Kinases in the Polyadenylation Complex

Abstract

After transcription of a gene, the abundance of a protein in a cell can be controlled through various mechanisms operating on the mRNA, including regulation of alternative splicing, polyadenylation, stability and degradation, nuclear export, and accessibility to translation factors. Star-PAP is a noncanonical poly(A) polymerase that has a proline-rich region (PRR), which splits the catalytic domain. Star-PAP functions as part of a larger macromolecular complex. However, in addition to the common proteins found in other polyadenylation complexes, the Star-PAP complex includes phosphatidylinositol-4-phosphate 5 kinase Iα (PIPKIα), and Star-PAP activity is stimulated by phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ]. Gonzales et al . showed that the Star-PAP complex isolated from transfected cells also includes protein kinase activity and that Star-PAP is phosphorylated by the associated kinase. The protein kinase activity of the Star-PAP complex was inhibited by the addition of PI(4,5)P 2 , which led the authors to identify the kinase as casein kinase Iα (CKIα), which is known to be inhibited by PI(4,5)P 2 . CKIα and Star-PAP colocalized in nuclear speckles, which are a site of pre-mRNA processing, and inhibitors of CKIα blocked Star-PAP phosphorylation in vitro. Star-PAP is responsible for increasing the abundance of many mRNAs based on microarray experiments; however, CKIα and PIPKIα appear to control the targeting of Star-PAP to a subset of mRNAs involved in oxidative stress. Cells in which CKIα or PIPKIα was decreased using siRNA showed a specific decrease in the abundance of a subset of mRNAs processed by Star-PAP, including two that encode enzymes involved in the response to oxidative stress (HO-1 and NQO1), without affecting other transcripts targeted by Star-PAP. Thus, the presence of a lipid kinase and a protein kinase in the Star-PAP complex appears to target Star-PAP to specific mRNA targets and connects nuclear lipid signaling to mRNA processing. M. L. Gonzales, D. L. Mellman, R. A. Anderson, CKIα is associated with and phosphorylates Star-PAP and is also required for expression of select Star-PAP target messenger RNAs. J. Biol. Chem. 283 , 12665-12673 (2008). [Abstract] [Full Text]