Proteolytic Maturation of the Outer Membrane -Type Cytochrome OmcZ by a Subtilisin-Like Serine Protease Is Essential for Optimal Current Production by Geobacter sulfurreducens.

An outer membrane -type cytochrome (OmcZ) in Geobacter sulfurreducens is essential for optimal current production in microbial fuel cells. OmcZ exists in two forms, small and large, designated OmcZ and OmcZ, respectively. However, it is still not known how these two structures are formed. A mutant with a disruption of the GSU2075 gene encoding a subtilisin-like serine protease (designated for the mcrotease), which is located downstream of , produced low currents at a level similar to that of the -deficient mutant strain. Biochemical analyses revealed that the mutant accumulated OmcZ and did not produce OmcZ, which is thought to be a mature form that is essential for the extracellular electron transfer to the electrode. A heterologous expression system cell lysate from an Escherichia coli strain producing OzpA cleaved OmcZ and generated OmcZ as the proteolytic product. Among the culture supernatant, loosely bound outer surface, and intracellular protein fractions from wild-type G. sulfurreducens, only the culture supernatant protein fraction showed OmcZ cleavage activity, indicating that the mature form of OmcZ, OmcZ, can be produced outside the cells. These results indicate that OzpA is an essential protease for current production via the maturation of OmcZ, and OmcZ is the key to the extracellular electron transfer to electrodes. This proteolytic maturation of OmcZ is a unique regulation among known -type cytochromes in G. sulfurreducens. Microbial fuel cells are a promising technology for energy generation from various waste types. However, the molecular mechanisms of microbial extracellular electron transfer to the electrode need to be elucidated. G. sulfurreducens is a common key player in electricity generation in mixed-culture microbial fuel cell systems and a model microorganism for the study of extracellular electron transfer. Outer membrane -type cytochrome OmcZ is essential for an optimal current production by G. sulfurreducens. OmcZ proteolytic cleavage occurs during maturation, but the underlying mechanism is unknown. This study identifies a subtilisin-like protease, OzpA, which plays a role in cleaving OmcZ and generating the mature form of OmcZ (OmcZ). OzpA is essential for current production and, thus, the proteolytic maturation of OmcZ. This is a novel regulation of the -type cytochrome for G. sulfurreducens extracellular electron transfer. This study also provides new insights into the design strategy and development of microbial extracellular electron transfer for an efficient energy conversion from chemical energy to electricity.