Molecular insights into the role of a disordered chaperone-like protein "Alr0806" in cyanobacterial stress responses.

Intrinsically Disordered Proteins (IDPs) lack a stable tertiary structure, which enables them to mediate flexible molecular interactions. As the biochemical functions of IDPs remain poorly understood, their physiological roles are largely unknown, particularly in photosynthetic organisms. Herein, Alr0806, a conserved salinity-induced putative IDP from the nitrogen-fixing cyanobacterium Anabaena was functionally characterized. Instead of the full-length protein predicted in databases, experimental analysis indicated this organism to express a shorter form of the Alr0806 protein, which was attributed to the mis-annotation of the translational start codon. Purified Alr0806, which was a highly thermostable protein, exhibited characteristic properties of highly disordered proteins, including anomalous migration on SDS-PAGE. Interestingly, CD analysis of Alr0806 revealed an increase in α-helicity at lower pH, suggesting a possible conformational plasticity, a property associated with IDPs. Notably, Alr0806 functioned as a chaperone-like protein and molecular shield, protecting proteins from aggregation. Furthermore, Anabaena strains deficient in Alr0806 showed compromised growth and diminished photosynthesis under standard growth conditions or in response to salt/heat stress. These findings provide critical insights into the chaperone-like activity of disordered proteins and enhance our understanding of the macromolecular adaptations that drive cyanobacterial survival under environmental stress.