Cooperative Subunit Refolding of a Light‐Harvesting Protein through a Self‐Chaperone Mechanism

The fold of a protein is encoded by its amino acid sequence, but how complex multimeric proteins fold and assemble into functional quaternary structures remains unclear. Here we show that two structurally different phycobiliproteins refold and reassemble in a cooperative manner from their unfolded polypeptide subunits, without biological chaperones. Refolding was confirmed by ultrafast broadband transient absorption and two‐dimensional electronic spectroscopy to probe internal chromophores as a marker of quaternary structure. Our results demonstrate a cooperative, self‐chaperone refolding mechanism, whereby the β‐subunits independently refold, thereby templating the folding of the α‐subunits, which then chaperone the assembly of the native complex, quantitatively returning all coherences. Our results indicate that subunit self‐chaperoning is a robust mechanism for heteromeric protein folding and assembly that could also be applied in self‐assembled synthetic hierarchical systems. Assisting each other: The folding and formation of quaternary proteins is currently poorly understood. The subunits in a hetero‐oligomeric light‐harvesting protein have now been shown to assist one another in folding: the large subunits (β) fold the small subunits (α) to form an α/β dimer, while the small subunits chaperone and prevent aggregation of the large subunits. Two α/β dimers then assemble into the heterodimeric protein.