Thermal stability of chicken brain α-spectrin repeat 17: a spectroscopic study

Spectrin is a rod-like multi-modular protein that is mainly composed of triple-helical repeats. These repeats show very similar 3D-structures but variable conformational and thermodynamical stabilities, which may be of great importance for the flexibility and dynamic behaviour of spectrin in the cell. For instance, repeat 17 (R17) of the chicken brain spectrin α-chain is four times less stable than neighbouring repeat 16 (R16) in terms of ∆G. The structure of spectrin repeats has mainly been investigated by X-ray crystallography, but the structures of a few repeats, e.g. R16, have also been determined by NMR spectroscopy. Here, we undertook a detailed characterization of the neighbouring R17 by NMR spectroscopy. We assigned most backbone resonances and observed NOE restraints, relaxation values and coupling constants that all indicated that the fold of R17 is highly similar to that of R16, in agreement with previous X-ray analysis of a tandem repeat of the two domains. However, (15)N heteronuclear NMR spectra measured at different temperatures revealed particular features of the R17 domain that might contribute to its lower stability. Conformational exchange appeared to alter the linker connecting R17 to R16 as well as the BC-loop in close proximity. In addition, heat-induced splitting was observed for backbone resonances of a few spatially related residues including V99 of helix C, which in R16 is replaced by the larger hydrophobic tryptophan residue that is relatively conserved among other spectrin repeats. These data support the view that the substitution of tryptophan by valine at this position may contribute to the lower stability of R17.