Figure 8

Molecular dynamics simulations predict enhanced flexibility of R1098Q spectrin with reduced calmodulin (CaM) binding and enhanced susceptibility to calpain cleavage. (a) Schematic of intermediate protein complexes simulated with molecular dynamics of spectrin repeats 9–10 with and without bound CaM. (Also see supplemental movie M2). (b) Convex bending of the spectrin scaffold (angle between adjacent monomers) is substantially shifted in R1098Q mutants, sampling both a more convex angle than WT spectrin and a broader distribution of values. (c) Hydrogen-bonding between D922 and either R1098, Q1098, or K1098 (a hypothetical variant) is markedly different between WT and variant spectrins. Salt-bridging between D922 and R1098 is strongest due to the presence of opposing charges and two hydrogen bond donors. Mutations that conserve native charge (e.g. R1098K) could still maintain intermediate hydrogen-bonding and scaffold flexibility, while pathological mutations such as R1098Q fully abrogate hydrogen-bonding and enhance spectrin flexibility. (d) The free energy of binding between CaM and spectrin is reduced by 88 kJ/mol in the R1098Q mutant, equivalent to a predicted 15% loss in binding affinity compared to WT complexes.