Casein kinase I from Trichosporon cutaneum ribosome-free extracts was purified. Its molecular mass was calculated for 33 kDa. It was shown that casein, phosvitin and Trichosporon cutaneum ribosomal protein of 15 kDa were preferable substrates for the enzyme. It was found that heparin can stimulate or inhibit CKI activity depending on the substrate used. Stimulation of casein and inhibition of phosvitin phosphorylation was observed. In addition it was shown that ribosomal proteins of 19 kDa and 38 kDa were phosphorylated by CKI only in the presence of heparin.
Casein kinases are second messengers independent Ser/Thr protein kinases, prefering acidic proteins such as casein or phosvitin as in vitro phosphorylation substrates. They are generally divided into two classes (casein kinase I and 11) according to their subunit structure, chromatographic behavior and catalytic properties.
Casein kinase I1 (CKII) is an oligomeric enzyme containing at least two different subunits, it accepts both ATP and GTP as phosphate donors, it binds tightly to anion exchange materials and is inhibited by low concentrations of heparin. In contrast casein kinase I (CKI) is a monomeric enzyme, it uses only ATP as phosphate donor and is not retained by anion exchange materials (TUAZON and TRAUGH 1991).
Several isoforms of CKI were identified recently. In mammals six forms encoded by six different genes were described (GRAVES and ROACH 1995). In Saccharomyces cerevisiae four different genes: HRR25, YCK1, YCK2 and YCK3 were identified (DE MAGGIO et al.