Effect of Ca2+ and cAMP on protein phosphorylation in mitochondria of maize seedlings

The effect of common intracellular signals (Ca2+ and cAMP) on the activity of protein phosphorylation in mitochondria was investigated in coleoptiles of maize (Zea mays L.). Treatment of isolated mitochondria with 2 mM CaCl2 brought about an increase in the level of phosphorylation of proteins with mol ws of 74, 60, and 33 kD but considerably reduced phosphorylation of the protein with a mol wt of 51.5 kD. In the presence of Ca2+, phosphorylation of polypeptides with mol wts of 59 and 66 kD was also detected. cAMP considerably reduced phosphorylation of essentially all the investigated proteins in isolated mitochondria, which could be explained by activation of their dephosphorylation. Phosphorylation of mitochondrial proteins involves a polypeptide of about 94 kD showing kinase activity, which may be proper protein kinase or one of the subunits of a compound structure. In maize mitochondria, PP1A phosphatases were found. A hypothesis was advanced that redox-dependent phosphorylation/dephosphorylation of mitochondrial proteins plays an important role in mitochondrial signaling in higher plants.