Interplay between Polo kinase, LKB1-activated NUAK1 kinase, PP1β^MYPT1 phosphatase complex and the SCF^βTrCP E3 ubiquitin ligase

NUAK1 and NUAK2 protein kinases are activated by the LKB1 tumour suppressor and have been implicated in regulating multiple processes such as cell survival, senescence, adhesion and polarity. Here we present evidence that expression of NUAK1 is controlled by cyclin dependent kinase (CDK), Polo kinase (PLK) and the Skp, Cullin, F-box^ßTrCP (SCF^ßTrCP) E3 ubiquitin ligase complex. Our data indicate that CDK phosphorylates NUAK1 at Ser445, triggering binding to PLK, which subsequently phosphorylates NUAK1 at two conserved non-catalytic Ser residues (Ser476 and Ser480). This induces binding of NUAK1 to ßTrCP, the substrate recognition subunit of the SCF^ßTrCP E3 ligase, resulting in NUAK1 becoming ubiquitylated and degraded. We also show that NUAK1 and PLK1 are reciprocally controlled in the cell cycle. In G2-M phase when PLK1 is most active, NUAK1 levels are low and vice versa in S-phase when PLK1 activity is low, NUAK1 is more highly expressed. Moreover, NUAK1 inhibitors (WZ4003 or HTH-01-015) suppresses proliferation by reducing the population of cells in S-phase and mitosis an effect that can be rescued by overexpression of a NUAK1 mutant in which the Ser476 and Ser480 residues are mutated to Ala. Finally, previous work has suggested that NUAK1 phosphorylates and inhibits PP1ß^MYPT1 and that a major role for the PP1ß^MYPT1 complex is to inhibit PLK1 by dephosphorylating its T-loop (Thr210). We demonstrate that activation of NUAK1 leads to a striking increase in phosphorylation of PLK1 at Thr210 an effect that is suppressed by NUAK1 inhibitors. Our data link NUAK1 to important cell cycle signalling components and suggest that NUAK1 plays a role in stimulating S-phase as well as PLK1 activity via its ability to regulate the PP1ß^MYPT1 phosphatase.  © 2014 The Authors Journal compilation © 2014 Biochemical Society