Characterization of an electron conduit between bacteria and the extracellular environment

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 (OM), 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-OM icosa-heme complex, MtrCAB, and demonstrate its capacity to move electrons across a lipid bilayer after incorporation into proteoliposomes. We also show that a stable MtrAB subcomplex can assemble in the absence of MtrC; an MtrBC subcomplex is not assembled in the absence of MtrA; and 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-OM spanning ␤-barrel protein that serves as a sheath, within which MtrA and MtrC exchange electrons. 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.cytochrome-c ͉ iron respiration ͉ protein film voltammetry ͉ electron paramagnetic resonance ͉ Shewanella

Proteins involved in electron transfer to Fe(III) and Mn(IV) oxides by Geobacter sulfurreducens and Geobacter uraniireducens

“…pccJ is a frameshifted gene that is intact in the KN400 strain, another wild-type isolate of G. sulfurreducens (Butler et al, 2012), but in the DL1 wild-type that was used in these experiments, the frameshift is predicted to result in a truncated protein with only 10 of the full 16 haem-binding sites. Monohaem c-type cytochrome OmcL, predicted to form a beta-barrel in the outer membrane, and the decahaem c-type cytochrome encoded by the adjacent omcK gene (GSU2203), which was also deleted, resemble MtrB and MtrA of Shewanella species, which are thought to interact with an extracellular electron shuttle (Hartshorne et al, 2009). None of the six deletion mutants differed from the wild-type during growth on soluble or insoluble electron acceptors (Table 5).…”

A dynamic periplasmic electron transfer network enables respiratory flexibility beyond a thermodynamic regulatory regime

“…An example is the MtrABC complex involved in the reduction of insoluble metals as well as shuttling compounds. It consists of one cytochrome each on the periplasmic (MtrA) and cell surface (MtrC) site of the outer membrane, with a membrane-associated b-barrel protein (MtrB) most likely acting as a connector between them (Ross et al, 2007;Shi et al, 2007;Hartshorne et al, 2009). Interestingly, the periplasmic space of S. oneidensis is with 235 A too wide for electron transfer to proceed directly between CymA and the periplasmic components of the above-mentioned multimeric complexes like MtrA.…”

Rapid electron exchange between surface-exposed bacterial cytochromes and Fe(III) minerals

“…Mtr pathways, also used by other Fe(III)-reducing bacteria, e.g., Geobacter metallireducens (3), and the Fe(II)-oxidizing bacterium Sideroxydans lithotrophicus ES-1 (6), use a porin-cytochrome complex in which a transmembrane β-barrel binds one or more multiheme cytochromes that transfer electrons across the membrane via a network of hemes. In S. oneidensis and other metal-reducing members of Shewanella, this function is performed by an MtrCAB complex (5,7). MtrA is an outer-membrane, periplasm-exposed, decaheme cytochrome (8); MtrB is predicted to be a 28-β-strand transmembrane porin (9); and MtrC is an extracellular, surface-exposed, decaheme cytochrome (10).…”

“…MtrA is an outer-membrane, periplasm-exposed, decaheme cytochrome (8); MtrB is predicted to be a 28-β-strand transmembrane porin (9); and MtrC is an extracellular, surface-exposed, decaheme cytochrome (10). Biochemical characterization of this complex indicates that MtrB functions as the transmembrane sheath into which both MtrC and MtrA are partially inserted (7,11). The arrangement of the two cytochromes within MtrB is proposed to be close enough to allow electron exchange between the hemes of both cytochromes, thus forming an efficient electron transfer conduit through the outer membrane of the cell (7).…”