Ribosome assembly factor; auxiliary component of snR4 and snR45 acetylation guide box C/D snoRNPs; recruits box C/D snoRNPs snR4 and snR45 to guide ribosomal RNA acetylation by Kre33p; associates with snR4 and snR45 in their free form and is required for their stable incorporation into 90S pre-ribosomes