Crystallization and X-ray analysis of the extracellular adhesion domain of Helicobacter pylori adhesin A: the significance of the cation composition in the crystallization precipitant

Adherence to host cells is a crucial step in the process of bacterial infection, which is usually mediated by a number of outer membrane proteins identified as adhesins. Helicobacter pylori adhesin A (HpaA) is a member of the adhesin family that mediates the adherence of Helicobacter pylori to gastric epithelial cells, and consequently assists the bacteria in becoming a life-long colonizer of the human stomach. In this study, two constructs were made for the production of truncated HpaA proteins comprising residues 31–260 and 53–260, respectively. The products of both constructs were crystallized, but only the protein from the shorter construct (residues 53–260) formed crystals that were capable of diffraction. In the subsequent optimization trials, crystals in different forms were unexpectedly obtained by using lithium sulfate and ammonium sulfate as the precipitant. An X-ray data set was collected to 1.95 Å resolution on beamline BL18U1 at SSRF using a crystal grown with 1.92 M lithium sulfate, which belonged to space group P6₅ with unit-cell parameters a = b = 95.42, c = 54.72 Å, γ = 120°, while another crystal grown with 1.9 M ammonium sulfate diffracted to 2.60 Å resolution and the collected data set was indexed in space group P2₁2₁2, with unit-cell parameters a = 121.01, b = 190.56, c = 106.31 Å. The collection of diffraction data has established a solid basis for structure determination.