Activation of C-Kinase η through Its Cholesterol-3-sulfate-Dependent Phosphorylation by Casein Kinase I in Vitro

The physiological correlation between casein kinase I (CK-I) and an isoform η of protein kinase C (C-kinase η) was investigated in vitro, since it has been reported that (i) cholesterol-3-sulfate (CH-3S) effectively activates C-kinase η rather than the other isoforms (C-kinase ε and C-kinase δ) in vitro; and (ii) CK-I efficiently phosphorylates CH-3S-binding proteins, such as high mobility group protein I (HMG1), in the presence of CH-3S in vitro. We found that (i) CK-I phosphorylated Thr in preference to Ser on recombinant human C-kinase isoform η (rhC-kinase n) in the presence of CH-3S, (ii) this phosphorylation was selectively inhibited by CK-I-7 (a CK-I inhibitor); and (iii) the activity (phosphorylation of protamine sulfate) of rhC-kinase η was approx. 3. 2-fold stimulated by its full phosphorylation by CK-I in the presence of 3 μM CH-3S. These results suggest that CK-I is a protein kinase responsible for the activation of rhC-kinase η in the presence of CH-3S in vitro.