Yeast Nop15p is an RNA-binding protein required for pre-rRNA processing and cytokinesis

Nop15p is an essential protein that contains an RNA recognition motif (RRM) and localizes to the nucleoplasm and nucleolus. Cells depleted of Nop15p failed to synthesize the 25S and 5.8S rRNA components of the 60S ribosomal subunit, and exonucleolytic 5' processing of 5.8S rRNA was strongly inhibited. Pre-rRNAs co-precipitated with tagged Nop15p confirmed its association with early pre-60S particles and Nop15p bound a pre-rRNA transcript in vitro. Nop15p-depleted cells show an unusually abrupt growth arrest prior to substantial depletion of ribosomal subunits. Following cell synchronization in mitosis, Nop15p-depleted cells undergo nuclear division with wild-type kinetics, activate the mitotic exit network and disassemble their mitotic spindle. However, they uniformly arrest at cytokinesis and fail to assemble a contractile actin ring at the bud neck. In dividing wild-type cells, segregation of nucleolar proteins to the daughter nuclei occurs after separation of the nucleoplasm. In these late mitotic cells, Nop15p was partially delocalized from the nucleolus to the nucleoplasm, consistent with a specific function in cell division in addition to its role in ribosome synthesis.