Association of CPI-17 with protein kinase C and casein kinase I.

The protein kinase C-potentiated inhibitor protein of 17kDa, called CPI-17, specifically inhibits myosin light chain phosphatase (MLCP). Phosphorylation of Thr-38 in vivo highly potentiates the ability of CPI-17 to inhibit MLCP. Thr-38 has been shown to be phosphorylated in vitro by a number of protein kinases including protein kinase C Rho-associated coiled-coil kinase (ROCK), and protein kinase N (PKN). In this study we have focused on the association of protein kinases with CPI-17. Using affinity chromatography and Western blot analysis, we found interaction with all isotypes and casein kinase I isoforms, CKIalpha and CKI. By contrast, ROCK and PKN did not associate with CPI-17, suggesting that duanyu1531 may be the relevant kinase that phosphorylates Thr-38 in vivo. CPI-17 interacted with the cysteine-rich domain of duanyu1531 and was phosphorylated by all duanyu1531 isotypes. We previously found that CPI-17 co-purified with casein kinase I in brain suggesting they are part of a complex and we now show that CPI-17 associates with the kinase domain of CKI isoforms.

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