Expression, purification and analysis of an Arabidopsis recombinant CBL-interacting protein kinase3 (CIPK3) and its constitutively active form.

CIPK3 is a member of CBL (calcineurin B-like)-interacting serine-threonine protein kinases which play an important role in many developmental and adaptation processes in Arabidopsis. Studies conducted on members of this family such as SOS2, PKS8 and PKS11 have provided insight into how these kinases interact with their target substrates in the signal-response process. Since SOS2, PKS8 and PKS11 have low enzymatic activities in vitro and their amino acid sequences are homologous to that of CIPK3, it was assumed that CIPK3 would have a low enzymatic activity. To enhance CIPK3 enzyme activity, a constitutively active form, CIPK3T183D, was generated by a Thr(183) to Asp(183) substitution in the activation loop. To obtain proteins for analysis, glutathione S-transferase (GST) fusion protein system was used. Although both CIPK3 and CIPK3T183D were successfully expressed, they were found in inclusion bodies with three truncated proteins. Since the truncated proteins had a similar affinity to the GST-Bind Resin as the target protein, the one-step affinity purification could no longer be used. As an alternative, His fusion protein expression system was employed for protein production. Although both His-CIPK3 and His-CIPK3T183D also accumulated in inclusion bodies, they were expressed as a single protein species. A method involving Sarkosyl was developed for isolating and purifying the His fusion proteins. His-CIPK3 and His-CIPK3T183D produced were highly purified and enzymatically active. In addition, a 9-fold increase in kinase activity in His-CIPK3T183D was observed, indicating that Thr(183) to Asp(183) substitution in the activation loop of CIPK3 had succeeded in enhancing the kinase activity.