The PHLPP1 N-Terminal Extension Is a Mitotic Cdk1 Substrate and Controls an Interactome Switch.

PH domain leucine-rich repeat protein phosphatase 1 (PHLPP1) is a tumor suppressor that directly dephosphorylates a wide array of substrates, most notably the prosurvival kinase Akt. However, little is known about the molecular mechanisms governing PHLPP1 itself. Here, we report that PHLPP1 is dynamically regulated in a cell cycle-dependent manner and deletion of PHLPP1 results in mitotic delays and increased rates of chromosomal segregation errors. We show that PHLPP1 is hyperphosphorylated during mitosis by Cdk1 in a functionally uncharacterized region known as the PHLPP1 N-terminal extension (NTE). A proximity-dependent biotin identification (BioID) interaction screen revealed that during mitosis, PHLPP1 dissociates from plasma membrane scaffolds, such as Scribble, by a mechanism that depends on its NTE and gains proximity to kinetochore and mitotic spindle proteins such as KNL1 and TPX2. Our data are consistent with a model in which phosphorylation of PHLPP1 during mitosis regulates binding to its mitotic partners and allows accurate progression through mitosis. The finding that PHLPP1 binds mitotic proteins in a cell cycle- and phosphorylation-dependent manner may have relevance to its tumor-suppressive function.

Pharmacology on Target.

Small-molecule inhibitors are a key resource in the cell signaling toolbox. However, because of their global distribution in the cell, they cannot provide a refined understanding of signaling at distinct subcellular locations. Bucko and colleagues have designed a novel tool to localize inhibitors to specific protein scaffolds, opening a new avenue to study localized kinase activity.

PHLPP1 counter-regulates STAT1-mediated inflammatory signaling.

Inflammation is an essential aspect of innate immunity but also contributes to diverse human diseases. Although much is known about the kinases that control inflammatory signaling, less is known about the opposing phosphatases. Here we report that deletion of the gene encoding PH domain Leucine-rich repeat Protein Phosphatase 1 (PHLPP1) protects mice from lethal lipopolysaccharide (LPS) challenge and live Escherichia coli infection. Investigation of PHLPP1 function in macrophages reveals that it controls the magnitude and duration of inflammatory signaling by dephosphorylating the transcription factor STAT1 on Ser727 to inhibit its activity, reduce its promoter residency, and reduce the expression of target genes involved in innate immunity and cytokine signaling. This previously undescribed function of PHLPP1 depends on a bipartite nuclear localization signal in its unique N-terminal extension. Our data support a model in which nuclear PHLPP1 dephosphorylates STAT1 to control the magnitude and duration of inflammatory signaling in macrophages.