Plays a central role in ribosomal RNA processing. Probable catalytic subunit of H/ACA small nucleolar ribonucleoprotein (H/ACA snoRNP) complex; which catalyzes pseudouridylation of rRNA. This involves the isomerization of uridine such that the ribose is subsequently attached to C5; instead of the normal N1. Pseudouridine ('psi') residues may serve to stabilize the conformation of rRNAs. May function as a pseudouridine synthase. Binds in vitro to centromeres and microtubules. It is a centromeric DNA-CBF3-binding factor which is involved in mitotic chromosome segregation. Essential for cell growth. {ECO:0000269|PubMed:10523634; ECO:0000269|PubMed:8336724; ECO:0000269|PubMed:9315678; ECO:0000269|PubMed:9472021; ECO:0000269|PubMed:9848653}.