Bms1p, a G-domain-containing protein, associates with Rcl1p and is required for 18S rRNA biogenesis in yeast

Maturation of 18S rRNA and biogenesis of the 40S ribosomes in yeast require s a large number of trans-acting factors, including the U3 small nucleolar ribonucleoprotein (U3 snoRNP), and the recently characterized cyclase-like protein RcI1p. U3 snoRNP is a key particle orchestrating early 35S rRNA cle avage events. A unique property of RcI1p is that it specifically associates with U3 snoRNP, but this association appears to occur only at the level of nascent ribosomes and not with the U3 monoparticle. Here we report the cha racterization of Bms1p, a protein that associates with RcI1p in multiple st ructures, including a specific complex sedimenting at around 10S. Like RcI1 p, Bms1p is an essential, evolutionarily conserved, nucleolar protein, and its depletion interferes with processing of the 35S pre-rRNA at sites A(0), A(1), and A(2), and the formation of 40S subunits. The N-terminal domain o f Bms1p has structural features found in regulatory GTPases and we demonstr ate that mutations of amino acids implicated in GTP/GDP binding affect Bms1 p activity in vivo. The results indicate that Bms1p may act as a molecular switch during maturation of the 40S ribosomal subunit in the nucleolus.