Expression and subcellular locations of two forms of nucleolar protein B23 in rat tissues and cells.

Protein B23 (Mr/pI = 38,000/5.1) is a major RNA-associated nucleolar phosphoprotein and putative ribosome assembly factor. Previous cDNA and genomic analyses indicated the presence of two forms of the protein (B23.1 and B23.2) which arise from a single gene via alternative splicing. Expression of the two B23 isoforms in rat tissues cells has been studied at the mRNA level using northern blot and RNA-polymerase chain reaction analyses and at the protein level by western blotting. In all tissues examined, the relative amount of B23.1 mRNA was much higher (3-6-fold) than B23.2 mRNA. For B23.1, the level of mRNA and protein expression was highest in Novikoff hepatoma cells, followed by testis, liver, and kidney. This suggests that B23.1 expression is correlated with the rate of proliferation of the tissue and/or the rate of ribosome biogenesis. In contrast, the amount of B23.2 mRNA did not vary significantly among the three normal tissues, although it was elevated in Novikoff hepatoma cells. Similarly, the B23.2 protein was barely detectable in the normal rat tissues, but significant quantities were found in the Novikoff cells. In cell fractionation experiments, B23.1 was predominantly found in nucleoli, whereas B23.2 was located in cytoplasmic fractions and appeared to be associated with cytoskeletal elements. These studies suggest that the two protein B23 isoforms are engaged in very different functions.