Details of the functional mechanisms of intrinsically disordered proteins (IDPs) in
living cells is an area not frequently investigated. Here, we dissect the molecular
mechanism of action of an IDP in cells by detailed structural analyses based on an
in-cell nuclear magnetic resonance experiment. We show that the ID stress protein
(IDSP) A. thaliana Early Response to Dehydration (ERD14) is capable of protecting
E. coli cells under heat stress. The overexpression of ERD14 increases the viability
of E. coli cells from 38.9% to 73.9% following heat stress (50 °C × 15 min). We also
provide evidence that the protection is mainly achieved by protecting the proteome
of the cells. In-cell NMR experiments performed in E. coli cells show that the protective
activity is associated with a largely disordered structural state with conserved,
short sequence motifs (K- and H-segments), which transiently sample helical conformations
in vitro and engage in partner binding in vivo. Other regions of the protein, such
as its S segment and its regions linking and flanking the binding motifs, remain unbound
and disordered in the cell. Our data suggest that the cellular function of ERD14 is
compatible with its residual structural disorder in vivo.
