Fig. 3.

FRAP indicators of reversible protein depolymerization (also Figs. S3C and S5). (A) Increase in FRAP rate of D4cpV-Casq1 ($D/D_{\text{resting}}\equiv f\prime \left(0\right)/f\prime {\left(0\right)}_{\text{resting}}$) vs. reduction in [Ca²⁺]_SR for individual experiments (symbols) and averages over 0.2-wide bins in abscissa. (B) Reversibility of the effect of exposure to solution 2. Circles represent the evolution of $f\left(t\right)$ of EYFP-Casq1 in a bleached region. Note the change in the slope of recovery after solution change. Squares represent the normalized recovery rate $k_e\left(t\right)$, showing reversion of effect on washout. (C) Initial rate $f\prime \left(0\right)$ of D4cpV-Casq1 in six successively bleached regions, and simultaneously measured [Ca²⁺]_SR. The blue bar on time axis marks the application of depleting solution 0. The $f\prime \left(0\right)$ rate increases as [Ca²⁺]_SR slowly decreases, and recovers on washout. The large increase in $f\prime \left(0\right)$ lags behind the decay in [Ca²⁺]_SR. (D) Green symbols show that [Ca²⁺]_SR undergoes a large oscillation upon exposure to solution 5 (additional examples are provided in Fig. S5). $f\prime \left(0\right)$ went from 0.85 in a cell at rest (open circles) to 1.8 upon depletion (filled circles. (Inset) Details of initial recovery. (E) Relationship between [Ca²⁺]_SR and the immobile fraction of fluorescence, defined as $1-f_{\infty }$, where $f_{\infty }$ is the limit value of $f$ at long times. The linear correlation coefficient ρ² is 0.61.