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 cyanobacterial IDP, was characterized from the nitrogen-fixing cyanobacterium Anabaena. 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, a highly thermostable protein, exhibited characteristic properties of highly disordered proteins, including anomalous migration on SDS-PAGE. Alr0806 displayed disorder-to-order transitions with decreasing pH, indicating structural flexibility. Notably, Alr0806 leveraged structural plasticity to function as a chaperone and molecular shield, protecting proteins from aggregation. Furthermore, consistent with this function, Anabaena strains deficient in Alr0806 showed compromised growth and diminished photosynthesis under standard conditions of growth or in response to salt/heat stress. These findings establish Alr0806 as a key player in cyanobacterial physiology and provide insights into the physiological functions of IDPs in photosynthetic organisms.
Hurali, D. T., Ballal, A., Banerjee, M.
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