Human autoantibodies identify a protein in dense fibrillar and granular components of the nucleolus

A novel nucleolar protein of an approximate molecular weight of 60 kDa was identified by immunoprecipitation in human cells with an autoimmune sclerod ermic serum. It maps at the ultrastructural level to nucleolar granular and dense fibrillar components. This 60 kDa protein could not be demonstrated in Western blots suggesting that the epitope structure is complex and/or is sensitive to the treatment of cells. The immunoprecipitation results indic ate that the 60 kDa protein is not a phosphoprotein and is not associated w ith a nucleolar RNA containing particle. The identified protein represents a new autoimmune marker in the field of systemic connective tissue diseases .