Expression of the human myotonic dystrophy kinase-related Cdc42-binding kinase gamma is regulated by promoter DNA methylation and Sp1 binding.

Myotonic dystrophy kinase-related Cdc42 binding kinases (MRCKs) are family members most related to the myotonic dystrophy kinase (DMPK), RhoA-binding kinase (ROK), and citron kinase. Two highly conserved members, MRCKalpha and -beta, have been previously identified and characterized. We now describe a novel isoform, MRCKgamma, which is functionally and structurally related to members of this kinase family. We show these kinases to have marked similarities in their genomic organization, substrate phosphorylation, and catalytic autoinhibition. Unlike MRCKalpha and -beta, which are expressed ubiquitously, MRCKgamma mRNA was only expressed in heart and skeletal muscle. In cultured cells, MRCKgamma showed differential expression with high levels of expression only in certain cell lines. DNA analysis showed that lack of expression is correlated with promoter DNA methylation. We have mapped the methylation sites in the MRCKgamma promoter. Significantly, agents that suppressed DNA methylation caused increases in the expression of the kinase in low-expressing cells, further supporting the notion that promoter DNA methylation plays an important role in the expression of MRCKgamma. Analysis of the MRCKgamma promoter has also revealed two proximal Sp1 sites that are essential for transcriptional activity. We conclude that both promoter DNA methylation and Sp1 binding are important regulators for MRCKgamma expression.