Studies of the structural properties and stability of the molybdenum transport protein ModA from Oleidesulfovibrio alaskensis G20 upon metal binding

Abstract

Molybdenum and tungsten are taken up by cells through highly specific transport systems known in bacteria as ModABC and Tup/WtpABC, respectively. Component A (Mod/Tup/WtpA) binds the metal in the periplasm and thus represents the first selective mechanism for the uptake of the correct metal. The genome of Oleidesulfovibrio alaskensis G20 contains both mod and tup genes. In a previous paper, we reported the structure, molybdenum and tungsten-binding constants, and biochemical properties of TupA isolated from this organism. In the current study, we used biochemical and spectroscopic techniques to explore changes in the structure and stability of ModA upon metal binding. We show that the OaModA is expressed as a monomeric and soluble protein. In addition, it binds both molybdate and tungstate with extremely high affinity with K_D constant in the nanomolar range and is unable to distinguish between them. The protein is more stable upon molybdenum binding, as demonstrated by differential scanning fluorescence studies. Such change is accompanied by a transition to a closed conformation with changes in secondary structure.