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J Biol Chem, Vol. 273, Issue 17, 10147-10152, April 24, 1998

Prothymosin  Stimulates Ca²⁺-dependent Phosphorylation of Elongation Factor 2 in Cellular Extracts

From the Departamento de Fisiología, Facultad de Veterinaria, Lugo and ^§ Laboratorios de Neurociencia "Ramón Domínguez," Departamento de Fisiología, Facultad de Medicina and Unidad Medicina Molecular, INGO, 15705 Santiago de Compostela, Spain and ^¶ Ludwig Institute for Cancer Research, 75124 Uppsala, Sweden

Prothymosin  (PTA) stimulates in a dose-dependent manner the phosphorylation of a 105-kDa protein (p105) in cell extracts from different cell types. Protein sequencing and immunological analysis indicated that this protein is elongation factor 2 (EF-2). We propose that calcium/calmodulin-dependent protein kinase III is responsible for the PTA-dependent EF-2 phosphorylation based on the following lines of evidence: (a) Ca²⁺ is required for the effect; (b) calmodulin enhances the reaction, and calmodulin inhibitors block the phosphorylation; and (c) no phosphorylation is seen in cell extracts depleted of calmodulin-binding proteins. To obtain a strong phosphorylated EF-2 band, we found it necessary to add PTA to cytosolic extracts from synchronized dividing cells in various phases of the cell cycle except in mitosis. Since PTA is a nuclear protein everywhere in the cell cycle except in mitosis, when it is found in the cytoplasm, we hypothesize that, if PTA activates EF-2 phosphorylation in vivo, as present data suggest, its presence in the cytoplasm during mitosis could explain why EF-2 phosphorylation is mainly restricted to that phase of the cell cycle. Moreover, other bands in addition to EF-2 were phosphorylated in a calmodulin- and PTA-dependent manner, and several of them (in a range between 50 and 60 kDa) have similar M_r to those that conform to the holoenzyme calcium/calmodulin dependent protein kinase II, suggesting that PTA could have a more general function modulating the activity of various Ca²⁺/CaM-dependent enzymes along the cell cycle.

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