In an initial study of anti-nuclear
antibodies in the chronic inflammatory
bladder disease interstitial cystitis, we reported that 7% of
interstitial cystitis patients studied had
autoantibodies to the nucleolus. We now report that, using an autoimmune serum from a patient with
interstitial cystitis, we have identified and partially characterized a novel
protein with an M(r) of approximately 55 kDa (hereafter referred to as No55) localized to the granular component of the nucleolus. No55 was initially characterized by diffuse nucleolar immunofluorescence staining in interphase cells and by Western blotting as a 55-kDa doublet on whole-
cell extracts. During mitosis, No55 was associated with chromosomes and appeared in prenucleolar bodies during telophase, but it did not colocalize with
p80-coilin in coiled bodies. Immunoelectron microscopy revealed that No55 was localized uniformly throughout the granular component of the nucleolus compared with a more peripheral localization of nucleolar granular component
protein B23. On segregation of the nucleolus with
actinomycin D, No55 remained with the granular component of the segregated nucleolus, whereas
protein B23 was found predominantly in the nucleoplasm. Finally, a
cDNA expression library was screened with the human
autoantibody against No55, and a 2.4-kb insert was isolated, subcloned to homogeneity, and then sequenced. Analysis of this sequence showed an open reading frame of approximately 1.3 kb coding for 437
amino acids with a predicted molecular weight of 50 kDa. A search of the gene sequence database indicated homology with SC65, a rat synaptonemal complex
protein. Therefore, on the basis of molecular weight, nucleolar sublocalization, response to
actinomycin D, and
cDNA sequence determination, No55 is a novel
protein of the interphase nucleolus.