Activation of a Ca²⁺-dependent Protein Kinase in Response to Heat Shock in the Myxoamoebae of a True Slime Mold, Physarum Polycephalum

Abstract

Protein kinase activities in myxoamoebae of a true slime mold, Physarum polycephalum, were investigated in response to heat shock. In-gel assay detected an apparent activation of a Ca²⁺-dependent, 53-kDa protein kinase that phosphorylated casein but not histone HI. This enzyme needed co-presence of Mg²⁺ ion with Ca²⁺ for its activity. Treatment with calf intestinal alkaline phosphatase did not affect the heat-inducible 53-kDa protein kinase activity at all. The effects of protein kinase inhibitors were examined, and staurosporine suppressed the activity of this enzyme completely. H-7 decreased the activity to about 20% and HA-1004 to 65%. These results suggest that this protein kinase that may phosphorylate tyrosine and serine/threonine residues of target proteins is activated by heat shock in Physarum cells, and that the activation is not regulated via phosphorylation by putative protein kinase(s) that may act at an upstream position in the signaling cascade(s).