Figure 6. Increased exposure of a calpain target site on desmoplakin due to a glycine substitution at residue 451.

(A) Ribbon model of desmoplakin showing location of affected calpain site (resides 447–451). Inset shows intramolecular interactions surrounding the putative calpain site. (B) Immunoblots of lysates from a donor control heart (OSU 294050) for changes in desmoplakin levels following the addition of exogenous Ca²⁺ or calpain. Addition of 10 mM Ca²⁺ over endogenous levels results in significant degradation of desmoplakin. Addition of exogenous calpain further reduced levels of desmoplakin at both endogenous levels and increased levels of Ca²⁺ (*P < 0.05, **P < 0.01 for 2-way ANOVA with Tukey’s multiple comparisons test; n = 3). (C) Histogram of the number of noncovalent interactions surrounding the calpain site, calculated from molecular dynamic simulations of WT desmoplakin (blue) and R451G desmoplakin (red). R451G produced a clear reduction in the average number of noncovalent interactions surrounding the calpain site, possibly increasing solvent accessibility degradation rate. (D) Surface model of desmoplakin showing the exposure of the affected calpain site in WT desmoplakin (left) and R451G desmoplakin (right). Red represents the exposed surface of calpain site residues. (E) Average exposed surface area of the calpain site over the length of the simulation (*P < 0.05, 2-tailed unpaired t test; n = 8). Error bars represent SEM.