A Novel PI4,5P2 Regulated Poly(A) Polymerase Controls the Expression of Select mRNAs

Abstract

Phosphoinositides are a family of lipid signalling molecules that regulate a plethora of cellular functions in eukaryotes. Phosphatidylinositol 4,5-bisphosphate (PI4,5P2), the central component in the phosphoinositide signalling circuitry is primarily generated by type I phosphatidylinositol 4-phosphate 5-kinases (PIPKIα, Iβ and Iγ). In addition to functions in the cytosol, phosphoinositides are present in the nucleus where they modulate a number of functions, however, the mechanism by which they directly regulate nuclear functions remains ambiguous. PIPKIs regulate cellular functions through interactions with protein partners, often PI4,5P2 effectors, that target PIPKIs to discrete sub-cellular compartments resulting in the spatial and temporal generation of PI4,5P2 required for the regulation of specific signalling pathways. Therefore, to determine roles for nuclear PI4,5P2 we set out to identify proteins which interacted with the nuclear PIPK, PIPKIα. In this study we demonstrate that PIPKIα co-localizes at nuclear speckles and interacts with a newly identified non-canonical poly(A) polymerase, nuclear Speckle Targeted PIPKIα Regulated-Poly(A) Polymerase (Star-PAP) and that the activity of Star-PAP can be specifically regulated by PI4,5P2. Star-PAP and PIPKIα function together in a complex to control the expression of select mRNAs, including the transcript encoding the key cytoprotective enzyme Heme Oxygenase-1 (HO-1) and other oxidative stress response genes by regulating the 3′-end formation of their mRNAs. Taken together, the data demonstrates a model whereby phosphoinositide signalling works in tandem with complement pathways to regulate the activity of Star-PAP and the subsequent biosynthesis of its target mRNA. The results reveal an unanticipated mechanism for the integration of nuclear phosphoinositide signals and a novel means for the regulation of gene expression.