Arabidopsis CBF5 interacts with the H/ACA snoRNP assembly factor NAF1

The conserved protein CBF5, initially regarded as a centromere binding protein in yeast and higher plants, was later found within nucleoli and in Cajal bodies of yeast and metazoa. There, it is assumed to be involved in post-transcriptional pseudouridinylation of various RNA species that might be important for RNA processing. We found EYFP-labeled CBF5 of A. thaliana to be located within nucleoli and Cajal bodies, but neither at centromeres nor somewhere else on chromosomes. Arabidopsis mutants carrying a homozygous T-DNA insertion at the CBF5 locus were lethal. Yeast two-hybrid and mRNA expression analyses demonstrated that AtCBF5 is co-expressed and interacts with a previously uncharacterized protein containing a conserved NAF1 domain, presumably involved in H/ACA box snoRNP biogenesis. The homologous yeast protein has been shown to contribute to RNA pseudouridinylation. Thus, AtCBF5 might have an essential function in RNA processing rather than being a kinetochore protein.