LC-MS/MS and in vitro phosphorylation assays, CPK3, CNGC6-N, and CNGC6-C, and point-mutated CNGC6-N-C were cloned into the vector pGEX4T-1 downstream of the GST tag, and the recombinants were expressed in and purified from E. coli BL21-Gold (DE3)