Identification of a putative Zn metallochaperone (ZCRP-A) in multiple marine diatoms and characterization in Phaeodactylum tricornutum

Dissolved concentrations of the essential trace element Zinc (Zn) in surface waters of the open ocean can reach as low as ~0.1nM, suggesting that low Zn availability may co-limit the growth of oceanic phytoplankton in some environments. Zn is particularly important in the operation of the carbon concentrating mechanism (CCM) in photosynthetic algae, as carbonic anhydrase (CA) enzymes utilize Zn²⁺ as a catalytic cofactor during the conversion of bicarbonate to carbon dioxide in the vicinity of Rubisco, a key enzyme in the Calvin cycle. Recent phylogenetic and genomic analyses of the largely uncharacterized and diverse COG0523 subfamily of GTPases has suggested that some COG0523 proteins are linked to Zn homeostasis pathways, although experimental evidence, especially in eukaryotes, is lacking. In this study, we identify homologs of a novel COG0523 protein in the marine diatoms Thalassiosira pseudonana CCMP1335, Phaeodactylum tricornutum CCMP632, Pseudo-nitzschia delicatissima UNC1205, and Chaetoceros sp. RS-19 that became abundant in all diatoms under Zn and Co limitation, but became scarce under replete Zn or Co conditions. As the expression of this protein is responsive to Zn and Co, we refer to it as ZCRP-A (Zn Co Responsive Protein A). To characterize this unknown protein, we have created zcrp-a knockout (KO) and zcrp-a overexpression lines of P. tricornutum and compared these genetically modified cell lines to the wild-type using proteomic analyses of Zn-limited cultures.