Expression, purification, crystallization and preliminary crystallographic analysis of PilA from the nontypeable Haemophilus influenzae type IV pilus

The type IV pili of nontypeable Haemophilus influenzae (NTHi) are involved in twitching motility, adherence, competence and biofilm formation. They are potential virulence factors for this important human pathogen and are thus considered to be vaccine targets. To characterize these pili, an attempt to solve the atomic structure of the major pilin subunit PilA was initiated. A 1.73 Å resolution X-ray diffraction data set was collected from native N-terminally truncated PilA (ΔN-PilA). Data processing indicated a hexagonal crystal system, which was determined to belong to space group P6₁ or P6₅ based on the systematic absences and near-perfect twinning of the crystal. The unit-cell parameters were a = b = 68.08, c = 197.03 Å with four molecules in the asymmetric unit, giving a solvent content of 50%. Attempts to solve the ΔN-­PilA structure by molecular replacement with existing type IV pilin and type II secretion pseudopilin structures are in progress.