DAF-16/FOXO requires Protein Phosphatase 4 to drive the transcription of stress resistance and longevity promoting genes

The transcription factor DAF-16/FOXO is one of the most powerful aging regulators known to date, relaying dire conditions into the expression of stress resistance and longevity promoting genes. For some of these functions, in particular the relay of low insulin/IGF signaling (IIS), DAF-16 depends on a protein called SMK-1/SMEK, but how SMK-1 exerts this role has long remained unknown. Using C. elegans, we show that SMK-1 functions as part of a specific Protein Phosphatase 4 complex (PP4SMK-1) which is essential for many of the beneficial gene expression changes that DAF-16 confers. At these genes, loss of PP4SMK-1 substantially impairs RNA polymerase II (Pol II) recruitment to the promoter; and more subtly, it may affect promoter clearance and transcriptional termination, too. Through a search for the relevant substrate of PP4SMK-1 by phospho-proteomics we identify the conserved transcriptional regulator SPT-5/SUPT5H, whose post-developmental knockdown phenocopies loss of PP4SMK-1. It is know that phosphorylation of SPT-5 in its C-terminal domain (CTD) promotes transcriptional elongation and that this phosphorylation needs to be reversed for efficient transcriptional termination. Our work now shows that dephosphorylation of SPT-5, namely by PP4SMK-1, can be crucial also for earlier transcriptional events which precede elongation. Notably, this mechanism affects expression of only parts of the genome, including many DAF-16 target genes, which renders PP4SMK-1 so important for DAF-16’s physiological roles.