Isoform 1: Required for ribosome biogenesis and telomere maintenance. 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. Each rRNA can contain up to 100 pseudouridine ('psi') residues; which may serve to stabilize the conformation of rRNAs. Also required for correct processing or intranuclear trafficking of TERC; the RNA component of the telomerase reverse transcriptase (TERT) holoenzyme.