Structural and energetic basis of infection by the filamentous bacteriophage IKe

Abstract

SummaryFilamentous phage use the two N-terminal domains of their gene-3-proteins to initiate infection of Escherichia coli. One domain interacts with a pilus, and then the other domain binds to TolA at the cell surface. In phage fd, these two domains are tightly associated with each other, which renders the phage robust but non-infectious, because the TolA binding site is inaccessible. Activation for infection requires partial unfolding, domain disassembly and prolyl isomerization. Phage IKe infects E. coli less efficiently than phage fd. Unlike in phage fd, the pilus-and TolAbinding domains of phage IKe are independent of each other in stability and folding. The site for TolA binding is thus always accessible, but the affinity is very low. The structures of the two domains, analysed by X-ray crystallography and by NMR spectroscopy, revealed a unique fold for the N-pilus-binding domain and a conserved fold for the TolA-binding domain. The absence of an activation mechanism as in phage fd and the low affinity for TolA probably explain the low infectivity of phage IKe. They also explain why, in a previous co-evolution experiment with a mixture of phage fd and phage IKe, all hybrid phage adopted the superior infection mechanism of phage fd.

Crystal Structures of a CTXφ pIII Domain Unbound and in Complex with a Vibrio cholerae TolA Domain Reveal Novel Interaction Interfaces

“…Thus, although TCP binding is important for bringing CTX to the V. cholerae surface, it may not be required to unfold pIII and expose the TolA binding site. This relationship is similar to that seen for the IF1 and IKe coliphage, where the N1 and N2 domains are not tightly apposed but instead are arranged like beads-on-a-string, and thus their TolA binding sites are accessible in the native pIII (54,64). The TolA binding domains of CTX, M13, fd, IF1, and IKe phage are all structurally similar regardless of whether their respective pilus binding domains are tightly associated or not.…”

Cryo-electron microscopy structure of the filamentous bacteriophage IKe

“…The wild-type viral genome has ∼6,880 nt and encodes 10 proteins. Similar to other filamentous phages, the encoded protein 3 (p3) is essential for host cell recognition and virus entry as it binds to a retracting pilus and the TolA receptor of the host cell through its two functional domains D1 and D2, respectively (20). Upon infection, the major capsid protein is inserted into the host cell membrane while changing its conformation (21), and the genome is released into the host cell cytoplasm.…”