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Nested Bacterial Boxes: Nuclear and Other Intracellular Compartments in Planctomycetes

Bacteria in the phylum Planctomycetes and some related phyla challenge our concept of the typical bacterium as consisting of cells without internal compartments or membrane-bounded organelles. Cells of all species of planctomycetes examined consist of at least two major compartments, and there are two other types of compartmentation in which a third compartment is formed either by a double-membrane envelope around the nucleoid in the case of the aerobic Gemmata obscuriglobus or by a single but potentially energized membrane in the case of the anaerobic ammonium-oxidizing anammox planctomycetes. We examine here the nature of these planctomycete compartments in relation to function and their relationship to the endomembranes defining them, and discuss the implications of the remarkable compartment-confined process of protein uptake in Gemmata, which resembles receptor- and clathrin-mediated endocytosis of eukaryotes. Planctomycetes have implications for our understanding of the evolution of membrane-bounded organelles, of endomembranes, transport across endomembranes and membrane trafficking, and for how the complexity of a eukaryote style of cell organization could have originated.