Immunodetection of poly(A) binding protein II in the cell nucleus.

During the polyadenylation of pre-mRNA in vitro, poly(A) binding protein II (PAB II) binds to the growing poly(A) tail, stimulating its extension. The subcellular localization of PAB II was investigated with an antibody affinity-purified from rabbit serum raised against the purified protein. Immunofluorescence microscopy detected PAB II exclusively in the cell nucleus, both in a widespread staining and in more intensely stained "speckles." PAB II was excluded from the nucleoli. By electron microscopy, PAB II was also found almost exclusively in the nucleus, predominantly in clusters of interchromatin granules, likely corresponding to the speckles observed by immunofluorescence microscopy, and in perichromatin fibrils, which represent nascent transcripts and probably the sites of pre-mRNA processing. In addition, electron microscopy also detected PAB II in nucleoli. The distribution corresponds largely to that of other factors involved in the processing of pre-mRNA and is thus in agreement with the proposed role of the protein in polyadenylation.