Fig. 6: Spy inhibits the folding of flavodoxin-fold substrates by acting as a holdase.
From: Mechanism of the small ATP-independent chaperone Spy is substrate specific
a Fluorescence traces of the refolding of 0.1 μM AzoFld C69A in the presence of various concentrations of Spy dimer (0–182.6 μM after mixing) in KP buffer. Time axis is shown in logarithmic scale. b A plot of the observed major rate constant for AzoFld refolding (kobs) as a function of Spy dimer concentration. c Mechanism of substrate-specific action of Spy. Simple substrates can fold while being continuously chaperone-bound because Spy binds the native (N) and unfolded (U) states with weak affinities. Even at saturating concentrations of Spy, folding occurs because the N–Spy complex is the most energetically stable form of the substrate. However, large and topologically complex substrates cannot fold while bound because of Spy’s strong affinity for (partially) unfolded states (denoted generically by U†). For such substrates, Spy acts as a holdase by sequestering aggregation-prone U† states. Kinetically, such holdase-like action makes Spy a competitive inhibitor of folding because the U†–Spy complex is the most energetically stable form of the substrate. Source data are provided as a Source Data file.
