Actin in Triton-insoluble cytoskeleton of thyroid.

Treatment of bovine thyroid with the non-ionic detergent Triton X-100 extracts most of the cell protein and leaves insoluble residue. This Triton-insoluble cytoskeleton consists of five major polypeptides on sodium dodecyl sulfate polyacrylamide gels. One of these polypeptides is actin. Based on DNase inhibition assay, 30% of the total actin is associated with the cytoskeleton as the filamentous form. Thyroid actin from the cytoskeleton has been solubilized by dialysis against a low ionic strength buffer at pH 8.0 and purified to homogeneity by a polymerizing-depolymerizing cycle. The overall purification was about 144-fold with a yield of 10%. Bovine thyroid actin is very similar to actins from other tissues on the basis of: (1) comigration with rabbit skeletal muscle actin during gel electrophoresis in sodium dodecyl sulfate, (2) its amino acid composition, which includes about 1 mole of 3-methylhistidine per 42,000 g, (3) its ability to bind and inhibit pancreatic deoxyribonuclease I, and (4) its ability to form 7-8 nm microfilaments which is similar to that of skeletal filamentous actin. Thyroid actin contains beta- and gamma-isoactins, with isoelectric points more alkaline than the alpha-actin of rabbit skeletal muscle.