Cell compartmentalization and endocytosis in planctomycetes: structure and function in complex bacteria

Planctomycetes are unique among the domain Bacteria in possessing cells with a complex plan defined by internal membranes forming separated compartments within the cell. They also possess other unique features such as cell walls composed of protein as a major polymer instead of the peptidoglycan typical of other bacteria. All species examined display an underlying shared cell organization in which an internal intracytoplasmic membrane separates two major cell compartments, an outer ribosome-free paryphoplasm and a more central ribosome-containing pirellulosome. Some planctomycete species have three compartments, where further membranes within the pirellulosome define another compartment, the anammoxosome in anammox planctomycetes and the membrane-bounded nuclear body in Gemmata obscuriglobus. Compartments are preserved when new cells are formed during division. Functional features which are correlated with structural compartmentalization in planctomycetes include in G. obscuriglobus the ability to take up proteins within the paryphoplasm of the cell by a mechanism similar to receptor-mediated endocytosis of eukaryotes. Novel molecular and cell biology features for bacteria can be predicted to accompany such structural and functional complexity and are discussed here