An intrinsically disordered domain in Polaribacter irgensii KOPRI 22228 CspB confers extraordinary freeze-tolerance

Organisms living in extremely cold environments possess mechanisms to survive low temperatures. Among the known cold-induced genes, cold-shock proteins (Csps) are the most prominent. A csp-homologous gene, cspBPi, has been cloned from the Arctic bacterium Polaribacter irgensii KOPRI 22228, and overexpression of this gene greatly increased the freezing tolerance of its host. This protein consists of a unique N-terminal domain and a well conserved C-terminal cold shock domain. To elucidate the detailed mechanisms involved in the extraordinary freeze-tolerance conferred by CspBPi, we identified the responsible domain by mutational analysis. Changes of residues in the cold shock domain that are crucial for binding RNA or single-stranded DNA did not impair the ability of the host to survive freezing stress. All domain-shuffled CspBPi variants containing the N-terminal domain retained the ability to confer superior freeze-tolerance. Slow electrophoretic mobility and far-UV circular dichroism spectra of the N-terminal domain suggested an intrinsically disordered structure for this region. The N-terminal domain also bound to lipid vesicles in vitro. This lipid vesicle binding characteristic is shared with other intrinsically disordered proteins, such as α-synuclein and plant dehydrins, known to confer cold-tolerance when overexpressed, suggesting a mechanism for cold-survival through membrane binding.