A Study of β-Actinin, Myofibrillar Protein from Rabbit Skeletal Muscle
1971 Volume 69 Issue 2 Pages 369-386
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1971 Volume 69 Issue 2 Pages 369-386
β-Actinin was purified by Sephadex G-200 column chromatography, and this preparation showed the same activity in keeping the dispersed state of sonicated F-actin as the β-actinin purified by repeated (NH4)2SO4 fractionation. However, the sedimentation coefficient was 4.5 S instead of 8S and the molecular weight was 62, 000 instead of 140, 000. The intrinsic viscosity was 0.05 dl/g. Helical content was 13-15%, estimated by the methods of optical rotary dispersion and circular dichroism.
In the presence of β-actinin, 70-100 S component of F-actin was changed into 40S component as revealed in the sedimentation pattern. The dynamic rigidity of F-actin was greatly reduced by the addition of βactinin, 5-10% by weight to F-actin, whereas viscosity was decreased down to 50% of the control value. The infinitely high value of viscosity at very low velocity gradients (<0.2 sec-1) was remarkably dropped by β-actinin. These facts clearly indicate that the inter-filamental interactions of F-actin in solution is inhibited by β-actinin.
Factors influencing the activity of β-actinin in dispersing F-actin after sonic treatment were investigated. It was very active in the presence of 0.1M KCl between pH 5.5 and 9.5, and at pH 7.2 between 0.05M and 0.2M KCl. β-Actinin was quickly inactivated at 60°C, and at 70-80°C it was completely denatured within 5min. ATP favored the action of β-actinin. p-Chloromercuribenzoate did not affect β-actinin, although it alone shortened F-actin when sonicated. 8M Urea inactivated β-actinin.
The effects of trypsin [EC 3. 4. 4. 4], chymotrypsin [EC 3. 4. 4. 5], papain [EC 3. 4. 4. 10] and Nagarse [EC 3. 4. 4. 16] on polymerizability of G-actin and on particle length of F-actin were examined to see if partially digested actin be active as β-actinin. F-actin partially digested by Nagarse showed a similar behavior after sonic treatment in the flow birefringence properties to the case with β-actinin. But it turned out that the Nagarse-treated F-actin particles were randomly aggregated under the electron microscope.
With the information of the localization of β-actinin in the I band of myofibrils, the physiological role of β-actinin is discussed.
