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We have investigated the size and shape of gelsolin, a calcium-dependent actin-binding protein, by means of combined photon correlation spectroscopy and depolarized dynamic light scattering. From the measured rotational and translational diffusion coefficients, a hydrodynamically equivalent ellipsoid of revolution with an axial ratio of 0.23 is obtained in the presence of EGTA. Such a distinctly elongated shape is suggested also by the depolarization ratio of 1.6 × 10−3. In the presence of Ca2+, when gelsolin has two Ca ions bound, a significant decrease in the translational diffusion coefficient and a smaller decrease in the rotational diffusion coefficient are observed. Both effects can be assigned to a change of shape from the elongated toward a more spherical form with an axial ratio of 0.37 and a hydrodynamic volume more than twice as large as in the absence of Ca2+. The apparent molecular mass as measured by static light scattering remains unchanged, indicating that the volume increase is not due to aggregation of gelsolin. The relatively large conformational changes observed upon binding of Ca2+, are probably related to an alteration of the surface charge of the molecule. As a result of these conformational changes, actin binding sites may become exposed and the molecule gains a high affinity for two actin molecules. |
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