Connections between the protein kinases that function within complex cell polarity networks are poorly understood. Rod-shaped fission yeast cells grow in a highly polarized manner, and genetic screens have identified many protein kinases, including the CaMKK-like Ssp1 and the MARK/PAR-1 family kinase Kin1, that are required for polarized growth and cell shape, but their functional mechanisms and connections have been unknown [1-5]. We found that Ssp1 promotes cell polarity by phosphorylating the activation loop of Kin1. Kin1 regulates cell polarity and cytokinesis through unknown mechanisms [4-7]. We performed a large-scale phosphoproteomic screen and found that Kin1 phosphorylates itself and Pal1 to promote growth at cell tips, and these proteins are interdependent for localization to growing cell tips. Additional Kin1 substrates for cell polarity and cytokinesis (Tea4, Mod5, Cdc15, and Cyk3) were also phosphorylated by a second kinase, the DYRK family member Pom1 [8]. Kin1 and Pom1 were enriched at opposite ends of growing cells, and they phosphorylated largely non-overlapping sites on shared substrates. Combined inhibition of both Kin1and Pom1 led to synthetic defects in their shared substrates Cdc15 and Cyk3, confirming a non-redundant functional connection through shared substrates. These findings uncover a new Ssp1-Kin1 signaling pathway, and define its functional and mechanistic connection with Pom1 signaling for cell polarity and cytokinesis. These kinases are conserved in many eukaryotes including humans, suggesting that similar connections and mechanisms might operate in a broad range of cells.

AMPK-related protein kinases (ARKs) coordinate cell growth, proliferation, and migration with environmental status. It is unclear how specific ARKs are activated at specific times. In the fission yeast Schizosaccharomyces pombe, the CaMKKlike protein kinase Ssp1 promotes cell cycle progression by activating the ARK Cdr2 according to cell growth signals. Here, we demonstrate that Ssp1 activates a second ARK, Ssp2/AMPK␣, for cell proliferation in low environmental glucose. Ssp1 activates these two related targets by the same biochemical mechanism: direct phosphorylation of a conserved residue in the activation loop (Cdr2-T166 and Ssp2-T189). Despite a shared upstream kinase and similar phosphorylation sites, Cdr2 and Ssp2 have distinct regulatory input cues and distinct functional outputs. We investigated this specificity and found that distinct protein phosphatases counteract Ssp1 activity toward its different substrates. We identified the PP6 family phosphatase Ppe1 as the primary phosphatase for Ssp2-T189 dephosphorylation. The phosphatase inhibitor Sds23 acts upstream of PP6 to regulate Ssp2-T189 phosphorylation in a manner that depends on energy but not on the intact AMPK heterotrimer. In contrast, Cdr2-T166 phosphorylation is regulated by protein phosphatase 2A but not by the Sds23-PP6 pathway. Thus, our study provides a phosphatase-driven mechanism to induce specific physiological responses downstream of a master protein kinase.KEYWORDS AMPK, Cdr2, Ssp1, fission yeast, kinase, phosphatase, pombe C ells control proliferation, polarized growth, and migration according to nutrient and environmental status. These processes must be tightly regulated during normal growth and development, and loss of this coordination is coupled with distinct steps in the initiation and maturation of tumors (1, 2). The AMP-activated protein kinase (AMPK)-related kinase (ARK) subfamily of protein kinases is essential for coordination of cell growth, proliferation, and migration with environmental status (3). Human cells express at least 13 members of this conserved kinase subfamily, including the metabolic sensor AMPK, the cytoskeletal kinase MARK1/Par-1, and the SAD kinases that regulate cell growth and polarity (4). Given their functions in diverse cell biological processes, it is remarkable that all ARKs can be activated by a shared upstream activating kinase, which phosphorylates the activation loop of ARK kinase domains to turn them "on" (4). To meet the needs of a rapidly changing environment, different ARKs need to be turned on and off at different times. This leads to a simple question: how does a common activation mechanism activate different ARKs at different times?The defining member of the ARK subfamily is AMPK. This conserved heterotrimeric complex is activated by low cellular energy status, typically the result of low levels of environmental nutrients (5). AMPK activation shifts cells toward catabolism and promotes the Warburg effect in cancer cells (5-7). The AMPK heterotrimer consists of a catalytic ␣ subun...