Two crystal structures of the cytoplasmic molybdate-binding protein ModG suggest a novel cooperative binding mechanism and provide insights into ligand-binding specificity

The X-ray structures of the cytoplasmic molybdate-binding protein ModG from Azotobacter vinelandii in two different crystal forms have been determined . For such a small protein it is remarkably complex. Each 14.3 kDa subunit contains two small beta -barrel domains, which display an OB-fold motif, al so seen in the related structure of ModE, a molybdenum-dependent transcript ional regulator, and very recently in the Mop protein that, like ModG, has been implicated in molybdenum homeostasis within the cell. In contrast to e arlier speculation, the functional unit of ModG is actually not a dimer (as in ModE), but a trimer capable of binding a total of eight molybdate molec ules that are distributed between two disparate types of site. All the bind ing sites are located at subunit interfaces, with one type lying on a cryst allographic 3-fold axis, whilst the other lies between pairs of subunits. T he two types of site are linked by short hydrogen bond networks that may su ggest a cooperative binding mechanism. A superposition of two subunits of t he ModG trimer on the apo-ModE dimer allows the probable locations of the m olybdate-binding sites of the latter to be assigned. Through structural com parisons with other oxyanion-binding proteins, including Mop and ModE, it i s possible to speculate about ligand-binding affinities, selectivity and ev olution. (C) 2001 Academic Press.