Crystallographic structure reveals phosphorylated pilin from Neisseria: phosphoserine sites modify type IV pilus surface chemistry and fibre morphology

Understanding the structural biology of type IV pill, fibres responsible fo r the virulent attachment and motility of numerous bacterial pathogens, req uires a detailed understanding of the three-dimensional structure and chemi stry of the constituent pilin subunit. X-ray crystallographic refinement of Neisseria gonorrhoeae pilin against diffraction data to 2.6 Angstrom resol ution, coupled with mass spectrometry of peptide fragments, reveals phospho serine at residue 68, Phosphoserine is exposed on the surface of the modell ed type IV pilus at the interface of neighbouring pilin molecules. The site -specific mutation of serine 68 to alanine showed that the loss of the phos phorylation alters the morphology of fibres examined by electron microscopy without a notable effect on adhesion, transformation, piliation or twitchi ng motility, The structural and chemical characterization of protein phosph oserine in type IV pilin subunits is an important indication that this modi fication, key to numerous regulatory aspects of eukaryotic cell biology, ex ists in the virulence factor proteins of bacterial pathogens, These O-linke d phosphate modifications, unusual in prokaryotes, thus merit study for pos sible roles in pilus biogenesis and modulation of pilin chemistry for optim al in vivo function.