The outer membrane cytochromes of Shewanella oneidensis MR-1 are lipoproteins

Aim: To determine if the outer membrane (OM) cytochromes OmcA and OmcB of the metal-reducing bacterium Shewanella oneidensis MR-1 are lipoproteins, and to assess cell surface exposure of the cytochromes by radioiodination. Methods and Results: In anaerobic MR-1 cells grown with H-3-palmitoleic acid, both OmcA and OmcB were radiolabelled. The identities of these bands were confirmed by the absence of each radiolabelled band in the respective mutants lacking individual OM cytochromes. Radioiodination of cell surface proteins in anaerobic cells resulted in I-125-labelled OmcA. The identity of this band was confirmed by its absence in an OmcA-minus mutant. A ubiquitous radioiodinated band that migrates similarly to OmcB precluded the ability to determine the potential cell surface exposure of OmcB by this method. Conclusions: Both OmcA and OmcB are lipoproteins, and OmcA is cell surface exposed. Significance: The lipoprotein modification of these OM cytochromes could be important for their localization or incorporation into the OM. The cell surface exposure of OmcA could allow it to directly transfer electrons to extracellular electron acceptors (e.g. manganese oxides) and is consistent with its in vivo role.