Proteome-wide quantification of inositol pyrophosphate-protein interactions

Inositol polyphosphates (InsPs) and inositol pyrophosphates (PP-InsPs) constitute a group of highly phosphorylated molecules that are involved in many cellular signaling processes. To characterize discrete signaling events of these structurally closely related molecules, a mass spectrometry approach was developed to derive apparent binding constants for these ligands on a proteome-wide scale. The method employed a series of chemically synthesized, biotinylated affinity reagents for inositol hexakisphosphate (InsP ₆ ), and the inositol pyrophosphates 1PP-InsP ₅ , 5PP-InsP ₅ and 1,5(PP) ₂ -InsP ₄ (also termed InsP ₈ ). Application of these affinity reagents at different concentrations, in combination with tandem mass tag (TMT) labeling, provided binding data for thousands of proteins from a mammalian cell lysate. Investigation of different enrichment conditions, where Mg ²⁺ ions were either available or not, showcased a strong influence of Mg ²⁺ on the protein binding capacities of PP-InsPs. Gene ontology analysis closely linked PP-InsP-interacting proteins to RNA processing in the nucleus and nucleolus. Subsequent data analysis enabled a targeted search for protein pyrophosphorylation among PP-InsP interactors, and identified four new pyrophosphorylated proteins. The data presented here constitute a valuable resource for the community, and application of the method reported here to other biological contexts will enable the exploration of PP-InsP dependent signaling pathways across species.