title: Outer Membrane Proteins Derived from Non-cyanobacterial Lineage Cover the Peptidoglycan of Cyanophora paradoxa Cyanelles and Serve as a Cyanelle Diffusion Channel.
abstract: The cyanelle is a primitive chloroplast that contains a peptidoglycan layer between its inner and outer membranes. Despite the fact that the envelope structure of the cyanelle is reminiscent of Gram-negative bacteria, the Cyanophora paradoxa genome appears to lack genes encoding homologs of putative peptidoglycan-associated outer membrane proteins and outer membrane channels. These are key components of Gram-negative bacterial membranes, maintaining structural stability and regulating permeability of outer membrane, respectively. Here, we discovered and characterized two dominant peptidoglycan-associated outer membrane proteins of the cyanelle (∼2 × 10(6) molecules per cyanelle). We named these proteins CppF and CppS (cyanelle peptidoglycan-associated proteins). They are homologous to each other and function as a diffusion channel that allows the permeation of compounds with Mr <1,000 as revealed by permeability measurements using proteoliposomes reconstituted with purified CppS and CppF. Unexpectedly, amino acid sequence analysis revealed no evolutionary linkage to cyanobacteria, showing only a moderate similarity to cell surface proteins of bacteria belonging to Planctomycetes phylum. Our findings suggest that the C. paradoxa cyanelle adopted non-cyanobacterial lineage proteins as its main outer membrane components, providing a physical link with the underlying peptidoglycan layer and functioning as a diffusion route for various small substances across the outer membrane.
authors: From the Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan and Graduate School of Life Sciences, Tohoku University, Sendai 980-8577, Japan skojima0801@gmail.com.
authors: Seiji
authors: S
authors: Kojima
authors: Graduate School of Life Sciences, Tohoku University, Sendai 980-8577, Japan.
authors: Koji
authors: K
authors: Muramoto
authors: Graduate School of Life Sciences, Tohoku University, Sendai 980-8577, Japan.
authors: Tomonobu
authors: T
authors: Kusano
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism*; D044424:Cyanophora / Q000235:genetics / Q000378:metabolism*; D010457:Peptidoglycan / Q000235:genetics / Q000378:metabolism*
chemical_list: D001425:Bacterial Outer Membrane Proteins; D010457:Peptidoglycan
keywords: Gram-negative bacteria; chloroplast; cyanelle; cyanobacteria; diffusion channel; outer membrane; peptidoglycan