Structure of the molybdate/tungstate binding protein Mop front Sporomusa ovata

Background: Transport of molybdenum into bacteria involves a high-affinity ABC transporter system whose expression is controlled by a repressor protei n called ModE. While molybdate transport is tightly coupled to utilization in some bacteria, other organisms have molybdenum storage proteins. One cla ss of putative molybdate storage proteins is characterized by a sequence co nsisting of about 70 amino acids (Mop). A tandem repeat of Mop sequences al so 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 symme tric 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 pepti des. All oxyanion binding sites were found to be occupied by WO4 rather tha n MoO4. Conclusions: The biological function of proteins containing only Mop sequen ces is unknown, but they have been implicated in molybdate homeostasis and molybdopterin cofactor biosynthesis. While there are few indications that t he S. ovata Mop binds pterin, the structure suggests that only the type-1 o xyanion binding sites would be sufficiently accessible to bind a cofactor. The observed occupation of the oxyanion binding sites by WO4 indicates that Mop might also be involved in controlling intracellular tungstate levels.