Structure of the catalytic domain of Streptococcus pneumoniae sialidase NanA

Streptococcus pneumoniae genomes encode three sialidases, NanA, NanB and NanC, which are key virulence factors that remove sialic acids from various glycoconjugates. The enzymes have potential as drug targets and also as vaccine candidates. The 115 kDa NanA is the largest of the three sialidases and is anchored to the bacterial membrane. Although recombinantly expressed full-length NanA was soluble, it failed to crystallize; therefore, a 56.5 kDa domain that retained full enzyme activity was subcloned. The purified enzyme was crystallized in 0.1 M MES pH 6.5, 30%(w/v) PEG 4000 using the sitting-drop vapour-diffusion method. Data were collected at 100 K to 2.5 Å resolution from a crystal grown in the presence of the inhibitor 2-deoxy-2,3-dehydro-N-acetyl neuraminic acid. The crystal belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 49.2, b = 95.6, c = 226.6 Å. The structure was solved by molecular replacement and refined to final R and R (free) factors of 0.246 and 0.298, respectively.