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Daniela Duy, Jrgen Soll, Katrin Philippar

Solute channels of the outer membrane: from bacteria to chloroplasts

Keywords: endosymbiosis, metabolite channel, outer envelope protein, plant, plastid, porin

Chloroplasts, unique organelles of plants, originated by endosymbiosis of an ancestor of today's cyanobacteria with a mitochondria-containing host cell. It is assumed that the outer envelope membrane, which delimits the chloroplast from the surrounding cytosol, was thereby inherited from its Gram-negative bacterial ancestor. This plastid-specific membrane is thus equipped with elements of prokaryotic and eukaryotic origin. In particular, the membraneintrinsic outer envelope proteins (OEPs) form solute channels with properties reminiscent of porins and channels in the bacterial outer membrane. OEP channels are characterised by distinct specificities for metabolites and a quite peculiar expression pattern in specialised plant organs and plastids, and thus disprove the assumption that the outer envelope is a non-specific molecular sieve. The same is true for the outer membrane of Gram-negative bacteria, which functions as permeability barrier in addition to the cytoplasmic membrane, and embeds different classes of channel pores. The channels of these prokaryotic prototype proteins, ranging from unspecific porins, specific channels to ligand-gated receptors, are exclusively built by ß-barrels. Although most of the OEP channels are formed by ß-strands as well, a phylogeny based on sequence homology alone is not feasible. Thus, the comparison of structural and functional properties of chloroplast outer envelope and bacterial outer membrane channels is required to pinpoint the ancestral 'OEP portrait galler'

Biological Chemistry, Walter de Gruyter

Print ISSN: 1431-6730
Volume: 2007
Pages: -

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