The Filamentous Phages fd and IF1 Use Different Mechanisms to Infect Escherichia coli

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.…”

“…This unnatural covalent linkage could potentially force an interaction between these proteins that is not biologically relevant. However, the crystal structure of a complex between phage IF1 pIII-N1 and E. coli TolA-C, obtained from proteins that were expressed separately (54), is highly similar to that of the M13 pIII-N1⅐TolA-C complex. Thus, it appears that CTX pIII and the coliphage indeed bind to distinct sites on TolA.…”

“…␤3a and ␤3b curve around the ␤-barrel at almost 90 o to each other. ␤3b is connected to another long strand, ␤4 (53)(54)(55)(56)(57)(58)(59)(60)(61)(62), by a type I hairpin turn. The ␤3␤4 hairpin extends away from the top of the ␤-barrel.…”

“…fd and M13 pIII proteins have 99% amino acid sequence identity, and their crystal structures (domains N1 and N2 or N1N2) are essentially identical (22,23). pIII of the related coliphage IF1 pIII shares the same domain organization as the Ff phage pIII proteins and binds to TolA-C via N1 and to the E. coli I pilus via N2, but its domains are more loosely associated, and no conformational change is required for TolA binding (54). IF1 pIII-N1 is 31% identical to fd pIII-N1, and their structures are highly similar.…”

Initiation of Phage Infection by Partial Unfolding and Prolyl Isomerization

“…1B). For the corresponding variant (G3P ⌬␤6), G3P IIHY was used as the parent protein as the wild-type form of G3P without strand ␤6 started to unfold already at 25°C (19).…”