Lhcx proteins in centric diatoms
Final Report Abstract
Diatoms are photosynthetic, unicellular algae only distantly related to higher plants. Although the proteins responsible for the collection of light energy, called fucoxanthin-chlorophyll proteins (FCP) due to their pigmentation, belong to the same protein family as those of higher plants, their supramolecular organisation and pigmentation differs. Like all photosynthetic organisms, diatoms need a mechanism to protect themselves against too much excitation energy, which is called non-photochemical quenching (NPQ). Members of the FCP family, the Lhcx proteins, were shown to be involved in this mechanism. Centric diatoms like Thalassiosira pseudonana and Cyclotella meneghiniana express several Lhcx proteins, whereby for Lhcx1 it had already been shown that it is a subunit of one of the major FCP complexes called FCPa. The localisation and function of the different Lhcx proteins was the focus of this project. For wild-type C. meneghiniana we could demonstrate two membrane domains consisting of different FCP complexes. One domain was characterised by complexes containing Lhcx1 subunits, whereby the amount was increased when cells were grown under high-light conditions. The second area was devoid of Lhcx1 containing complexes, but both domains contained complexes with Lhcx6_1. Due to this analysis we could establish that not only the trimeric FCPa complexes contain Lhcx subunits, but that a second, previously unknown FCPb complex is present that contains Lhcx proteins as well. We elucidated the function of Lhcx1 further using knock-down mutants and could show that it plays a more structural role, enabling the aggregation of FCPa complexes that is needed for NPQ. We could also demonstrate that the complexes change their spectroscopic properties under NPQ conditions, whereby energy is converted to heat instead of fluorescence because fluorescence emitting Chl a molecules change to more red-emitting species with a low fluorescence yield. We could further show that this is enabled by an extreme flexibility of the FCPa complexes, whereby fluctuations in the protein environment lead to switches in the spectral properties of the Chl a molecules, enabling efficient excitation energy transfer but also fluorescence quenching. This is supported by binding of additional diatoxanthin molecules that contribute to the quenching abilities. Concerning the other Lhcx proteins, for T. pseudonana the approach of using knock-down mutants failed since knocking down one Lhcx gene led to an unexpectedly strong up-regulation of the remaining ones. Due to this, most knock-down mutants displayed an even stronger NPQ, probably due to over-compensation. This is a strong hint for the importance of Lhcx4, Lhcx6 and Lhcx6_1 in T. pseudonana, but hampered further analysis of single effects. Since now two FCPb complexes are known, the question of the precise oligomeric state got more important. It had been known that FCPbs contain more subunits than the trimeric FCPa, but the nonameric state had been only a hypothesis. Using electron microscopy based method we could now demonstrate that the FCPb complexes that are a peculiarity of centric diatoms are indeed nonamers.
Publications
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How reduced excitonic coupling enhances light harvesting in the main photosynthetic antennae of diatoms. Proc. Natl. Acad. Sci. USA 114 (52), E11063-E11071 (2017)
Krüger TPJ, Malý P, Alexandre MTA, Mančal T, Büchel C, van Grondelle R
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The structure of FCPb, a light-harvesting complex in the diatom Cyclotella meneghiniana. Photosynth. Res. 135 (1), 203-211 (2018)
Röding A, Boekema E, Büchel C
