Bms1p, a novel GTP-binding protein, and the related Tsr1p are required fordistinct steps of 40S ribosome biogenesis in yeast

Bms1p and Tsr1p define a novel family of proteins required for synthesis of 40S ribosomal subunits in Saccharomyces cerevisiae. Both are essential and localize to the nucleolus. Tsr1p shares two extended regions of similarity with Bms1p, but the two proteins function at different steps in 40S riboso me maturation. Inactivation of Bms1p blocks at an early step, leading to di sappearance of 20S and 18S rRNA precursors. Also, slight accumulation of an aberrant 23S product and significant 35S accumulation are observed, indica ting that pre-rRNA processing at sites Ao, Ai, and A(2) is inhibited. In co ntrast, depletion of Tsr1p results in accumulation of 20S rRNA. Because pro cessing of 20S to 18S rRNA occurs in the cytoplasm, this suggests that Tsr1 p is required for assembly of a transport- or maturation-competent particle or is specifically required for transport of 43S pre-ribosomal particles, but not 60S ribosome precursors, from the nucleus to the cytosol. Finally, Bms1p is a GTP-binding protein, the first found to function in ribosome ass embly or rRNA processing.