The thermal denaturation of a 31-kDa soluble fragment derived from the Escherichia coli aspartate receptor cytoplasmic region (c-fragment) was found to be reversible. Denaturation monitored by differential scanning calorimetry (DSC) and circular dichroism (CD) was typically over 90% reversible in pH 7.0 buffer. The wild-type c-fragment exhibited one transition (Tm = 51 °C), which was taken as the main denaturation transition. c-Fragments derived from signaling mutants, shown to form oligomers by gel filtration chromatography (GFC), displayed a second low-temperature transition that correlated with the disappearance of the oligomeric form in the GFC traces over the same temperature range. The CD and DSC experiments also indicated that oligomers were more folded than monomers, observations that may provide an explanation for the structural basis of the smooth-swimming signaling state of the receptor. Octyl glucoside (OG), phospholipid (PL), and glycerol were added to characterize factors that contribute to c-fragment stability. At 10 mg/mL OG, the van't Hoff enthalpy of unfolding was reduced ca. 10-fold, although at room temperature the CD spectrum indicated little change in the secondary structure. The van't Hoff enthalpy was not affected by 35% (w/v) glycerol, but the Tm increased by ca. 18 °C. Cooperative transitions were detected in buffer containing OG, PL, and glycerol (10 mg/mL, 2 mg/mL, 35%, respectively). The correlation between conditions where cooperative transitions are observed, and where aspartate-modulated receptor signaling has been previously observed, provides an explanation for the inhibition of signaling in OG-containing buffers without glycerol and PL.
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