Phosphorylation of high mobility group protein 1 (HMG1) by casein kinase I (CK-I) and potent effecters (inhibitors and activators) of this phosphorylation were investigated in vitro. We found that (i) CK-I phosphorylates specifically threonine residues on HMG1 when incubated with cholesterol-3-sulfate (CH-3S), but no phosphorylation of HMG1 is detected in the presence of other cholesterol related compounds or their sulfated derivatives; (ii) this phosphorylation is selectively inhibited by heparin, but stimulated significantly by 3 ' ,4 ' ,7-trihydroxy-isofavone at low doses (0.1-3 muM); and (iii) CH-3S directly induces a drastic conformational change in HMG1. The latter finding provides a mechanism to explain how CH-SS alone can induce the phosphorylation of HMG1 by CK-I in vitro.