Regulation of Mammalian Poly(A) Polymerase Activity

Poly(A) polymerase (PAP) is the enzyme catalyzing the synthesis of the adenine tail to the 3’-end of mRNA. This A-tail is present on the majority of the primary RNA transcripts of protein-coding genes, and is important for mRNA stability, export to the cytoplasm and translation. Therefore, PAP is a key regulator of eukaryotic gene expression. This thesis describes the heterogeneity of PAP and the functional significance of multiple isoforms of PAP. PAP exists in many different isoforms generated by three different mechanisms, gene duplication, alternative mRNA processing and post-translational modification. In HeLa cell extracts three different forms of PAP being 90, 100 and 106 kDa in size have been detected, where the 106 kDa isoform is a phosphorylated version of the 100 kDa species. It is shown that the N-terminal region of PAP contains a region required for catalysis, while the C-terminal end is important for the interaction with the cleavage and polyadenylation specificity factor (CPSF). Interestingly, it was found that also the extreme N-terminal end is important for the interaction with CPSF. This region is post-translationally modified by phosphorylation. Five alternatively spliced forms of PAP mRNAs are encoded by the PAPOLA gene while one unique species is encoded by the PAPOLG gene. The analysis showed that the exact structure of the alternatively spliced C-terminal end of PAP played an important role for catalytic efficiency. Thus, the C-terminal end contains a region important for modulating the catalytic efficiency of PAP. Aminoglycoside antibiotics inhibit PAP activity, most likely by displacement of catalytically important divalent metal ions. Data shows that different aminoglycosides inhibit PAP activity by different mechanisms suggesting that the binding sites for the different aminoglycosides do not completely overlap. It is concluded that aminoglycosides interfere with enzymes important for housekeeping functions in mammalian cell, which may explain some of the toxic side effects caused by aminoglycoside antibiotics in clinical practice