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

The topology of 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 eznymic 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. [ABSTRACT FROM AUTHOR]