Bop1 is a mouse WD40 repeat nucleolar protein involved in 28S and 5.8S rRNA processing and 60S ribosome biogenesis

We have identified and characterized a novel mouse protein, Bop1, which contains WD40 repeats and is highly conserved through evolution. bop1 is ubiquitously expressed in all mouse tissues examined and is upregulated during mid-G(1) in serum-stimulated fibroblasts. Immunofluorescence analysis shows that Bop1 is localized predominantly to the nucleolus. In sucrose density gradients, Bop1 from nuclear extracts cosediments with the 50S-80S ribonucleoprotein particles that contain the 32S rRNA precursor. RNase A treatment disrupts these particles and releases Bop1 into a low-molecular-weight fraction. A mutant form of Bop1, Bopl1 Delta, which lacks 231 amino acids in the N-terminus, is colocalized with wild-type Bop1 in the nucleolus and in ribonucleoprotein complexes, Expression of Bop1 Delta leads to cell growth arrest in the G(1) phase and results in a specific inhibition of the synthesis of the 28S and 5.8S rRNAs without affecting 18S rRNA formation. Pulse-chase analyses show that Bop1 Delta expression results in a partial inhibition in the conversion of the 36S to the 328 pre-rRNA and a complete inhibition of the processing of the 328 pre-rRNA to form the mature 28S and 5.8S rRNAs, Concomitant with these defects in rRNA processing, expression of Bop1 Delta in mouse cells leads to a deficit in the cytosolic 60S ribosomal subunits. These studies thus identify Bop1 as a novel, nonribosomal mammalian protein that plays a kev role in the formation of the mature 288 and 5.8S rRNAs and in the biogenesis of the 60S ribosomal subunit.