Peptide and Antibody Fragment Display is achieved through conjugation to the minor coat protein pIII of filamentous phage M13, enabling presentation of desired molecules. Located at one end of the phage capsid, pIII comprises three functionally autonomous domains (D1, D2, and D3) connected by glycine-rich linkers. During Gram-negative bacterial infection, the N-terminal D1 domain mediates viral DNA translocation into the host cytoplasm. The D2 domain binds the bacterial F-pilus and executes essential functions in infection. The C-terminal D3 domain is indispensable for stable capsid assembly and serves as a prerequisite for phage production. In most phage display vectors, deletion of D1 and D2 domains in pIII yields phage particles with reduced infectivity.

A randomly synthesized 12-mer peptide-encoding oligonucleotide fragment is chemically synthesized and inserted into the gene encoding the phage coat protein. The engineered gene is then transformed into E. coli using recombinant DNA technology. A random 12-mer peptide is fused to the M13 bacteriophage minor coat protein (pIII), creating a combinatorial library. The amplified phage particles display the foreign peptides on their surface, with each phage expressing one unique foreign peptide. This collection of phage constitutes a pre-made random peptide library. The phage vector contains both phage and bacterial origins of replication, a phage packaging signal, a selectable marker gene, and the gene for the chosen coat protein fused to the insert. This design facilitates easy vector preparation and maintenance, high yields of double-stranded DNA (dsDNA), and improved transformation efficiency. 

To assemble functional phage particles, co-infection with a helper phage (e.g., M13K07) is required. The physical link between the phage-encoded genotype and the displayed phenotype is maintained because the helper phage has an origin of replication and a packaging signal, but its gene III protein function is compromised. Consequently, the phage vector is preferentially packaged into new viral particles over the helper phage genome. For multivalent display purposes, alternative helper phages, such as hyperphage, are used.

Using client-provided target molecules, high-affinity binding phage clones are isolated from pre-fabricated randomized 12-mer peptide libraries through screening, amplification, and panning. Determination of phage DNA sequences reveals the amino acid sequences of displayed peptides.

The overarching principle of Phage Peptide Library Screening exploits the varying affinities existing between certain biomolecules. Using a biomolecule as the target, specific binding peptides with desired affinity can be isolated from the peptide library through single or multiple rounds of screening. The Phage Display Screening method typically employs a biological selection technique termed biopanning. This process utilizes biological manipulation to isolate the desired phage-displayed peptides from a random phage peptide library. The specific screening methodology may differ based on the nature of the target, the scale of the screening, and operator preferences.

Classical screening methods primarily consist of solid-phase screening and solution-phase screening. 

In Solid-Phase Screening, the antigen is coated onto a solid support (e.g., microtiter plates, nitrocellulose membranes, or immunotubes). The antigen is then incubated with the peptide library, allowing specific phage-displayed peptides to bind. Its advantage is the capacity to adsorb more target protein per unit area of the solid support. However, a limitation of solid-phase screening is that the target molecule—particularly conformationally labile targets—may undergo conformational changes during the immobilization process. 

Solution-phase screening was developed to address this limitation. In this method, the target molecule is typically biotinylated, mixed with the peptide library in solution to allow interaction, and the mixture is then added to immobilized avidin. High-affinity phage-displayed peptides are enriched through the binding of biotin to avidin.

Two drugs developed by screening phage-displayed random peptide libraries are the pegylated erythropoietin receptor agonist peginesatide (Hematide, Affymax) and the Fc-fused thrombopoietin receptor agonist romiplostim (Nplate, Amgen). Additionally, two other drugs—the anti-TNF-α antibody adalimumab and the small protein inhibitor of plasma protease kallikrein, ecallantide—have been developed using phage display technology, establishing this technique alongside other established methods in the drug discovery field.