Actin in triton-insoluble cytoskeleton of thyroid

Abstract Treatment of bovine thyroid with the non-ionic detergent Triton X-100 extracts most of the cell protein and leaves an 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 thyroid 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 β- and γ-isoactins, with isoelectric points more alkaline than the α-actin of rabbit skeletal muscle.