Box C/D small nucleolar RNA trafficking involves small nucleolar RNP proteins, nucleolar factors and a novel nuclear domain

Nucleolar localization of box C/D small nucleolar (sno) RNAs requires the b ox C/D motif and, in vertebrates, involves transit through Cajal bodies (CB ). We report that in yeast, overexpression of a box C/D reporter leads to a block in the localization pathway with snoRNA accumulation in a specific s ub-nucleolar structure, the nucleolar body (NB). The human survival of moto r neuron protein (SMN), a marker of gems/CB, specifically localizes to the NB when expressed in yeast, supporting similarities between these structure s. Box C/D snoRNA accumulation in the NB was decreased by mutation of Srp40 and increased by mutation of Nsr1p, two related nucleolar proteins that ar e homologous to human Nopp140 and nucleolin, respectively. Box C/D snoRNAs also failed to accumulate in the NB, and became delocalized to the nucleopl asm, upon depletion of any of the core snoRNP proteins, Nop1p/fibrillarin, Snu13p, Nop56p and Nop5p/Nop58p. We conclude that snoRNP assembly occurs ei ther in the nucleoplasm, or during transit of snoRNAs through the NB, follo wed by routing of the complete snoRNP to functional sites of ribosome synth esis.