The g3p protein (also known as attachment protein or coat protein A) of filamentous phage such as M13, phage fd and phage f1, is an essential coat protein for the infection of Escherichia coli. The g3p protein consists of three domains: two N-terminal domains (N1 and N2) with a similar beta-barrel fold, and a C-terminal domain . The N-terminal domains protrude from the phage surface, while the C-terminal domain acts as an anchor embedded in the phage coat, together forming a horseshoe-like structure . The g3p protein exists as 3-5 copies at the tip of the phage particle.
Infection by filamentous phage occurs in two steps, both of which are mediated by the g3p protein: phage attachment to the F-pilus of the host cell as the primary receptor, followed by attachment to the C-terminal domain of the periplasmic protein TolA as a co-receptor.
This entry represents the two N-terminal domains, N1 and N2, of the filamentous phage coat protein g3p.