ISOLATION OF U3 SNORNP FROM CHO CELLS - A NOVEL 55 KDA PROTEIN BINDS TO THE CENTRAL PART OF U3 SNORNA

U3 snoRNP, the most abundant of the small nucleolar ribonucleoprotein particles (snoRNPs), has previously been demonstrated to participate i n pre-rRNA maturation. Here we report the purification of U3 snoRNP fr om CHO cells using anti-m3G-immunoaffinity and mono Q anion-exchange c hromatography. Isolated U3 snoRNPs contain three novel proteins, of 15 , 50 and 55 kDa respectively. These proteins may represent core U3 sno RNP proteins whose binding mediates the association of other proteins, such as fibrillarin, that are lost during purification. Using a rabbi t antiserum raised against the 55 kDa protein, and an in vitro reconst itution assay, we have localised the 55 kDa protein binding site on th e U3 snoRNA. Stable binding of the 55 kDa protein requires sequences l ocated between nucleotides 97 and 204 of the human U3 snoRNA, includin g the evolutionarily conserved B and C sequence motifs.