A conserved C-terminal extension stabilizes Msps TOG3.
A, Msps TOG3 has a 12-residue C-terminal extension that makes multiple contacts with the ultimate four HRs. B, CD spectra of native (green) and point mutant R816A (purple) Msps TOG3 constructs indicate α-helical character at 20 °C, as indicated by signature minima at 208 and 222 nm. C, CD thermal melts of native and R816A TOG3 were taken in 1 °C steps from 20 to 94 °C. Native TOG3 has a cooperative unfolding profile with an inflection point at 49 °C. Msps TOG3 R816A has a noncooperative unfolding curve, which suggests that the Arg-816 side chain in the conserved C-terminal extension promotes domain stabilization though its interactions with HRs C and D.