A filamentous bacteriophage is a type of bacteriophage, defined by its filamentous or rod-like shape. Filamentous bacteriophages usually contain single-stranded DNA genomes and infect Gram-negative bacteria. The Ff, M13, fd, and f1 families are important phages that can be used for phage display, of which M13 phage is the most commonly used. The M13 phage is cylindrical, 880nm long, and 6nm in diameter. It encapsulates a single-stranded genome that encodes five different capsid proteins, including two groups, the major coat protein (pVIII) and the minor coat protein (pVII, pIX, pVI, and pIII). E. coli filamentous phages is commonly used for phage display. Most of the antibodies and peptides are displayed on the phage protein pIII and pVIII, and they construct the pIII and pVIII display systems. In addition, the hybrid phage system can display large proteins with all five M13 coat proteins as N-terminal fusions to pIII, pVIII, pVII, and pIX, also as C-terminal fusions to pVI, pIII, and pVIII.
Fig 1. Schematic representation of bacteriophage M13 and different monovalent display types based on pIII coat protein (Georgieva, et al. Y-2011)
M13 phage is E. coli-specific filamentous bacteriophage, which infects E. coli through contact with fimbriae. It is composed of a protein shell and its coated circular single-stranded DNA. The whole genome contains 11 genes, 5 of which encode capsid proteins, namely pIII, pVI, pVII, pVIII and pIX proteins. Our phage display systems are listed as follows:
pIII is a protein that determines the infectivity of virus particles. It consists of 406 amino acid residues and appears on the tip of the phage in 3 to 5 copies. One advantage of using pIII instead of pVIII is that when phagemid and helper phage are used in combination, pIII allows monovalent display
pVIII is the main coat protein of Ff bacteriophage, expressed by gene 8, and there are 2700 copies. Therefore, when the phage-displayed antibody binds to the antigen, it is used to enhance the detection signal.
The pVII protein and pIX protein are located at the same end of the M13 phage, while the pIII protein and pVI protein are co-located at the other end. The four are all minor capsid proteins, each with 5 copies.
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