The poly(A) polymerase Star-PAP is regulated by stably associated phosphoinositide messengers

Star-PAP is a noncanonical poly(A) polymerase that controls gene expression. Star-PAP was previously reported to bind PIPKI⍺ and its product PI(4,5)P₂, which regulate Star-PAP activity and expression of specific genes. Recent studies have revealed a nuclear p53-phosphoinositide signaling pathway in which the phosphatidylinositol transfer proteins (PITPs) and phosphoinositide kinases/phosphatases bind p53 to sequentially modify p53-linked phosphoinositides and regulate p53 function. Here, we demonstrate that multiple phosphoinositides are also coupled to Star-PAP in response to stress. This pathway is initiated by PITP⍺/β binding to Star-PAP, and the Star-PAP-phosphoinositide complexes are sequentially modified by PI4KII⍺, PIPKI⍺, IPMK, and PTEN. The formation of Star-PAP-phosphoinositide complexes enhances the association of the small heat shock proteins HSP27 and ⍺B-crystallin with Star-PAP. Knockdown of the PITPs, PIP kinases, or HSP27 reduces the expression of Star-PAP targets. Our results demonstrate that PITP⍺/β play a key role in the assembly of Star-PAP-phosphoinositide complexes that are sequentially interconverted by PIP kinases/phosphatases and recruit the small heat shock proteins to these complexes to regulate Star-PAP activity in response to stress.