Phosphorylation of p27 KIP 1 homologs KRP 6 and 7 by SNF 1‐related protein kinase–1 links plant energy homeostasis and cell proliferation

Summary SNF 1‐related protein kinase–1 (Sn RK 1), the plant kinase homolog of mammalian AMP ‐activated protein kinase ( AMPK ), is a sensor that maintains cellular energy homeostasis via control of anabolism/catabolism balance. AMPK ‐dependent phosphorylation of p27 KIP 1 affects cell‐cycle progression, autophagy and apoptosis. Here, we show that Sn RK 1 phosphorylates the Arabidopsis thaliana cyclin‐dependent kinase inhibitor p27 KIP 1 homologs At KRP 6 and At KRP 7, thus extending the role of this kinase to regulation of cell‐cycle progression. At KRP 6 and 7 were phosphorylated in vitro by a recombinant activated catalytic subunit of Sn RK 1 (AtSn RK 1α1). Tandem mass spectrometry and site‐specific mutagenesis identified Thr152 and Thr151 as the phosphorylated residues on At KRP 6‐ and At KRP 7, respectively. AtSn RK 1 physically interacts with At KRP 6 in the nucleus of transformed BY –2 tobacco protoplasts, but, in contrast to mammals, the At KRP 6 Thr152 phosphorylation state alone did not modify its nuclear localization. Using a heterologous yeast system, consisting of a cdc28 yeast mutant complemented by A. thaliana CDKA ;1, cell proliferation was shown to be abolished by At KRP 6 WT and by the non‐phosphorylatable form At KRP 6 T152A , but not by the phosphorylation‐mimetic form At KRP 6 T152D . Moreover, A. thaliana Sn RK 1α1/ KRP 6 double over‐expressor plants showed an attenuated At KRP 6‐associated phenotype (strongly serrated leaves and inability to undergo callogenesis). Furthermore, this severe phenotype was not observed in At KRP 6 T152D over‐expressor plants. Overall, these results establish that the energy sensor AtSn RK 1 plays a cardinal role in the control of cell proliferation in A. thaliana plants through inhibition of At KRP 6 biological function by phosphorylation.