Modification of the Transcription Factor FOXL2 at Serines 101 and 107 Disables DNA Binding, Leads to Nucleolar Relocalization, and Rewires Granulosa‐Cell Programs
TL;DR
It is shown that protein kinase C (PKC) phosphorylates FOXL2 in vitro, and hypothesized that PKC‐dependent phosphorylation within the FOXL2 DNA‐recognition helix would underlie a rapid, reversible switch, weakening DNA binding, redirecting subnuclear partitioning, and rewiring protein–protein interactions, thereby reshaping FOXL2‐dependent gene regulation in granulosa cells.
Abstract
ABSTRACT FOXL2 is a forkhead transcription factor (TF) essential for granulosa‐cell identity and function, yet how post‐translational modifications tune its activity remains incompletely understood. Here, we show that protein kinase C (PKC) phosphorylates FOXL2 in vitro. Two of the four phosphorylation sites, notably Ser101 and Ser107, map to the forkhead DNA‐recognition helix. Phosphomimetic substitutions (S → D) at these positions (S101D/S107D) abolish binding to a consensus DNA sequence recognized by FOXL2 and luciferase reporter activation, whereas alanine substitutions are rather neutral. In HeLa cells, the S101D mutant and, to a lesser extent, S107A/S107D, relocalize at least partially to nucleoli and exhibit increased mobility consistent with reduced DNA engagement. This pattern was recapitulated in stably transduced KGN granulosa cells. RNA‐seq of such KGN cells revealed that S101D and a C‐terminal truncation (ΔC) induce a massive loss‐of‐function (LOF) relative to wild‐type (WT) FOXL2. The LOF affects sets of genes involved in pathways central to granulosa physiology including ECM organization, cell migration/adhesion, and MAPK cascades, whereas S101A is largely WT‐like. An analysis of the FOXL2 interactome in the transduced cells by mass spectrometry (MS) showed that S101D loses numerous interactions with TFs and chromatin remodelers, and Pol I/III regulators such as UBTF and TFIIIC components, while it gains other partners. By contrast, ΔC retains many of the protein–protein contacts of WT and preferentially loses ribosomal/TFIII interactions. Together, these data allow us to hypothesize that PKC‐dependent phosphorylation within the FOXL2 DNA‐recognition helix would underlie a rapid, reversible switch, weakening DNA binding, redirecting subnuclear partitioning, and rewiring protein–protein interactions, thereby reshaping FOXL2‐dependent gene regulation in granulosa cells.