IMMUNODETECTION OF POLY(A) BINDING-PROTEIN-II IN THE CELL-NUCLEUS

During the polyadenylation of pre-mRNA in vitro, poly(A) binding prote in II (PAB II) binds to the growing poly(A) tail, stimulating its exte nsion. The subcellular localization of PAB II was investigated with an antibody affinity-purified from rabbit serum raised against the purif ied protein. Immunofluorescence microscopy detected PAB II exclusively in the cell nucleus, both in a widespread staining and in more intens ely stained ''speckles.'' PAB II was excluded from the nucleoli. By el ectron microscopy, PAB II was also found almost exclusively in the nuc leus, predominantly in clusters of interchromatin granules, likely cor responding 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 micro scopy also detected PAB II in nucleoli. The distribution corresponds l argely 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 pol yadenylation. (C) 1994 Academic Press, Inc.