Quaternary structure of human nucleoside diphosphate kinase isoforms HA and HB in solution

Human isoforms of nucleoside diphosphate kinase, NDPK-HA and NDPK-HB, have been expressed in E. coli and purified. Their apparent molecular masses have been determined by FPLC gel filtration. Absolute molecular masses were measured by equilibrium ultracentrifugation and sedimentation coefficients determined from the sedimentation velocity. Under near-physiological conditions, NDPK-HA has a mass of 101 +/- 3 kDa, close to that calculated for a hexamer (102.11 kDa), whilst NDPK-HB has a mass of 71 +/- 3 kDa, close to a tetramer (68.67 kDa). The sedimentation coefficients, 5.15 +/- 0.2 and 3.41 +/- 0.1 x 10(-13) s, for HA and HB also indicate a hexamer and a tetramer respectively. This suggests, although the crystal structure shows a hexameric quaternary arrangement [Webb et al. (1995) J. Mol. Biol. 251, 574-587], that NDPK-HB forms tetramers in solution like bacterial NDPK [Williams et al. (1993) J. Mol. Biol. 234, 1230-1247].