Abstract
BackgroundAlthough our knowledge about diatom photosynthesis has made huge progress over the last years, many aspects about their photosynthetic apparatus are still enigmatic. According to published data, the spatial organization as well as the biochemical composition of diatom thylakoid membranes is significantly different from that of higher plants.ResultsIn this study the pigment protein complexes of the diatom Thalassiosira pseudonana were isolated by anion exchange chromatography. A step gradient was used for the elution process, yielding five well-separated pigment protein fractions which were characterized in detail. The isolation of photosystem (PS) core complex fractions, which contained fucoxanthin chlorophyll proteins (FCPs), enabled the differentiation between different FCP complexes: FCP complexes which were more closely associated with the PSI and PSII core complexes and FCP complexes which built-up the peripheral antenna. Analysis by mass spectrometry showed that the FCP complexes associated with the PSI and PSII core complexes contained various Lhcf proteins, including Lhcf1, Lhcf2, Lhcf4, Lhcf5, Lhcf6, Lhcf8 and Lhcf9 proteins, while the peripheral FCP complexes were exclusively composed of Lhcf8 and Lhcf9. Lhcr proteins, namely Lhcr1, Lhcr3 and Lhcr14, were identified in fractions containing subunits of the PSI core complex. Lhcx1, Lhcx2 and Lhcx5 proteins co-eluted with PSII protein subunits. The first fraction contained an additional Lhcx protein, Lhcx6_1, and was furthermore characterized by high concentrations of photoprotective xanthophyll cycle pigments.ConclusionThe results of the present study corroborate existing data, like the observation of a PSI-specific antenna complex in diatoms composed of Lhcr proteins. They complement other data, like e.g. on the protein composition of the 21 kDa FCP band or the Lhcf composition of FCPa and FCPb complexes. They also provide interesting new information, like the presence of the enzyme diadinoxanthin de-epoxidase in the Lhcx-containing PSII fraction, which might be relevant for the process of non-photochemical quenching. Finally, the high negative charge of the main FCP fraction may play a role in the organization and structure of the native diatom thylakoid membrane. Thus, the results present an important contribution to our understanding of the complex nature of the diatom antenna system.
Highlights
Our knowledge about diatom photosynthesis has made huge progress over the last years, many aspects about their photosynthetic apparatus are still enigmatic
Further evidence for the spatial separation of Photosystem II (PSII) and Photosystem I (PSI) stems from the work of Bina et al [4] who showed that the thylakoid membranes of the pennate diatom P. tricornutum contain large areas which are exclusively occupied by a supercomplex of PSI with its associated antenna composed of Lhcr proteins when the algae are cultivated with red light of a low light intensity
They corroborate existing data like the observation of a PSI-specific antenna complex in diatoms composed of Lhcr proteins
Summary
Our knowledge about diatom photosynthesis has made huge progress over the last years, many aspects about their photosynthetic apparatus are still enigmatic. The photosynthetic pigment protein complexes comprising the photosystem (PS) II and PSI core complexes with their specific light-harvesting complexes (LHC) are embedded into the thylakoid membrane. Despite the regular arrangement recent results have proposed that a heterogeneous distribution of PSII and PSI, as it exists in the grana and stroma membranes, is present in the diatom thylakoid membranes. According to the model of Lepetit et al [22] PSI with its specific FCP complex is mainly located in the peripheral membrane regions together with an enrichment of the negatively charged membrane lipid sulfoquinovosyldiacylglycerol (SQDG). In accordance with the model of Lepetit et al [22] they propose that PSII is located in the core membranes whereas PSI is enriched in the peripheral, stroma-facing membranes
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