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

Abstract

Maturation of 18S rRNA and biogenesis of the 40S ribosomes in yeast requires a large number of trans-acting factors, including the U3 small nucleolar ribonucleoprotein (U3 snoRNP), and the recently characterized cyclase-like protein Rcl1p. U3 snoRNP is a key particle orchestrating early 35S rRNA cleavage events. A unique property of Rcl1p 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 characterization of Bms1p, a protein that associates with Rcl1p in multiple structures, including a specific complex sedimenting at around 10S. Like Rcl1p, 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 of Bms1p has structural features found in regulatory GTPases and we demonstrate that mutations of amino acids implicated in GTP/GDP binding affect Bms1p activity in vivo. The results indicate that Bms1p may act as a molecular switch during maturation of the 40S ribosomal subunit in the nucleolus.

Role of GTPases in ribosome assembly

“…24,25 Initial experiments provided strong evidence that Bms1 was involved in assembly of the 40S subunit. 24,25 Using purified recombinant proteins, it was subsequently shown that Bms1 hydrolyzes GTP, binds directly to Rcl1, a putative endonuclease, and the essential snoRNA U3.…”

Proteomic snapshot analyses of preribosomal ribonucleoprotein complexes formed at various stages of ribosome biogenesis in yeast and mammalian cells

“…It had been shown by conventional biochemical and genetic analyses that the snoRNP that contains the U3 snoRNA differed in structure from other C/D-box snoRNPs; in addition to the core proteins that are common to C/D-box snoRNPs, U3 snoRNP contained at least eight more proteins-Sof1p, Mpp10p, Rrp9p, Imp3p, Imp4p, Lcp5p, Rcl1p, and Bms1p (Fig. 3A) (Billy et al, 2000;Wegierski et al, 2001). However, the conventional biochemical and genetic analyses have not shown the total picture of this snoRNP until the proteomic approach was applied to the analysis of the U3 snoRNP complexes described in a later section; briefly, the U3 snoRNA was found to form a large 80S RNP complex that contained at least 28 proteins.…”

Rcl1 Protein, a Novel Nuclease for 18 S Ribosomal RNA Production

“…Rcl1 (RNA 3Ј-phosphate cyclase-like) is a conserved S. cerevisiae nucleolar protein (7,8) named for its similarity to RNA cyclases. Cyclases catalyze the ATP-dependent formation of a 2Ј,3Ј-cyclic phosphodiester from a 3Ј-phosphate (9).…”

“…However, the Rcl1 protein contains an insertion not present in cyclases, and thus, Rcl1 lacks cyclase activity (7). Rcl1 is required for co-transcriptional steps in 18 S rRNA biogenesis (7) and is delivered to pre-ribosomes by Bms1, an essential GTPase that is also required for assembly of the 40 S ribosomal subunit (8,10,11). The regulation of this protein by a GTPase suggests that its function may represent an irreversible gatekeeper in ribosome assembly.…”