ASE-1 - A NOVEL PROTEIN OF THE FIBRILLAR CENTERS OF THE NUCLEOLUS ANDNUCLEOLUS ORGANIZER REGION OF MITOTIC CHROMOSOMES

A novel nucleolar component has been identified and cloned using a hum an autoimmune serum. This antigen, as inferred from the cDNA sequence, is an Mr 55000 protein. Immune blot analysis, however, of both the na tive protein and the in vitro translation products of the cDNA showed that they migrate on SDS-PAGE at an apparent molecular mass of 90000 A BLAST search using the cDNA sequence indicated that it is in an antis ense orientation to and overlaps the gene of the DNA repair enzyme ERC C-1. An open reading frame, without a translational start site, had be en observed by others in this region of the chromosome 19 (19q13.3) an d the putative protein was termed ASE-1 (Anti-Sense to ERCC-1). Our cD NA is a full-length equivalent of that open reading frame. ASE-1 was f ound to contain two domains that are present in a number of nucleolar specific proteins originating from a variety of organisms: a glycine-, arginine-and phenylalanine-rich putative nucleotide interaction domai n and an alternating basic/acidic region. Indirect immunofluorescence analysis using antibodies generated to cloned regions of ASE-1 indicat ed that this protein occurs at the fibrillar centres of the nucleolus in interphase, the putative sites of rDNA transcription, and during ce ll division it is localized to the nucleolus organizer regions of the chromosomes. ASE-1 co-localises with the RNA polymerase I transcriptio n initiation factor UBF/NOR-90 throughout all stages of the cell cycle and these two proteins associate with each other in vitro.