Colocalization within the nucleolus of two highly related IFN-induced human nuclear phosphoproteins with nucleolin

We have previously reported the identification of two interferon (IFN)-indu ced cDNAs which code for two proteins, named 41 and 75, which have homology to a number of proteins involved in regulating gene expression. Here we es tablish that these cDNAs correspond to in vivo synthesized mRNAs. Expressio n of the 41 and 75 cDNAs, both in vitro and in vivo, generated proteins of 30 and 68 kDa, respectively. In a variety of mammalian cells, 41 and 75 wer e found to be located in the nucleus, with 41 being localized to the nucleo lus, whereas 75, although it is mainly concentrated at the periphery of the nucleolus, is also found throughout the nucleoplasm. Treatment with interf eron results in a translocation of 41 to the periphery of the nucleolus and it is in this region that the two proteins colocalize. 41 and 75 were foun d to colocalize with nucleolin but not with B23 or fibrillarin, three nucle olar proteins involved in ribosome synthesis. This colocalization was not a ffected by low concentrations of actinomycin D. In view of this and since 4 1 and 75 have homology to proteins involved in regulating gene expression, we suggest that, in association with nucleolin, they play a role in ribosom e biogenesis. (C) 1999 Academic Press.