Critical interactions in the stability control region of tropomyosin

Our laboratory has recently described a stability control region in the two-stranded α-helical coiled-coil α-tropomyosin that accounts for overall protein stability but is not required for folding (Hodges et al., 2009). We have used a synthetic peptide approach to investigate three stability control sites within the stability control region (residues 97-118). Two of the sites, electrostatic cluster 1 (97-104, EELDRAQE) and electrostatic cluster 2 (112-118, KLEEAEK), feature sequences with unusually high charge density and the potential to form multiple intrachain and interchain salt bridges (ionic attractions). A third site (105-111, RLATALQ) features an e position Leu residue, an arrangement known previously to enhance coiled-coil stability modestly. A native peptide and seven peptide analogs of the tropomyosin sequence 85-119 were prepared by Fmoc solid-phase peptide synthesis. Thermal stability measurements by circular dichroism (CD) spectroscopy revealed the following Tm values for the native peptide and three key analogs: 52.9 °C (Native), 46.0 °C (R101A), 45.3 °C (K112A/K118A), and 27.9 °C (L110A). The corresponding ΔTm values for the analogs, relative to the native peptide, are −6.9 °C, −7.6 °C, and −25.0 °C, respectively. The dramatic contribution to stability made by L110e is three times greater than the contribution of either electrostatic cluster 1 or 2, likely resulting from a novel hydrophobic interaction not previously observed. These thermal stability results were corroborated by temperature profiling analyses using reversed-phase high-performance liquid chromatography (RP-HPLC). We believe that the combined contributions of the interactions within the three stability control sites are responsible for the effect of the stability control region in tropomyosin, with the Leu110e contribution being most critical.