Catalyzes the transfer of one adenosine molecule from an ATP to an mRNA poly(A) tail bearing a 3'-OH terminal group and enhances mRNA stability and gene expression. Can also elongate RNA oligos ending with uridine molecule, provided that the sequence is adenosine-rich. Mainly targets mRNAs encoding endoplasmic reticulum-targeted protein. ATP + RNA(n) = diphosphate + RNA(n)-3'-adenine ribonucleotide Interacts with BCCIP and PABPC1; the interaction has no effect on TENT5C poly(A) polymerase function. Interacts with PLK4; this interaction leads to the TENT5C recruitment into the centrosome. Recruited into the centrosome through its interaction with PLK4. Belongs to the TENT family.