Fig. 3.

Effects of phosphorylation and calcium on the structure of the M-domain. (A and B) AFM force:extension curves for single unphosphorylated wild-type C1C2 (A) and phosphomimetic C1C2^4D (B) molecules in the absence of calcium. The initial contour length of the freely extensible components (L_C1) and final contour length of the fully extended molecules (L_C3) are indicated, as is the change in contour length associated with the preceding unfolding event (ΔL_C). (C) The final contour lengths of the C1C2 and C1C2^4D fragments (L_C3 in A and B) in the absence and presence 0.1 mM free calcium. *P < 0.01, L_C3 vs. C1C2, Student’s t test. (D) The relative stability of the M-domain within the single C1C2 and C1C2^4D molecules in the absence and presence of 0.1 mM free calcium as determined from the number of molecules showing three (freely extensible M-domain) or four (partially stable M-domain) peaks in the force:extension curves.