AcmB is an amphipathic protein located in the cytoplasm or periplasmic space, and it binds to the cytoplasmic membrane through weak dispersion interactions . We also observed a high aggregation tendency of AcmB, which was not reported in the case of KstD1 . Thus, it could be hypothesized that an additional domain is responsible for anchoring the protein to the bacterial cytoplasmic membrane. Probably, the binding of the protein to the membrane occurs through a longer amphipathic α-helix (H9) (Figure ). Since helix H9 is parallel to the lipid bilayer, its hydrophobic part is separated from the water environment, whereas the polar surface of the helix interacts with the water phase and negatively charged phosphoryl groups, the membrane-associated domain can penetrate the area of polar bilayer groups. Such an orientation of the enzyme would also locate the active site near the membrane allowing steroids to dissolve in it. The association of proteins with the membrane is quite common among steroidtransforming enzymes such as cholesterol oxidase .