Abstract
BackgroundCyanelles, the peptidoglycan-armored plastids of glaucocystophytes, occupy a unique bridge position in between free-living cyanobacteria and chloroplasts. In some respects they side with cyanobacteria whereas other features are clearly shared with chloroplasts. The Sec translocase, an example for "conservative sorting" in the course of evolution, is found in the plasma membrane of all prokaryotes, in the thylakoid membrane of chloroplasts and in both these membrane types of cyanobacteria.ResultsIn this paper we present evidence for a dual location of the Sec translocon in the thylakoid as well as inner envelope membranes of the cyanelles from Cyanophora paradoxa, i. e. conservative sorting sensu stricto. The prerequisite was the generation of specific antisera directed against cyanelle SecY that allowed immunodetection of the protein on SDS gels from both membrane types separated by sucrose density gradient floatation centrifugation. Immunoblotting of blue-native gels yielded positive but differential results for both the thylakoid and envelope Sec complexes, respectively. In addition, heterologous antisera directed against components of the Toc/Tic translocons and binding of a labeled precursor protein were used to discriminate between inner and outer envelope membranes.ConclusionThe envelope translocase can be envisaged as a prokaryotic feature missing in higher plant chloroplasts but retained in cyanelles, likely for protein transport to the periplasm. Candidate passengers are cytochrome c6 and enzymes of peptidoglycan metabolism. The minimal set of subunits of the Toc/Tic translocase of a primitive plastid is proposed.
Highlights
Cyanelles, the peptidoglycan-armored plastids of glaucocystophytes, occupy a unique bridge position in between free-living cyanobacteria and chloroplasts
Thylakoid membranes appeared in the 45% layer and the inner envelope membrane band was found between 39% and 15% sucrose
It was mandatory that thylakoid membranes and inner envelope membranes were subjected to western blotting with antisera directed against constituents of the Sec translocase of cyanelles
Summary
The peptidoglycan-armored plastids of glaucocystophytes, occupy a unique bridge position in between free-living cyanobacteria and chloroplasts. In some respects they side with cyanobacteria whereas other features are clearly shared with chloroplasts. The core of the translocase consists of SecY, SecE, and SecG [1]. This SecYEG complex and the hydrophilic protein SecA, which acts as the translocation. During the integration step of membrane proteins, YidC (a bacterial homolog of mitochondrial Oxa and chloroplast Alb3) is required, eventually in addition to the Sec translocase [2,3]. A Sec-specific (page number not for citation purposes)
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