Acidocalcisomes and Polyphosphate Granules Are Different Subcellular Structures in Agrobacterium tumefaciens.

Acidocalcisomes are membrane-enclosed, polyphosphate-containing acidic organelles in lower but have also been described for (M. Seufferheld, M. Vieira, A. Ruiz, C. O. Rodrigues, S. Moreno, and R. Docampo, J Biol Chem 278:29971-29978, 2003, https://doi.org/10.1074/jbc.M304548200). This study aimed at the characterization of polyphosphate-containing acidocalcisomes in this alphaproteobacterium. Unexpectedly, fluorescence microscopic investigation of cells using fluorescent dyes and localization of constructed fusions of polyphosphate kinases (PPKs) and of vacuolar H-translocating pyrophosphatase (HppA) with enhanced yellow fluorescent protein (eYFP) suggested that acidocalcisomes and polyphosphate are different subcellular structures. Acidocalcisomes and polyphosphate granules were frequently located close together, near the cell poles. However, they never shared the same position. Mutant strains of with deletions of both genes () were unable to form polyphosphate but still showed cell pole-located eYFP-HppA foci and could be stained with MitoTracker. In conclusion, forms polyP granules that are free of a surrounding membrane and thus resemble polyP granules of and other bacteria. The composition, contents, and function of the subcellular structures that are stainable with MitoTracker and harbor eYFP-HppA remain unclear. The uptake of alphaproteobacterium-like cells by ancestors of eukaryotic cells and subsequent conversion of these alphaproteobacterium-like cells to mitochondria are thought to be key steps in the evolution of the first eukaryotic cells. The identification of acidocalcisomes in two alphaproteobacterial species some years ago and the presence of homologs of the vacuolar proton-translocating pyrophosphatase HppA, a marker protein of the acidocalcisome membrane in eukaryotes, in virtually all species within the alphaproteobacteria suggest that eukaryotic acidocalcisomes might also originate from related structures in ancestors of alphaproteobacterial species. Accordingly, alphaproteobacterial acidocalcisomes and eukaryotic acidocalcisomes should have similar features. Since hardly any information is available on bacterial acidocalcisomes, this study aimed at the characterization of organelle-like structures in alphaproteobacterial cells, with as an example.