Structure and assembly of the pseudopilin PulG

Summary The pseudopilin PulG is one of several essential com- ponents of the type II pullulanase secretion machin- ery (the Pul secreton) of the Gram-negative bacterium Klebsiella oxytoca . The sequence of the N-terminal 25 amino acids of the PulG precursor is hydrophobic and very similar to the corresponding region of type IV pilins. The structure of a truncated PulG (lacking the homologous region), as determined by X-ray crystallography, was found to include part of the long N-terminal a -helix and the four internal anti-parallel b - strands that characterize type IV pilins, but PulG lacks the highly variable loop region with a disulphide bond that is found in the latter. When overproduced, PulG forms flexible pili whose structural features, as visu- alized by electron microscopy, are similar to those of bacterial type IV pili. The average helical repeat com- prises 17 PulG subunits and four helical turns. Elec- tron microscopy and molecular modelling show that PulG probably assembles into left-handed helical pili with the long N-terminal a a a -helix tightly packed in the centre of the pilus. As in the type IV pilins, the hydro- phobic N-terminal part of the PulG a -helix is neces- sary for its assembly. Subtle sequence variations within this highly conserved segment seem to deter- mine whether or not a type IV pilin can be assembled into pili by the Pul secreton.