Changes in K⁺, Na⁺-Sensitive Actin Gelation Factor during the Differentiation of Myeloid Leukemia Cells

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A homodimer protein consisting of two 38, 000 dalton peptides was isolated from a murine leukemia cell line (Ml). The binding molar ratio of the 38K-dimer protein to purified skeletal muscle actin was saturated at 1:3, and when the 38K-dimer/actin ratio exceeded 1:12, gelation occurred. This gelation was completely inhibited by the presence of either 10mM KCl or 20mM NaCl. The protein induced actin filament bundling, which required a higher 38K-dimer/actin ratio and was not affected by the presence of monovalent cations. During the differentiation of Ml cells, the sensitivity of the 38K protein to monovalent cations was decreased; that is 20mM KCl or 50mM NaCl was required to inhibit the gelation by the 38K protein isolated from differentiated cells. On the other hand, the intracellular K⁺ content of Ml cells decreased from 70±5mM to 18±3mM, and Na⁺ increased from 10±5mM to 40±10mM during the differentiation. These findings suggest that the differentiation brought about conditions favourable for the 38K protein to induce actin gelation, and in turn, the locomotive and phagocytic activities which were induced only after differentiation in this cell line.