Involvement of the Lhcx protein Fcp6 of the diatom Cyclotella meneghiniana in the macro-organization and structural flexibility of thylakoid membranes

Artur Ghazaryana, Parveen Akhtarb, Gyozo Garabb, Petar H. Lambrevb, Claudia Büchela, supporting author(s)b
aInstitute of Molecular Biosciences, Goethe University, Max von Laue Str 9, 60438 Frankfurt, Germany; bHungarian Academy of Sciences, Biological Research Centre, Temesvári krt. 62, 6726 Szeged, Hungary

Diatoms are unicellular, eukaryotic algae that possess membrane intrinsic light-harvesting complexes called fucoxanthin-chlorophyll complexes (FCP) due to their pigmentation. The photoprotection mechanism of energy-dependent non-photochemical quenching (NPQ) is present in diatoms as well, and depends on special light-harvesting proteins, Lhcx. These Lhcx proteins were shown to be subunits of the main trimeric FCPa complex in centric diatoms, and the fluorescene yield of isolated complexes depends on the pH, the diatoxanthin content and the aggregation state [1], whereby the two former to factors are also important for NPQ in vivo. The quenched state of FCPa is in addition characterised by the appearance of a long wavelength emission band of low yield [2]. To get further insight into the role of Lhcx proteins, we investigated the influence of Fcp6, an orthologue to Lhcx1 of Thalassiosira pseudonana in the diatom Cyclotella meneghiniana, by reducing its amount using an antisense approach.

Knock-down mutants showed reduced NPQ, but FCP levels stayed the same, except for Fcp6 which was significantly reduced. Whereas the pigment interactions inside FCPa were not influenced by the presence or absence of Fcp6, as demonstrated by unaltered spectra of circular dichroism, changes could be observed on the level of thylakoids and cells in the mutants compared to WT [3]. This fits to recent models of NPQ in diatoms, where FCP aggregation or supramolecular reorganisation is thought to be a major feature. Thus, Fcp6 (Lhcx1) appears to alter pigment-pigment interactions inside FCP aggregates, but not inside (un-aggregated) FCPa itself.

References

[1] Gundermann K, Büchel C, Biochimica Biophysica Acta 2012, 1817, 1044-1052.
[2] Wahadoszamen M et al., Biochimica Biophysica Acta 2013, 1837, 193-200.
[3] Ghazaryan A et al., Biochimica Biophysica Acta 2016, in press.