daf-2;par-5

14-3-3 proteins are evolutionarily conserved and ubiquitous proteins that function in a wide variety of biological processes. Here we define a new role for C. elegans 14-3-3 proteins in life span regulation. We identify two C. elegans 14-3-3 proteins as interacting proteins of a major life span regulator, the C. elegans SIR2 ortholog, SIR-2.1. Similar to sir-2.1, we find that overexpression of either 14-3-3 protein (PAR-5 or FTT-2) extends life span and that this is dependent on DAF-16, a forkhead transcription factor (FOXO), another important life span regulator in the insulin/IGF-1 signaling pathway. Furthermore, we show that both 14-3-3 proteins are co-expressed with DAF-16 and SIR-2.1 in the tissues critical for life span regulation. Finally, we show that DAF-16/FOXO also physically interacts with the 14-3-3 proteins. These results suggest that C. elegans 14-3-3 proteins can regulate longevity by cooperating with both SIR-2.1 and DAF-16/FOXO.

The longevity of Caenorhabditis elegans is promoted by extra copies of the sir-2.1 gene in a manner dependent on the forkhead transcription factor DAF-16. We identify two C. elegans 14-3-3 proteins as SIR-2.1 binding partners and show that 14-3-3 genes are required for the life-span extension conferred by extra copies of sir-2.1. 14-3-3 proteins are also required for SIR-2.1-induced transcriptional activation of DAF-16 and stress resistance. Following heat stress, SIR-2.1 can bind DAF-16 in a 14-3-3-dependent manner. By contrast, low insulin-like signaling does not promote SIR-2.1/DAF-16 interaction, and sir-2.1 and the 14-3-3 genes are not required for the regulation of life span by the insulin-like signaling pathway. We propose the existence of a stress-dependent pathway in which SIR-2.1 and 14-3-3 act in parallel to the insulin-like pathway to activate DAF-16 and extend life span.

The Caenorhabditis elegans daf-2/insulin-like signaling pathway is critical for regulating development, longevity, metabolism and stress resistance. We identified the 14-3-3 protein FTT-2 to be a new regulatory component of this pathway. We found that RNAi knock down of ftt-2 specifically enhanced the daf-2-mediated dauer formation phenotype. Furthermore, ftt-2 knock down caused the nuclear accumulation of DAF-16/FOXO, the forkhead transcription factor that is the major downstream effecter of daf-2/insulin-like signaling, and enhanced the transcriptional activities of DAF-16. In contrast to ftt-2, RNAi knock down of par-5/ftt-1, the only other gene predicted to encode a 14-3-3 protein in C. elegans, did not show any notable effect on dauer formation, DAF-16 localization, or DAF-16 downstream gene transcription, underscoring the functional specification of FTT-2 and PAR-5 despite their high sequence homology. Using co-immunoprecipitation, we revealed that FTT-2 formed a complex with GFP-fused DAF-16 in C. elegans. Our results indicate that FTT-2 binds to DAF-16 in C. elegans and regulates DAF-16 by sequestering it in the cytoplasm. A similar mechanism of regulation of FOXO by 14-3-3zeta has been reported in mammalian cells, highlighting the high degree of conservation of the daf-2/insulin-like signaling pathway.