Deficient Polymerization In Vitro of a Point-Mutated β-Actin Expressed in a Transformed Human Fibroblast Cell Line1

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Abstract
HUT-14 cells, tumorigenic human fibroblasts, express a mutant β-actin which has a single amino acid substitution at position 244 (glycine to aspartic acid), in addition to normal β- and γ-actin. In order to characterize the biochemical function of the mutant β-actin, actins were extracted and purified from HUT-14 cells. The partially purified actin fraction contained β-,γ-, and mutant β-actins in the ratio of 1: 1: 1, the same ratio as in the cells. When the actin of this fraction was purified through a polymerization step, mutant β-actin was always less incorporated into actin filaments than β- and γ-actin. When the polymerization ability of purified HUT-14 actins was examined by sedimentation technique, it was lower than those of muscle and of cytoplasmic actins from another human cell line (HUT-11) which expresses only normal β- and γ-actin, in the ratio of 2: 1. The deficient polymerization of mutant β-actin was also observed by examining the ratio of β-, γ-, and mutant -actins incorporated into actin filaments. The ratio of mutant $-actin in polymerized actins under all conditions examined was always less than that before polymerization. These results indicate that the single amino acid substitution at position 244 caused the reduction of incorporation of the mutant β-actin into actin filaments in vitro.