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The unusual properties of the photosynthetic cytochrome c550 from the red algal lineage evidence additional biological roles

ABSTRACT: Cytochrome c550 is an extrinsic component in the luminal side of photosystem II in cyanobacteria, as well as in eukaryotic algae from the red photosynthetic lineage including, among others, diatoms. By using strong and specific protease inhibitors we have confirmed that cytochrome c550 from the diatom Phaeodactylum tricornutum is expressed as a truncated form lacking the last hydrophobic residues at the C-terminal.. Phaeodactylum tricornutum cytochrome c550 seems to be modified at a post-translational level as the gene is appropriately transcribed. In addition, co-immunoprecipitation and mass spectrometry experiments have allowed the identification of novel interactions of Phaeodactylum tricornutum cytochrome c550 with proteins related to the CO2 concentrating mechanism and assimilation located in the chloroplast pyrenoid. Furthermore, immunoelectron microscopy analysis indicates that although cytochrome c550 is mainly located in the thylakoid domain of the chloroplast, it can also be found in the pyrenoid. The results are interpreted in terms of a specific protein modification associated to a higher PSII turnover or to new alternative functions of the heme protein, maybe related to carbon fixation.

INSTRUMENT(S): TripleTOF 5600

ORGANISM(S): Phaeodactylum Tricornutum

SUBMITTER: Jose Antonio Navarro

LAB HEAD: Jose Antonio Navarro Carruesco

PROVIDER: PXD008763 | Pride | 2022-03-03

REPOSITORIES: Pride

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Project description:The slow kinetics and poor substrate specificity of the key photosynthetic CO2-fixing enzyme Rubisco have prompted the repeated evolution of Rubisco containing compartments known as pyrenoids in diverse algal lineages and carboxysomes in prokaryotes. Inside these compartments actively transported bicarbonate is converted into CO2 gas, which saturates the carboxylase with its substrate. Using co-immunoprecipitation experiments in Phaeodactylum tricornutum we have identified the Rubisco linker protein PYCO1. Similar to the green algal Rubisco linker protein EPYC1, PYCO1 is intrinsically disordered, possesses repeats and is positively charged at physiological pH. However, it possesses no sequence similarity to EPYC1, as expected for convergent evolution of a red Rubisco containing pyrenoid. Fluorescent PYCO1 fusion proteins localize as a rod shaped structure in the diatom chloroplast, consistent with the shape of the pyrenoid defined by transmission electron microscopy. To test the hypothesis that PYCO1 is the diatom pyrenoid scaffold we produced pure protein in Escherichia coli. Recombinant PYCO1 protein undergoes homotypic liquid liquid phase separation in a salt dependent manner. Diatom Rubisco specifically partitions into PYCO1 condensates. Heterotypic PYCO1-Rubisco condensates can bind up to three Rubisco hexadecamers per PYCO1 protein. Rubisco carboxylase function is unaffected in the condensates. PYCO1 is highly mobile in homotypic condensates. In contrast PYCO1 condensates saturated with diatom Rubisco have greatly reduced dynamics, with both PYCO1 and Rubisco becoming immobile. Consistently, FRAP experiments indicate that PYCO1 is not mobile in vivo. A combination of Cryo-electron microscopy and site-directed mutagenesis data show that the KWSP motif found in PYCO1 repeats binds to small subunits at the entrance of the Rubisco hexadecamer’s solvent channel. Analysis of mutant PYCO1 proteins show that both the “KWSP” tryptophan and another repeating tyrosine are essential for homotypic phase separation. We speculate that the unusual material properties of the PYCO1-Rubisco condensate are necessary to support the unusual non-spherical shape of the Phaeodactylum pyrenoid. Careful characterization of multiple diverse Rubisco condensates will strengthen translational approaches aiming to introduce pyrenoids and other metabolic condensates into new host organisms.

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The unusual properties of the photosynthetic cytochrome c550 from the red algal lineage evidence additional biological roles

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