Full-length nisin immunity protein NisI from Lactococcus lactis in a lipid-free form: crystallization and X-ray analysis.

NisI is a lantibiotic-binding lipoprotein that is specific for nisin. Nisin-producing microorganisms use NisI as an immunity protein for self-protection against nisin. Here, the purification, crystallization and preliminary X-ray diffraction of full-length NisI from Lactobacillus lactis in a lipid-free form (NisI) are reported. Importantly, reductive methylation of the lysine residues in NisI was essential for initial crystallization. Only methylated NisI crystallized. The optimized crystals of methylated NisI were grown in 30-40 mM ammonium sulfate, 0.1 M sodium acetate pH 4.6, 16-18% PEG 4000 at 295 K and diffracted to 1.9 Å resolution. The crystal belonged to space group P222, with unit-cell parameters a = 45.99, b = 76.67, c = 76.39 Å, α = β = γ = 90.0°. Assuming the presence of one molecule in the asymmetric unit, the estimated Matthews coefficient (V) is 2.58 Å Da and the estimated solvent content is 52.3%.