Structure, Function and Formation of Bacterial Intracytoplasmic Membranes

Abstract

A variety of prokaryotes increase their membrane surface area for energy transduction processesby developing an intracytoplasmic membrane (ICM), in the form of tubules, interconnected vesicles, and single,paired or stacked lamellae. Recent images of the ICM of the photosynthetic bacterium Rhodobactersphaeroides, obtained by atomic force microscopy, have provided the first submolecular surfaceviews of any complex multi-component membrane. These topographs revealed rows of dimeric core light-harvesting1 (LH1) (RC) complexes, interconnected by the peripheral light-harvesting 2 (LH2) complex, which also existedin separate clusters. In addition, polarized light spectroscopy demonstrated that this optimal functionalarrangement is extended into a long-range pattern of membrane organization. Functional insights providedby the detailed structures of the light-harvesting, RC and cytochrome bc 1complexes are also discussed, including how LH1 is organized to facilitate ubiquinone exchange. It wasshown that LH1-RC core structures are inserted initially into the cytoplasmic membrane, which upon additionof LH2, invaginates to form the ICM, with LH2 packing between rows of dimeric core complexes, and ultimatelyforming separate bulk LH2 clusters. The ICM of the ecologically important methanotrophs, and chemolithotrophicnitrifying bacteria that convert ammonia to nitrite, is also discussed. The recent determination of thecrystal structure of the major ICM protein, methane monooxygenase, and the complete genome sequence of Methylococcus capsulatus, are providing further insights into the molecular detailsof both methane oxidation and utilization of the resulting methanol as a sole source of carbon andenergy.