Yeast Nop3p has structural and functional similarities to mammalian pre-mRNA binding proteins

Indirect immunofluorescence shows the yeast protein Nop3p/Npl3p/ Mts1p to be localized both in the nucleoplasm and the nucleolus. Here we show that Nop3p can be efficiently UV cross-linked in vivo to poly(A)⁺ RNA. In higher eukaryotes it would therefore be classed as a heterogenous nuclear ribonucleoprotein (hnRNP) component. As judged by immunofluorescence and in situ hybridization, a temperature sensitive allele of nop3 leads to the accumulation of poly(A)⁺ RNA in the nucleus at non permissive temperature. The sequence of Nop3p shows homology to the larger members of the SR protein family of splicing factors, particularly in the RRMH domain. We have, however, detected no splicing defects in strains genetically depleted to Nop3p or carrying a temperature-sensitive allele.