Deficient polymerization in vitro of a point-mutated beta-actin expressed in a transformed human fibroblast cell line

HUT-14 cells, tumorigenic human fibroblasts, express a mutant beta-actin which has a single amino acid substitution at position 244 (glycine to aspartic acid), in addition to normal beta- and gamma-actin. In order to characterize the biochemical function of the mutant beta-actin, actins were extracted and purified from HUT-14 cells. The partially purified actin fraction contained beta-, gamma-, and mutant beta-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 beta-actin was always less incorporated into actin filaments than beta- and gamma-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 beta- and gamma-actin, in the ratio of 2:1. The deficient polymerization of mutant beta-actin was also observed by examining the ratio of beta-, gamma-, and mutant beta-actins incorporated into actin filaments. The ratio of mutant beta-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 beta-actin into actin filaments in vitro.