We have produced monoclonal antibodies against purified nuclei from the yeast Saccharomyces cerevisiae and have characterized three different antibodies that recognize a protein with an apparent molecular weight of 38,000, termed p38. Subcellular fractionation shows that virtually all of p38 occurs in the nuclear fraction. High concentrations of salt (1 M) or urea (6 M) effectively solubilize p38 from a nuclear envelope fraction prepared by digestion of nuclei with DNase. Indirect immunofluorescence demonstrates a crescent shaped distribution of p38 at the inner periphery of the nucleus, with p38 extending between dividing pairs of cells during (closed) mitosis. Postembedding immunogold electron microscopy shows decoration of the densely stained "crescent" region of the yeast nucleus, confirming the localization of p38 to the nucleolus. One of the monoclonals, D77, cross reacts on immunoblots with a single protein of molecular weight 37,000 from purified rat liver nuclei. Indirect immunofluorescence localizes this protein to the nucleolus, and shows that it is dispersed throughout the cell during mitosis. The yeast and rat liver nucleolar proteins behave similarly when electrophoresed in two dimensions, and appear to have basic pI values. Analysis of immunological cross-reactivity using D77, and antibodies specific for nucleolar proteins from other sources, suggests that the rat liver protein is fibrillarin, and demonstrates that p38 shares epitopes with fibrillarin, as well as with other vertebrate nucleolar proteins.

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