Organelle-like structures in Agrobacterium tumefaciens and Ralstonia eutropha

Polyhydroxybutyrate (PHB) and polyphosphate (polyP) are storage compounds for carbon and phosphorous, respectively, and are intracellularly deposited as insoluble granule-like structures in a large variety of prokaryotic species. PHB granules are supramolecular complexes consisting of a polymer core that is surrounded by a proteinaceous surface layer of several proteins with different functions. PolyP is an inorganic anionic polymer (previously designated as volutin) and consists of linear chains of condensed phosphate residues with Ca2+ and Mg2+ as counter-ions. Similar to PHB, polyP granules in R. eutropha also have several proteins (at least eight) specifically attached to the polyP surfaces. In A. tumefaciens, polyP together with Ca2+ ions is present in membrane-enclosed acidic compartments called acidocalcisomes. I will provide an overview on the properties (structure, composition and function) of PHB and polyP granules/acidocalcisomes in A. tumefaciens and R. eutropha as model organisms of alpha- and beta-protobacteria.