In plants, calcium-dependent protein kinases (CDPKs) make up a large family that is characterized by a C-terminal calmodulin(CaM)-like domain. Recently, a novel carrot cDNA clone encoding an atypical CDPK, which has a significantly degenerate sequence in the CaM-like domain, was found and named CDPK-related protein kinase (CRK) [Lindzen, E. and Choi, J.H. (1995) Plant Mol. Biol. 28, 785-797]. We obtained two different cDNA clones from maize which encode CRKs. For the first enzymatic characterization of CRK, a maize cDNA clone was expressed in E. coli. The recombinant protein efficiently phosphorylated casein, a conventional protein substrate, Notably, in this in vitro phosphorylation assay, the kinase activity did not require calcium as an activator. Thus, CRKs were suggested to be novel calcium-independent protein kinases having a degenerate CaM domain, the function of which remains to be elucidated.
Furumoto, T., Ogawa, N., Hata, S., & Izui, K. (1996). Plant calcium-dependent protein kinase-related kinases (CRKs) do not require calcium for their activities. FEBS Letters, 396(2–3), 147–151. https://doi.org/10.1016/0014-5793(96)01090-3