Thermus thermophilus (Tth) HB8 glycine decarboxylase (P-protein) is an alpha(2)beta(2) tetrameric enzyme with a total molecular mass of 200 kDa. The alpha- and beta-subunits of the Tth P-protein have been coexpressed in Escherichia coli and purified as a stable complex. Dynamic light-scattering measurements indicated the recombinant protein to be monodisperse and its size to be consistent with an alpha(2)beta(2) tetrameric composition. Crystals of the protein have been grown in polyethylene glycol 3350 using the vapour-diffusion method at 291 K. Synchrotron radiation from BL45XU at SPring-8 was used to measure a complete native data set to 2.4 A resolution. The crystals belong to the trigonal space group P3(1)21 or P3(2)21, with unit-cell parameters a = b = 89.5, c = 371.0 A. Estimation of the crystal packing (V(M) = 2.15 A(3) Da(-1)) and self-rotation function analysis suggest the presence of one alpha(2)beta(2) tetramer per asymmetric unit, with the molecules related by non-crystallographic twofold symmetry.