Star-PAP is a nuclear non-canonical poly(A) polymerase (PAP) that shows 
specificity toward mRNA targets. Star-PAP activity is stimulated by lipid 
messenger phosphatidyl inositol 4,5 bisphoshate (PI4,5P2) and is regulated by 
the associated Type I phosphatidylinositol-4-phosphate 5-kinase that synthesizes 
PI4,5P2 as well as protein kinases. These associated kinases act as coactivators 
of Star-PAP that regulates its activity and specificity toward mRNAs, yet the 
mechanism of control of these interactions are not defined. We identified a 
phosphorylated residue (serine 6, S6) on Star-PAP in the zinc finger region, the 
domain required for PIPKIα interaction. We show that S6 is phosphorylated by 
CKIα within the nucleus which is required for Star-PAP nuclear retention and 
interaction with PIPKIα. Unlike the CKIα mediated phosphorylation at the 
catalytic domain, Star-PAP S6 phosphorylation is insensitive to oxidative stress 
suggesting a signal mediated regulation of CKIα activity. S6 phosphorylation 
together with coactivator PIPKIα controlled select subset of Star-PAP target 
messages by regulating Star-PAP-mRNA association. Our results establish a novel 
role for phosphorylation in determining Star-PAP target mRNA specificity and 
regulation of 3'-end processing.