Extracellular Streptomyces vesicles: amphorae for survival and defence

Blue-pigmented exudates arise as droplets on sporulated lawns of Streptomyces coelicolor M110 grown on agar plates. Our electron microscopical and biochemical studies suggest that droplets contain densely packed vesicles with large assemblies of different protein types and/or the polyketide antibiotic actinorhodin. Frozen-hydrated vesicles were unilamellar with a typical bilayer membrane, and ranged from 80 to 400 nm in diameter with a preferred width of 150-300 nm. By means of cryo-electron tomography, three types were reconstructed three-dimensionally: vesicles that were filled with particulate material, likely protein assemblies, those that contained membrane-bound particles, and a vesicle that showed a higher contrast inside, but lacked particles. Our LC/MS analyses of generated tryptic peptides led to the identification of distinct proteins that carry often a predicted N-terminal signal peptide with a twin-arginine motif or lack a canonical signal sequence. The proteins are required for a range of processes: the acquisition of inorganic as well as organic phosphate, iron ions, and of distinct carbon sources, energy metabolism and redox balance, defence against oxidants and tellurites, the tailoring of actinorhodin, folding and assembly of proteins, establishment of turgor, and different signalling cascades. Our novel findings have immense implications for understanding new avenues of environmental biology of streptomycetes and for biotechnological applications.