Nop1p / Fibrillarin was originally identified
as a nucleolar protein of bakers yeast, Saccharomyces cerevisiae
(accession P15646). The Nop1p protein is essential for yeast viability
and is localized in the nucleoli. The human homologue of Nop1p
is fibrillarin (accession P22087) a component of the nucleolar
small nuclear ribonucleoprotein (snRNP) particle. The human fibrillarin
gene is located on chromosome 19 (19q13.1). Fibrillarin proteins
have been cloned and sequenced from several other species (Mouse,
accession P35550, Xenopus accession P22232, C. elegans accession
Q22053, and S. pombe accession P35551). The N terminal 80 amino
acids contain multiple copies based on the peptide RGG, and the
remaining 240 amino acids consist of the fibrillarin domain. A
fibrillarin homologue has also been identified in the genome of
the archean Methanococcus (accession NC_000909). This protein lacks
the RGG rich N-terminal extension but is clearly homologues to
the other sequences throughout all of the fibrillarin domain. The
structure of this molecule has been determined and shown to consist
of 2 extended b-sheets flanked by 4 a-helixes. Patients with the
autoimmune disease scleroderma often have strong circulating autoantibodies
to a 34kDa protein which was subsequently found to be fibrillarin.
Fibrillarin is an excellent marker for the nucleolus.
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