Lipid Transfer Proteins and PI4KIIα Generate a Phosphoinositide-Linked Proteome

Phosphoinositide (PIP _n ) lipid second messengers in membranes regulate a myriad of cellular processes. In the cytosol, the phosphatidylinositol (PI) 3-kinase (PI3K)/Akt pathway is scaffolded on IQGAP1 to facilitate the activation of Akt by the synthesis of PI3,4,5P ₃ . In the nucleus, PIP _n signaling occurs in regions devoid of membranes via their stable association with proteins. While several of these proteins have been identified, understanding the extent and impact of protein-linked PIP _n signaling warrants further investigation. The tumor suppressor p53, was shown in the companion paper to be regulated by PI transfer proteins (PITPs) and a PI 4-kinase (PI4KIIα), which are required to form p53-PIP _n complexes that assemble a nuclear PI3K/Akt pathway. Here we report that class I PITPs (PITPα/β) and PI 4-kinase initiate PIP _n linkages to many different proteins. PITPα/β and PI4KIIα accumulate in the nucleoplasm in response to stress and are necessary to synthesize nuclear PIP _n s linked to proteins. These PITPα/β-dependent protein-PIP _n complexes are detected by metabolically labeling cells with the PIP _n precursor [H ³ ]- myo -inositol and resist denaturation and SDS-PAGE, indicating that these protein-PIP _n complexes represent a putative posttranslational modification. Proteomic analyses of proteins that are regulated by PITPα/β and/or are linked to PI4,5P ₂ have identified an emerging PIPylome that is enriched in metabolic, signaling, cytoskeletal and DNA repair pathway components. Taken together, these data provide evidence for an emerging proteome with linked PIP _n s that represent a PIP _n signaling paradigm that is distinct from the membrane-localized pathway but utilizes many of the same PIP kinases and phosphatases.

Phosphatidylinositol transfer proteins and PI 4-kinase initiate a PIP _n -linked protein network in membrane-free regions.