Expression, purification, crystallization and preliminary X-ray analysis of the cathelicidin motif of the protegrin-3 precursor

Numerous precursors of antibacterial peptides with unrelated sequences share a similar prosequence which belongs to the cathelicidin family of proteins. The three-dimensional structure of this cathelicidin motif, which contains two disulfide bonds, has not yet been reported. The cathelicidin motif (ProS) of the protegrin-3 precursor was overexpressed in Escherichia coli as a His-tagged protein. The His₆ tag was removed by thrombin cleavage. ProS was purified to homogeneity and single crystals were obtained by the hanging-drop vapour-diffusion method at pH 3-4. Preliminary X-ray diffraction analysis indicated that these crystals belong to the hexagonal space group P6₁22 or P6₅22, with unit-cell parameters a = b = 51.42, c = 134.25 Å. These crystals diffracted beyond 2.75 Å (1.9 Å at ESRF) and contain one molecule per asymmetric unit.