piiI

pIII is the protein product of gene III in filamentous bacteriophages such as M13, f1, and fd. It is a minor coat protein localized at the distal tip of the virion and is essential for initiating infection of F+ Escherichia coli cells. The pIII molecule binds to the F pilus, the bacterial adhesion organelle, thereby anchoring the phage to the cell surface. Following attachment, conformational changes and interactions with other phage proteins facilitate entry of the phage genome into the bacterial cytoplasm without lysis of the cell.

In addition to its native role in infection, pIII is a cornerstone of phage display technology. The N-terminus of pIII is commonly fused to foreign peptides or proteins, producing virions that present the attached cargo on their surface. This enables high-throughput library screening (panning) to identify ligands for target molecules. Because large fusions can reduce infectivity, display levels and library size are often balanced against phage viability, and many experiments utilize low-valency display on pIII or alternative coat proteins.

Structurally, pIII is a relatively large, modular protein comprising domains for receptor binding and for assembly at the phage tip. In typical M13-related phages there are only a few copies of pIII per virion, distinguishing it from the abundant major coat protein pVIII.

pIII thus serves both a biological role in phage infection and a practical role in biotechnology, particularly in ligand discovery and affinity maturation workflows.