NAP57, A MAMMALIAN NUCLEOLAR PROTEIN WITH A PUTATIVE HOMOLOG IN YEASTAND BACTERIA

We report the identification and molecular characterization of a novel nucleolar protein of rat liver. As shown by coimmunoprecipitation thi s protein is associated with a previously identified nucleolar protein , Nopp140, in an apparently stoichiometric complex and has therefore b een termed NAP57 (Nopp140-associated protein of 57 kD). Immunofluoresc ence and immunogold electron microscopy with NAP57 specific antibodies show colocalization with Nopp140 to the dense fibrillar component of the nucleolus, to coiled bodies, and to the nucleoplasm. Immunogold st aining in the nucleoplasm is occasionally seen in the form of curvilin ear tracks between the nucleolus and the nuclear envelope, similar to those previously reported for Nopp140. These data suggest that Nopp140 and NAP57 are indeed associated with each other in these nuclear stru ctures. The cDNA deduced primary structure of NAP57 shows a protein of a calculated molecular mass of 52,070 that contains a putative nuclea r localization signal near its amino and carboxy terminus and a hydrop hobic amino acid repeat motif extending across 84 residues. Like Nopp1 40, NAP57 lacks any of the known consensus sequences for RNA binding w hich are characteristic for many nucleolar proteins. Data bank searche s revealed that NAP57 is a highly conserved protein. A putative yeast (S. cerevisiae) homolog is 71% identical. Most strikingly, there also appears to be a smaller prokaryotic (E. coli and B. subtilis) homolog that is nearly 50% identical to NAP57. This indicates that NAP57 and i ts putative homologs might serve a highly conserved function in both p ro- and eukaryotes such as chaperoning of ribosomal proteins and/or of preribosome assembly.