Heterogeneous nucleation is required for crystallization of the ZnuA domain of pneumococcal AdcA

Zn2+is an essential nutrient for all known forms of life. In the major human pathogenStreptococcus pneumoniae, the acquisition of Zn2+is facilitated by two Zn2+-specific solute-binding proteins: AdcA and AdcAII. To date, there has been a paucity of structural information on AdcA, which has hindered a deeper understanding of the mechanism underlying pneumococcal Zn2+acquisition. Native AdcA consists of two domains: an N-terminal ZnuA domain and a C-terminal ZinT domain. In this study, the ZnuA domain of AdcA was crystallized. The initial crystals of the ZnuA-domain protein were obtained using dried seaweed as a heterogeneous nucleating agent. No crystals were obtained in the absence of the heterogeneous nucleating agent. These initial crystals were subsequently used as seeds to produce diffraction-quality crystals. The crystals diffracted to 2.03 Å resolution and had the symmetry of space groupP1. This study demonstrates the utility of heterogeneous nucleation. The solution of the crystal structures will lead to further understanding of Zn2+acquisition byS. pneumoniae.