UNLABELLED: 1. A myosin-actin hybrid complex was used to study actin-associated calcium sensitivity of a "cytoplasmic" actomyosin. The approach should be generally applicable. 2. Low salt extracts of Physarum polycephalum contain actin which remains in solution after centrifugation at 46 000 times g or at 100 000 times g for 1 h. The actin was precipitated by the addition of muscle myosin to the supernatants and detected in the hybrid complex by electron microscopy, sodium dodecyl sulfate gel analysis, super-precipitation and activation of the myosin ATPase activity. Actin was also precipitable from high speed supernatants of brain tissue or platelets. 3. The hybrid complexes from Physarum possessed 1.5-5-fold calcium dependency which could be removed by washing. Reincubation of the washed complex with concentrated wash solution resulted in high calcium sensitivity. On sodium dodecyl sulfate gels, unwashed complexes from Physarum contained high molecular weight material in addition to bands of molecular weights less than actin. The bands in the size range of 39 000 to 18 000 were primarily lost from the Physarum complex concomitantly with loss of calcium dependence. 4. When the Physarum supernatants were made 40 mM in MgCl2, precipitates were formed containing actin which possessed calcium sensitivity which was also lost on washing with low ionic strength solutions. This calcium dependency was partially reversed by the addition of desensitized rabbit actin to the precipitate before assay. 5. CONCLUSION: calcium regulation of actomyosin in Physarum is mediated primarily by factors that are bound to the actin component. The regulatory factors are soluble in low salt buffers. The molecular weights of the polypeptide chains of several of these factors are similar to those of the troponin polypeptides of striated muscle. In Physarum but not in platelet or brain a prominent polypeptide chain of approx. 55 000 molecular weight also occurs which coprecipitates with the hybrid complex and which is not easily removed.
UNLABELLED: 1. A myosin-actin hybrid complex was used to study actin-associated calcium sensitivity of a "cytoplasmic" actomyosin. The approach should be generally applicable. 2. Low salt extracts of Physarum polycephalum contain actin which remains in solution after centrifugation at 46 000 times g or at 100 000 times g for 1 h. The actin was precipitated by the addition of muscle myosin to the supernatants and detected in the hybrid complex by electron microscopy, sodium dodecyl sulfate gel analysis, super-precipitation and activation of the myosin ATPase activity. Actin was also precipitable from high speed supernatants of brain tissue or platelets. 3. The hybrid complexes from Physarum possessed 1.5-5-fold calcium dependency which could be removed by washing. Reincubation of the washed complex with concentrated wash solution resulted in high calcium sensitivity. On sodium dodecyl sulfate gels, unwashed complexes from Physarum contained high molecular weight material in addition to bands of molecular weights less than actin. The bands in the size range of 39 000 to 18 000 were primarily lost from the Physarum complex concomitantly with loss of calcium dependence. 4. When the Physarum supernatants were made 40 mM in MgCl2, precipitates were formed containing actin which possessed calcium sensitivity which was also lost on washing with low ionic strength solutions. This calcium dependency was partially reversed by the addition of desensitized rabbitactin to the precipitate before assay. 5. CONCLUSION:calcium regulation of actomyosin in Physarum is mediated primarily by factors that are bound to the actin component. The regulatory factors are soluble in low salt buffers. The molecular weights of the polypeptide chains of several of these factors are similar to those of the troponin polypeptides of striated muscle. In Physarum but not in platelet or brain a prominent polypeptide chain of approx. 55 000 molecular weight also occurs which coprecipitates with the hybrid complex and which is not easily removed.