FlgM gains structure in living cells

Abstract

Intrinsically disordered proteins such as FlgM play important roles in biology, but little is known about their structure in cells. We use NMR to show that FlgM gains structure inside living Escherichia coli cells and under physiologically relevant conditions in vitro, i.e., in solutions containing high concentrations (>/=400 g/liter) of glucose, BSA, or ovalbumin. Structure formation represents solute-induced changes in the equilibrium between the structured and disordered forms of FlgM. The results provide insight into how the environment of intrinsically disordered proteins could dictate their structure and, in turn, emphasize the relevance of studying proteins in living cells and in vitro under physiologically realistic conditions.

Figure 1

FlgM is structured in E. coli. The HSQC spectrum of FlgM in dilute solution (Left) and in living E. coli (Right). Red brackets surround some of the crosspeaks from the C-terminal half in FlgM that disappear in E. coli. The number near each bracket is the residue number (8). Dilute solution sample: 400 μM FlgM/10 mM sodium acetate, pH 5.0/10 mM NaCl/0.02% NaN₃/10% (vol/vol) D₂O at 298 K. The E. coli spectrum was acquired as described by Serber et al. (9).

Figure 2

Glucose induces structure in FlgM. Overlaid HSQC spectra of FlgM in dilute solution (red) and in 450 g/liter 2.5 M glucose (black). Conditions: 400 μM FlgM/10 mM sodium acetate, pH 5.0/10 mM NaCl/0.02% NaN₃/10% (vol/vol) D₂O at 298 K.

Figure 3

Glucose induces α-helix in FlgM. Far-UV CD spectra of 15 μM FlgM in dilute solution (solid line) and in 2.5 M (450 g/liter) glucose (dotted line). Both samples contain 10 mM sodium acetate (pH 5.0), 0.02% NaN₃.

Figure 4

BSA induces structure in FlgM. HSQC spectrum of FlgM in 400 g/liter BSA. Conditions: 400 μM FlgM/10 mM sodium acetate, pH 5.0/10 mM NaCl/0.02% NaN₃/10% (vol/vol) D₂O at 298 K.