Origin of chaperone dependence and assembly complexity in Rubisco’s biogenesis

Molecular chaperones assist with the folding and assembly of protein clients. Consequently, they are essential to diverse cellular functions. In most aerobic photosynthetic organisms such as β-cyanobacteria and all plants, the key CO₂-fixing enzyme Rubisco strictly relies on dedicated chaperones for its assembly. Why these chaperones were recruited is unclear, as they are absent and not required in other prokaryotic autotrophs. By retracing the evolution of Rubisco-chaperone interactions, we find that Rubisco’s dependence on chaperones evolved neutrally and not by adaptive optimization, and that this may be reversed without compromising catalytic function. Our work shows how chaperones are broad modifiers of their clients’ sequence spaces that can become totally essential through non-adaptive processes.