PABP: Lysates

The poly(A)-binding protein (PABP), which is found complexed to the 3-prime poly(A) tail of eukaryotic mRNA, is required for poly(A) shortening and translation initiation. Grange et al. (1987) isolated a melanoma cell cDNA encoding human PABP. The predicted 633-amino acid protein contains 4 repeats of an approximately 80-amino acid unit in its N-terminal half. The authors found that this repeat region is highly conserved between human and yeast PABP and is sufficient for poly(A) binding. In vitro translation of the human PABP cDNA yielded a protein with an apparent molecular mass of 73 kD by SDS-PAGE. Northern blot analysis indicated that PABP is expressed as a 2.9-kb mRNA in human melanoma cells. Gorlach et al. (1994) noted that each of the 4 repeats of PABP is a ribonucleoprotein (RNP) consensus sequence RNA-binding domain. They determined that PABP has a pI of approximately 10.3 and is a very abundant, stable protein. Immunofluorescence studies of mammalian cells indicated that PABP is located exclusively in the cytoplasm. However, using both indirect immunofluorescence and tagging of PABP1 by fusion to the green fluorescent protein (GFP), Afonina et al. (1998) demonstrated that PABP1 shuttles between the nucleus and cytoplasm. PABP1 accumulated in the nucleus when transcription was inhibited, suggesting that active transcription is required for nuclear export of PABP1.