Top figure:
a, b, ultrathin section of Methanobacterium thermoautotrophicum. The arrows point to particulate entities seen close to the cytoplasmic membrane. They are interpreted to the methanoreductosomes attached to the cytoplasmic membrane by a stalk (s. below). One of them (x) exhibits clear symmetric substructures. Dimensions are given in µm.

Bottom figure:
Diagrammatic view of the enzyme complex consisting of a head part (containing several copies of the enzyme methyl-CoM reductase; MCR), a mediating part, and a stalk which links the particle to a component of the system in the cytoplasmic membrane (a).
b, isolated enzyme methyl-CoM reductase, which was used for reconstitution experiments and which is assumed to also occur in the cell cytoplasm.
c, result of reconstitution experiments: MCR particles assemble to form hollow spheres („shells“) similar in size to head parts of methanoreductosomes isolated from the cells; however, the reconstituted head parts contain MCR only and they did not show any polarity, as is the case for in vivo head parts. The interaction of the end of the stalk with a membrane-integrated corrinoid protein (methyltransferase) has not been shown. (Upper figure: original micrograph taken by F. Mayer and M. Hoppert; bottom figure: from Mayer, F., 1993, ASM News 59, 346-350; additional references therein).