Proximity proteomics in a marine diatom reveals a putative cell surface-to-chloroplast iron trafficking pathway.

Jernej Turnšek,Maria Del Pilar Martinez Viedma,Andrew Ellis Allen,John K Brunson,Aleš Horák,Thomas J Deerinck,Miroslav Oborník,Vincent A Bielinski

doi:10.7554/elife.52770

Abstract

Iron is a biochemically critical metal cofactor in enzymes involved in photosynthesis, cellular respiration, nitrate assimilation, nitrogen fixation, and reactive oxygen species defense. Marine microeukaryotes have evolved a phytotransferrin-based iron uptake system to cope with iron scarcity, a major factor limiting primary productivity in the global ocean. Diatom phytotransferrin is endocytosed; however, proteins downstream of this environmentally ubiquitous iron receptor are unknown. We applied engineered ascorbate peroxidase APEX2-based subcellular proteomics to catalog proximal proteins of phytotransferrin in the model marine diatom Phaeodactylum tricornutum. Proteins encoded by poorly characterized iron-sensitive genes were identified including three that are expressed from a chromosomal gene cluster. Two of them showed unambiguous colocalization with phytotransferrin adjacent to the chloroplast. Further phylogenetic, domain, and biochemical analyses suggest their involvement in intracellular iron processing. Proximity proteomics holds enormous potential to glean new insights into iron acquisition pathways and beyond in these evolutionarily, ecologically, and biotechnologically important microalgae.

Highlights

Iron (Fe) likely played an important role in the origin of life (Bonfio et al, 2017; Jin et al, 2018) and is fundamental to almost all extant metabolisms (Aguirre et al, 2012) serving as a cofactor in enzymes involved in DNA replication, photosynthesis, cellular respiration, nitrate assimilation, nitrogen fixation, and reactive oxygen species defense (Crichton, 2016)
We provide further evidence for P. tricornutum phytotransferrin pTF endocytosis, demonstrate correct subcellular localization and activity of its APEX2 fusion, and identify nearly 40 likely proximal proteins through a proximity-dependent proteomic mapping experiment conducted in quintuplicate
APEX2 fused with P. tricornutum phytotransferrin is enzymatically active in vivo

Summary

Introduction

Iron (Fe) likely played an important role in the origin of life (Bonfio et al, 2017; Jin et al, 2018) and is fundamental to almost all extant metabolisms (Aguirre et al, 2012) serving as a cofactor in enzymes involved in DNA replication, photosynthesis, cellular respiration, nitrate assimilation, nitrogen fixation, and reactive oxygen species defense (Crichton, 2016). Anoxic oceans were rich in readily bioavailable ferrous, Fe2+, iron, but with the rise of oxygenic photosynthesis and the Great Oxygenation Event (GOE) in the Paleoproterozoic ~2.3 billion years ago followed by the Neoproterozoic Oxygenation Event (NOE) ~1.5 billion years later, most of the ferrous iron oxidized into insoluble ferric, Fe3+, minerals which are not bioavailable (Camacho et al, 2017; Knoll et al, 2017; Och and Shields-Zhou, 2012) These large global shifts in ocean chemistry could have had a major role in driving the evolution of novel iron uptake mechanisms. Phytotransferrin sequences have a broad taxonomic distribution and are abundant in marine transcriptomic datasets (Bertrand et al, 2015; Marchetti et al, 2012)

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Conclusion