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_22-C) are reported. Importantly, reductive methylation of the lysine residues in NisI_22-C was essential for initial crystallization. Only methylated NisI_22-C crystallized. The optimized crystals of methylated NisI_22-C 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 P2₁2₁2₁, 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_M) is 2.58 ų Da⁻¹ and the estimated solvent content is 52.3%.