Structure and properties of casein kinase Its from Allomyces arbuscula phosphorylating serine residues. Experimental Mycology 18, 349–362. Two casein kinase (CK) activities have been purified from the aquatic fungus Allomyces arbuscula and have been referred to as CK IIA and CK IIB. Both enzymes have the properties characteristic of animal and yeast casein kinase II, i.e., tetrameric holoenzyme, ability to use ATP as well as GTP as donor of phosphate group in phosphotransferase reactions, inhibition of kinase activity by heparin, and inability to use basic protein like histone H1 as substrate. The two enzymes differ from each other by their affinity to DE-52-cellulose, CK IIA does not bind to DE-52 while CK IIB does. The kinetic parameters of the two enzymes also differ. The K_m of CK IIA has been found to be 7.7 μM, 20.83 μM, and 0.96 mM and that of CK IIB 22.2 νM, 41.7 ~LM, and 1.86 mM, for casein, ATP, and Mgt²⁺ , respectively. Both α and β subunits of CK IIA and CK IIB undergo autophosphorylation. Polylysine is inhibitory to autophosphorylation of a subunit whereas spermine, protamine, and histone H1 are stimulatory. Phosphoamino acid analysis showed that serine was the phospho-accepting amino acid. The phosphopeptide analysis showed that the trypsin recognition sites of the α and β subunits in CK IIA and CK IIB are different.