pH regulation of the F-actin binding properties of Dictyostelium elongation factor 1α

ABP50, an F-actin bundling protein from Dictyostelium, is also the protein synthesis co-factor, elongation factor 1α (EF1α). Concomitant with cAMP stimulation in Dictyostelium is a cytoplasmic alkalinization (Aerts, R. J., DeWit, R. J. W., and Van Lookeren Campagne, M. M. (1987) FEBS Lett. 220, 366- 370) and a redistribution of EF1α (Dharmawardhane, S., Demma, M., Yang, F., and Condeelis, J. (1991) Cell Motil. Cytoskel. 20, 279-288). In addition, others have shown a correlation between intracellular pH and the level of protein synthesis in Dictyostelium (Aerts, R. J., Durston, A. J., and Moolenaar, W. H. (1985) Cell 43, 653-657). The present study investigates the relationship between pH and the F-actin binding properties of EF1α. We found that increasing pH over the physiological range 6.2-7.8 causes a loss of EF1α-mediated F-actin bundling and single filament binding, with corresponding increases in the amount of free EF1α in vitro. Similar results also were obtained by cell fractionation and confocal immunofluorescence microscopy. The EF1α binding constant (K(d)) for F-actin is increased from 0.2 μM to > 2.2 μM over the same pH range. In addition, EF1α-induced actin bundle formation is freely reversible by changes in pH. Thus, pH may be a potent modulator of cytoarchitecture in Dictyostelium and may also influence mRNA translation rates by modifying the interactions between the protein synthetic machinery and the actin cytoskeleton.