title: Structure of the molybdate/tungstate binding protein mop from Sporomusa ovata.
abstract: BACKGROUND
  Transport of molybdenum into bacteria involves a high-affinity ABC transporter system whose expression is controlled by a repressor protein called ModE. While molybdate transport is tightly coupled to utilization in some bacteria, other organisms have molybdenum storage proteins. One class of putative molybdate storage proteins is characterized by a sequence consisting of about 70 amino acids (Mop). A tandem repeat of Mop sequences also constitutes the molybdate binding domain of ModE.
  
  
  RESULTS
  We have determined the crystal structure of the 7 kDa Mop protein from the methanol-utilizing anaerobic eubacterium Sporomusa ovata grown in the presence of molybdate and tungstate. The protein occurs as highly symmetric hexamers binding eight oxyanions. Each peptide assumes a so-called OB fold, which has previously also been observed in ModE. There are two types of oxyanion binding sites in Mo at the interface between two or three peptides. All oxyanion binding sites were found to be occupied by WO(4) rather than MoO(4).
  
  
  CONCLUSIONS
  The biological function of proteins containing only Mop sequences is unknown, but they have been implicated in molybdate homeostasis and molybdopterin cofactor biosynthesis. While there are few indications that the S. ovata Mop binds pterin, the structure suggests that only the type-1 oxyanion binding sites would be sufficiently accessible to bind a cofactor. The observed occupation of the oxyanion binding sites by WO(4) indicates that Mop might also be involved in controlling intracellular tungstate levels.
authors.affiliation: Institut für Chemie Strukturbiologie Karl-Franzens-Universität A-8010, Graz, Austria.
authors.forename: U G
authors.initials: UG
authors.lastname: Wagner
authors.forename: E
authors.initials: E
authors.lastname: Stupperich
authors.forename: C
authors.initials: C
authors.lastname: Kratky
mesh_terms: D000595:Amino Acid Sequence; D001421:Bacteria, Anaerobic / Q000737:chemistry* / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D047908:Intracellular Signaling Peptides and Proteins; D008958:Models, Molecular; D008969:Molecular Sequence Data; D008982:Molybdenum / Q000378:metabolism*; D011487:Protein Conformation; D017510:Protein Folding; D012645:Selenomethionine / Q000737:chemistry; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics; D014414:Tungsten / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D047908:Intracellular Signaling Peptides and Proteins; C101050:ModE protein, bacteria; C420111:Mop protein, Sporomusa; D014157:Transcription Factors; D008982:Molybdenum; D012645:Selenomethionine; D014414:Tungsten