Figure 10: Proposed model for Cu homeostasis during eukaryogenesis.

Eukaryotes are thought to have evolved from ancient archaea that had initially differentiated to gain a nuclear membrane and acquire endocytic machinery. Thereafter, the endocytosis of an α-proteobacterium started a symbiotic relationship between the two organisms that resulted in permanent innovation, with retention of the α-proteobacterium as a mitochondrion and differentiation into a multi-organelle protoeukaryote. During its adaptation and development, we speculate that COX was one of the major Cu requirements for the protoeukaryote given its role as a co-factor in the primitive enzyme. As its evolution continued, numerous intermediate steps selected for other Cu-requiring processes and the metallochaperones required to enhance their activity and Cu recruitment. The exact order of gains and losses has not been investigated and therefore is shown here with dashed arrows. One of the final steps based on preliminary investigation of diverse genomes from the tree of life was the acquisition of a high affinity Cu transport system at the plasma membrane to fulfill these Cu needs. We propose that to maintain prioritization of Cu for mitochondria, the protoeukaryote then refined the hierarchical regulation of Cu handling around signals originating from the organelle.