This family includes C-type outer membrane cytochrome S (OmcS) which plays an important role in extracellular electron transfer. OmcS can transfer electrons to insoluble Fe(3+) oxides as well as other extracellular electron acceptors, including Mn(4+) oxide and humic substances. Recent studies show that Geobacter sulfurreducens hexaheme cytochrome OmcS proteins can assemble into filaments, known as microbial nanowires, similar to type IV pili composed of PilA protein. The coordination of a histidine in one subunit with the iron in the heme of an adjacent subunit is an important stabilizing element. The capacity of these bacteria to transport electrons to remote electron acceptors via these protein nanowires is of interest because of the environmental and practical significance of these microbes in soil.