Assembly of a transmembrane b-type cytochrome is mainly driven by transmembrane helix interactions.

Folding, assembly and stability of alpha-helical membrane proteins is still not very well understood. Several of these membrane proteins contain cofactors, which are essential for their function and which can be involved in protein assembly and/or stabilization. The effect of heme binding on the assembly and stability of the transmembrane b-type cytochrome b'559 was studied by fluorescence resonance energy transfer. Cytochrome b'559 consists of two monomers of a 44 amino acid long polypeptide, which contains one transmembrane domain. The synthesis of two variants of the b'559 monomer, each carrying a specific fluorescent dye, allowed monitoring helix-helix interactions in micelles by resonance energy transfer. The measurements demonstrate that the transmembrane peptides dimerize in detergent in the absence and presence of the heme cofactor. Cofactor binding only marginally enhances dimerization and, apparently, the redox state of the heme group has no effect on dimerization.