PⅢ is the minor capsid protein of filamentous phages, located at the terminus of the virus particles. It is necessary for E. coli phage infection. Each of the virus particles has three to five copies of PⅢ proteins, whose structure can be classified into the three functional domains of N1, N2, and CT3. These domains are linked by G1 and G2, two sections of linker peptides rich in glycine. N1 acts on protein TolA on the membrane of E. coli, related to phage invasion. N2 serves as the receptor binding region, responsible for binding to the F pilus. CT adheres to the bacterial intima before assembly, associated with the suspension of phage assembly.