PNNL

Abstract

A number of species of Gram-negative bacteria can use insoluble minerals of Fe(III) and Mn(IV) as extracellular respiratory electron acceptors. In some species of Shewanella deca-heme electron transfer proteins lie at the extracellular face of the outer membrane where they can interact with insoluble substrates. To reduce extracellular substrates, these redox proteins must be charged by the inner membrane/periplasmic electron transfer system. Here we present a spectro-potentiometric characterization of a trans-outer membrane icosa-heme protein complex, MtrCAB. Incorporation into proteoliposomes demonstrates its functional capacity to move electrons across a lipid bilayer. We also show that a stable MtrAB sub-complex can assemble in the absence of MtrC, that an MtrBC sub-complex was not assembled in the absence of MtrA and that MtrA is only associated to the membrane in cells when MtrB is present. We propose a model for the modular organization of the MtrCAB complex in which MtrC is an extracellular element that mediates electron transfer to extracellular substrates and MtrB is a trans-outer membrane spanning ?-barrel protein that serves as a sheath to house the deca-heme MtrA allowing it to span the membrane to form a trans-membrane electron delivery module that services MtrC. We have identified the MtrAB module in a range of bacterial phyla suggesting that it is widely used in electron exchange with the extracellular environment