Immunoelectron microscopy and epitope mapping with monoclonal antibodies suggest the existence of an additional N-terminal transmembrane helix in the cytochrome b subunit of bacterial ubiquinol:cytochrome-c oxidoreductases.

The topology of the ubiquinol:cytochrome-c oxidoreductase (cytochrome bc1 complex) from Paracoccus denitrificans was investigated by immunoelectron microscopy with sequence-specific murine monoclonal antibodies. Epitope mapping with synthetic peptides and enzymic proteolytic cleavage of the cytochrome bc1 complex were employed to localize precisely the respective antibody epitopes on the subunits of this membrane protein complex. Localization of defined epitopes on the cytochrome bc1 complex by immunoelectron microscopy clearly demonstrates that the N-terminus of the cytochrome b subunit is exposed to the periplasmic space. This finding is in agreement with a nine-transmembrane-helices topology model (I-IX) as predicted before for cytochrome b. However, due to other published evidence we favour the existence of an additional transmembrane helix (helix 0) complementing a more recently published eight-helices model (A-C,cd, D-H), at least for prokaryotes.