PIP₃-binding proteins promote age-dependent protein aggregation and limit survival in C. elegans

Class-I phosphatidylinositol 3-kinase (PI3K_I) converts phosphatidylinositol 4,5-bisphosphate (PIP₂) to phosphatidylinositol 3,4,5-triphosphate (PIP₃). PIP₃ comprises two fatty-acid chains that embed in lipid-bilayer membranes, joined by glycerol to inositol triphosphate. Proteins with domains that specifically bind that head-group (e.g. pleckstrin-homology [PH] domains) are thus tethered to the inner plasma-membrane surface where they have an enhanced likelihood of interaction with other PIP₃-bound proteins, in particular other components of their signaling pathways. Null alleles of the C. elegans age-1 gene, encoding the catalytic subunit of PI3K_I, lack any detectable class-I PI3K activity and so cannot form PIP₃. These mutant worms survive almost 10-fold longer than the longest-lived normal control, and are highly resistant to a variety of stresses including oxidative and electrophilic challenges. Traits associated with age-1 mutation are widely believed to be mediated through AKT-1, which requires PIP₃ for both tethering and activation. Active AKT complex phosphorylates and thereby inactivates the DAF-16/FOXO transcription factor. However, extensive evidence indicates that pleiotropic effects of age-1- null mutations, including extreme longevity, cannot be explained by insulin likereceptor/ AKT/FOXO signaling alone, suggesting involvement of other PIP₃-binding proteins. We used ligand-affinity capture to identify membrane-bound proteins downstream of PI3K_I that preferentially bind PIP₃. Computer modeling supports a subset of candidate proteins predicted to directly bind PIP₃ in preference to PIP₂, and functional testing by RNAi knockdown confirmed candidates that partially mediate the stress-survival, aggregation-reducing and longevity benefits of PI3K_I disruption. PIP₃-specific candidate sets are highly enriched for proteins previously reported to affect translation, stress responses, lifespan, proteostasis, and lipid transport.