Exosome complexes are 3 to 5 exoribonucleases composed of subunits that are critical for numerous distinct RNA metabolic (ribonucleometabolic) pathways. Several studies have implicated the exosome subunits Rrp6 and Dis3 in chromosome segregation and cell division but the functional relevance of these findings remains unclear. Here, we report that, in Drosophila melanogaster S2 tissue culture cells, dRrp6 is required for cell proliferation and error-free mitosis, but the core exosome subunit Rrp40 is not. Micorarray analysis of dRrp6-depleted cell reveals increased levels of cell cycleand mitosis-related transcripts. Depletion of dRrp6 elicits a decrease in the frequency of mitotic cells and in the mitotic marker phospho-histone H3 (pH3), with a concomitant increase in defects in chromosome congression, separation, and segregation. Endogenous dRrp6 dynamically redistributes during mitosis, accumulating predominantly but not exclusively on the condensed chromosomes. In contrast, core subunits localize predominantly to MTs throughout cell division. Finally, dRrp6-depleted cells treated with microtubule poisons exhibit normal kinetochore recruitment of the spindle assembly checkpoint protein BubR1 without restoring pH3 levels, suggesting that these cells undergo premature chromosome condensation. Collectively, these data support the idea that dRrp6 has a core exosome-independent role in cell cycle and mitotic progression.
INTRODUCTIONExosome complexes are critical players in the processing and degradation of many RNA species (Houseley et al., 2006). Found in both archaebacteria and eukaryotes, these complexes are composed of a "core" set of subunits. With respect to the yeast and human core complexes, they consist of six RNase PH domain-containing proteins (Rrp41, Rrp42, Rrp43, Rrp45, Rrp46, and Mtr3) and three S1 domain-containing proteins (Rrp4, Rrp40, and Csl4). Our understanding of exosome complex form and function has advanced greatly though the report of the crystal structures of archaebacterial (Buttner et al., 2005; and eukaryotic exosome core (Liu et al., 2006) complexes. The RNase PH subunits assemble into a hexameric ring upon which the S1 domain subunits reside. It has been proposed that the cylindrical shape and central channel of exosome complexes have evolved as a means to tightly regulate RNA recognition and subsequent decay (Lorentzen and Conti, 2006).Eukaryotes have compartment-specific exosome subunits and cofactors in addition to the core subunits. Although the eubacterial RNase R homolog Dis3 (also referred to as Rrp44) interacts with the yeast core (Mitchell et al., 1997;Allmang et al., 1999b;Dziembowski et al., 2007), it does not associate with either the human or trypanosome core (Chen et al., 2001;Estevez et al., 2003). Moreover, many archaebacteria lack an RNase R-like protein (Koonin et al., 2001). Thus, strictly speaking, Dis3 is not a core exosome component. Another exosome subunit missing in archaebacteria but present in eukaryotes is Rrp6, an RNase D homolog. Rrp6 associates specifical...