A conserved infection pathway for filamentous bacteriophages is suggested by the structure of the membrane penetration domain of the minor coat protein g3p from phage fd.
BACKGROUND
. Gene 3 protein (g3p), a minor coat protein from bacteriophage fd mediates infection of Escherichia coli bearing an F-pilus. Its N-terminal domain (g3p-D1) is essential for infection and mediates penetration of the phage into the host cytoplasm presumbly through interaction with the Tol complex in the E. coli membranes. Structural knowledge of g3p-D1 is both important for a molecular understanding of phage infection and of biotechnological relevance, as g3p-D1 represents the primary fusion partner in phage display technology.


RESULTS
. The solution structure of g3p-D1 was determined by NMR spectroscopy. The principal structural element of g3p-D1 is formed by a six-stranded beta barrel topologically identical to a permutated SH3 domain but capped by an additional N-terminal alpha helix. The presence of structurally similar domains in the related E. coli phages, lke and 12-2, as well as in the cholera toxin transducing phage ctx&phi; is indicated. The structure of g3p-D1 resembles those of the recently described PTB and PDZ domains involved in eukaryotic signal transduction.


CONCLUSIONS
. The predicted presence of similar structures in membrane penetration domains from widely diverging filamentous phages suggests they share a conserved infection pathway. The widespread hydrogen-bond network within the beta barrel and N-terminal alpha helix in combination with two disulphide bridges renders g3p-D1 a highly stable domain, which may be important for keeping phage infective in harsh extracellular environments.
Holliger
P
P
MRC Centre for Protein Engineering, MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.
Riechmann
L
L
D000327:Adsorption; D000595:Amino Acid Sequence; D001483:Base Sequence; D036022:Capsid Proteins; D002462:Cell Membrane / Q000821:virology; D003090:Coliphages / Q000737:chemistry / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000502:physiology; D004926:Escherichia coli / Q000821:virology; D010861:Fimbriae, Bacterial / Q000821:virology; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D014760:Viral Fusion Proteins / Q000737:chemistry* / Q000502:physiology
D036022:Capsid Proteins; D004268:DNA-Binding Proteins; D014760:Viral Fusion Proteins