Balancing cellular copper levels via the post-translational regulation of copper transport.

Copper (Cu) is essential to life as an enzymatic cofactor, powering a plethora of biological processes. However, cells teeter on a delicate balance where both too much and too little Cu can lead to dysfunction. These risks are exemplified by Menkes disease patients, where Cu deficiency leads to neurodevelopmental delay and early childhood mortality, and those affected by Wilson's disease, where Cu overload results in liver damage, behavioural changes and movement disorders. Cu is also a major cellular stressor, with roles in many other pathologies including neurodegeneration and cancer. Consequently, cells tightly regulate Cu load by highly conserved import, export, and distribution mechanisms. While these have been well documented, less is known about how cells sense and respond to Cu deviations. This function may lie in the extensive post-translational modifications (PTMs) that control the Cu transport proteins, where a chorus of glycosylation, phosphorylation, and ubiquitination affects their activity, intracellular trafficking, and stability. Cu also affects the machinery that applies these PTMs, providing a potential mechanism by which cells 'sense' Cu levels. This review explores this hypothesis, examining our current knowledge of how PTMs are modulated by Cu, and how they control the responses of the Cu transporters to fluctuating Cu load. Further probing of how cells sense and respond to Cu deviations may aid the pursuit of effective diagnosis and treatment of the many disorders associated with mismanaged Cu levels.