Chlamydomonas ATX1 is essential for Cu distribution towards the secretory pathway and maintenance of biomass in conditions demanding cupro-enzyme dependent metabolic pathways

Copper (Cu) chaperones, of which yeast ATX1 is a prototype, are small proteins with a Cu(I) binding Mx-CxxC motif, and are responsible for directing intracellular Cu towards specific client protein targets that use Cu as a cofactor. The Chlamydomonas reinhardtii ATX1 (CrATX1) was identified because of its high sequence similarity with yeast ATX1. Like the yeast homologue, CrATX1 accumulates in iron-deficient cells (but is not impacted by other metal-deficiencies), and YFP-ATX1 is distributed in the cytoplasm. Reverse genetic analysis using artificial microRNA (amiRNA) to generate lines with reduced CrATX1 abundance and CRISPR/CPF1 to generate ATX1 knock out lines validated a function for ATX1 in iron-poor cells, most likely because of an impact on metalation of the multicopper oxidase FOX1, which is an important component in high-affinity iron uptake. A more general impact on the secretory pathway is indicated by reduced growth of ATX1 mutant lines on guanine as a sole nitrogen source, which we attribute to loss of function of UOX1, a urate oxidase involved in guanine assimilation. The block of Cu trafficking towards the secretory pathway in ATX1 mutants is strikingly evident by a reduced amount of intracellular Cu in all conditions probed in this work.