Functional properties of LptA and LptD in Anabaena sp. PCC 7120.

Lipopolysaccharides (LPS) are central components of the outer membrane and consist of Lipid A, the core polysaccharide, and the O-antigen. The synthesis of LPS is initiated at the cytosolic face of the cytoplasmic membrane. The subsequent transport to and across the outer membrane involves multiple lipopolysaccharide transport (Lpt) proteins. Among those proteins, the periplasmic-localized LptA and the outer membrane-embedded LptD participate in the last steps of transfer and insertion of LPS into the outer membrane. While the process is described for proteobacterial model systems, not much is known about the machinery in cyanobacteria. We demonstrate that anaLptD (alr1278) of Anabaena sp. PCC 7120 is important for cell wall function and its pore domain shows a Lipid A sensitive cation-selective gating behavior. The N-terminal domain of anaLptD recognizes anaLptA (alr4067), but not ecLptA. Furthermore, anaLptA specifically interacts with the Lipid A from Anabaena sp. PCC 7120 only, while anaLptD binds to Lipid A isolated from Escherichia coli as well. Based on the comparative analysis of proteins from E. coli and Anabaena sp. we discuss the properties of the cyanobacterial Lpt system.

Structure and conservation of the periplasmic targeting factor Tic22 protein from plants and cyanobacteria.

Mitochondria and chloroplasts are of endosymbiotic origin. Their integration into cells entailed the development of protein translocons, partially by recycling bacterial proteins. We demonstrate the evolutionary conservation of the translocon component Tic22 between cyanobacteria and chloroplasts. Tic22 in Anabaena sp. PCC 7120 is essential. The protein is localized in the thylakoids and in the periplasm and can be functionally replaced by a plant orthologue. Tic22 physically interacts with the outer envelope biogenesis factor Omp85 in vitro and in vivo, the latter exemplified by immunoprecipitation after chemical cross-linking. The physical interaction together with the phenotype of a tic22 mutant comparable with the one of the omp85 mutant indicates a concerted function of both proteins. The three-dimensional structure allows the definition of conserved hydrophobic pockets comparable with those of ClpS or BamB. The results presented suggest a function of Tic22 in outer membrane biogenesis.