Abstract
BackgroundIn vertebrates, poly(A) binding protein (PABP) is known to exist in five different isoforms. PABPs are primarily cytosolic with the exception of the nuclear PABP (PABPN1), which is located in the nucleus. Within the nucleus, PABPN1 is believed to bind to the poly(A) tail of nascent mRNA and along with cleavage and polyadenylation specificity factor (CPSF) define the length of the newly synthesized poly(A) tail.Methodology/Principal FindingsThe cellular role of PABP1 has been extensively studied over the years; however, the function of other PABPs remains poorly defined. In order to understand the role of PABPN1 in cellular mRNA metabolism and it’s interrelation with other PABPs, we depleted PABPN1 using RNAi in HeLa and HEK293 cells. Our results show that PABPN1 depletion did not have any effect on the poly(A) tail length, nuclear export of mRNA, mRNA translation, and transcription. Rather, PABPN1 depletion resulted in a compensatory response as observed by increased level of PABP5 and nuclear accumulation of PABP4. In addition, PABP4 was associated with the poly(A) tract of pre-mRNA and CPSF in PABPN1 depleted cells. Nevertheless, PABPN1 depletion significantly affected cell survival as evidenced by an increase in apoptosis markers: phosphorylated p53 and PUMA and as judged by the expression of ER stress marker GRP78.ConclusionOur results suggest that although function of PABPN1 may be compensated by nuclear translocation of PABP4 and perhaps by increase in the cytoplasmic abundance of PABP5, these were not sufficient to prevent apoptosis of cells. Thus PABPN1 may have a novel anti apoptotic role in mammalian cells.
Highlights
Mammalian nuclear poly(A) binding protein (PABPN1) is a highly conserved nuclear RNA binding protein with specificity towards the poly(A) tract of eukaryotic mRNAs
Our results suggest that function of PABPN1 may be compensated by nuclear translocation of PABP4 and perhaps by increase in the cytoplasmic abundance of PABP5, these were not sufficient to prevent apoptosis of cells
PABPN1 Depletion Several biochemical studies have implicated a role of PABPN1 in assisting poly(A) polymerase in the addition of poly(A) tracts at the 3` end of mRNA, and as a ruler to control the length of the poly(A) tract to approximately 250 adenines [28]
Summary
Mammalian nuclear poly(A) binding protein (PABPN1) is a highly conserved nuclear RNA binding protein with specificity towards the poly(A) tract of eukaryotic mRNAs. It consists of one typical RNA recognition motifs (RRM) domain with consensus RNP1 and RNP2 motifs in the central region of the polypeptide, and an arginine rich C- terminal domain [1] Both RNP domains and the C-terminal region of PABPN1 are required for binding to RNA and its polypeptide partners respectively. The fold of the third loop and dimerization of the crystal are distinct features of PABPN1 [2].The nuclear localization signal is located between amino acids 289–306 and overlaps with the oligomerization domain [3,4]. PABPN1 is believed to bind to the poly(A) tail of nascent mRNA and along with cleavage and polyadenylation specificity factor (CPSF) define the length of the newly synthesized poly(A) tail
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