA than did the motif phage. Both classes of SA-binding phage failed to bind native and non-glycosylated forms of avidin, even though SA and avidin are structurally similar and both proteins possess extraordinary affinities for biotin. The polyclonal goat anti-mouse IgG Fc Ab preparation selected phage displaying sequences similar to a region of the mouse IgG Fc. Thus, a single immunodominant epitope on the mouse IgG Fc was identified. Furthermore, a second phage displaying peptides with no discernible sequence similarities to mouse IgG Fc was isolated. Thus, an M13 library displaying 38-aa peptides can yield phage with affinity for various targets. Finally, we have observed a biological bias against odd numbers of Cys residues in the displayed peptides.