Molecular architectures and mechanisms involved in protein-mediated molybdate/tungstate acquisition and storage in prokaryotes

Molybdenum and tungsten are essential trace metals whose biological utilization requires strict and selective control of their uptake and intracellular availability. Prokaryotes achieve this through highly specialized systems, involving the oxyanion-specific ATP-binding cassette transporters (ModABC, WtpABC, and TupABC), which discriminate between molybdate and tungstate via distinct periplasmic-binding mechanisms. Following molybdenum import, the molybdenum storage proteins store up to ∼130 molybdate ions as polyoxometalate clusters within a protein cage via an ATP-dependent sequestration mechanism. These transport and storage strategies maintain metal homeostasis even under nutrient-limiting conditions, allowing sustained biosynthesis of molybdoenzymes and illustrating an advanced paradigm for microbial trace metal regulation.