Interaction of NtCDPK1 calcium‐dependent protein kinase with NtRpn3 regulatory subunit of the 26S proteasome in Nicotiana tabacum

Using a yeast two‐hybrid system, we identified NtRpn3, a regulatory subunit of 26S proteasome, as an interacting protein of NtCDPK1 calcium‐dependent protein kinase in Nicotiana tabacum. Rpn3 in yeast is an essential protein involved in proteolysis of cell cycle regulatory proteins, and the carrot homolog of Rpn3 was previously isolated as a nuclear antigen that is mainly expressed in the meristem. NtCDPK1 physically interacts with NtRpn3 in vitro in a Ca²⁺‐independent manner and phosphorylates NtRpn3 in a Ca²⁺‐dependent manner with Mg²⁺ as a cofactor. NtCDPK1 and NtRpn3 are co‐localized in the nucleus, nuclear periphery, and around plasma membrane in vivo. Both NtCDPK1 and AtRpn3, an NtRpn3 homolog of Arabidopsis, are mainly expressed in the rapidly proliferating tissues including shoot and root meristems, and developing floral buds. Virus‐induced gene silencing of either NtRpn3 or NtCDPK1 resulted in the phenotypes of abnormal cell morphology and premature cell death in newly emerged leaves. Finally, NtCDPK1 interacts with NtRpn3 in vivo as shown by co‐immunoprecipitation. Based on these results, we propose that NtCDPK1 and NtRpn3 are interacting in a common signal transduction pathway possibly for regulation of cell division, differentiation, and cell death in tobacco.