Poster abstracts

Poster number 6 submitted by Amy Graham

Dynamic Form and Function of Exosome Subunit Complexes during Mitosis and Cytokinesis

Amy C. Graham (Molecular Biology and Microbiology; Case Western Reserve University), Daniel L. Kiss (Molecular Biology and Microbiology; Case Western Reserve University), Sarah B. Smith (Molecular Biology and Microbiology; Case Western Reserve University), Erik D. Andrulis (Molecular Biology and Microbiology; Case Western Reserve University)

Abstract:
Exosome complexes are composed of 3’ to 5’ exoribonucleases that are essential for viability, conserved in eukaryotes and archaea, and critical for numerous distinct RNA metabolic pathways. Previous studies have implicated specific exosome subunits in chromosome segregation and cell division, but the exact mechanisms underlying these functions have remained unclear or considered indirect. We find that endogenous exosome subunits dynamically and differentially redistribute during mitosis and cytokinesis in Drosophila melanogaster S2 tissue culture cells. For example, dRrp6 is recruited to centrosomes during prophase and prometaphase, then accumulates on the perichromosomal layer during metaphase, anaphase, and telophase, and finally is targeted to the midbody and partitioned centrosomes during cytokinesis. In contrast, dRrp4, dRrp40, and dRrp46 are almost exclusively, yet differentially, enriched on microtubular structures including the spindle, astral microtubules, spindle remnant, and midbody. Treatment of S2 cells with drugs that perturb microtubule architecture elicits subunit- and stage-specific localization patterns. RNA interference of certain exosome subunits leads to a decrease in mitotic cells with an increase in aberrant chromosome segregation events. These results suggest that exosome subunits have discrete interactions and functions during distinct stages of cell division and that the signaling pathways regulating subunit mobilization and interplay are complex and tightly regulated.

We further explored exosome subunit function in cell division in the budding yeast, Saccharomyces cerevisiae. Notably, several yeast exosome subunit mutants are defective in mitotic progression as evidenced by an increase in cells harboring pre-anaphase spindles. These spindles are frequently misoriented along the mother-bud axis and mispositioned away from the bud neck. Moreover, exosome subunit mutant cells contain elongated astral microtubules. Finally, consistent with a role for exosome subunits in regulating microtubule structure, exosome mutant cell growth is inhibited by microtubule destabilizing agents. Given that RNA is required for spindle assembly in Xenopus (Blower et al., 2005), we hypothesize that exosome subunits directly participate in a conserved microtubule-based mechanism that is required for proper mitotic progression.

References:
M. Blower, M. Nachury, R. Heald, K. Weis. (2005) A Rae1-Containing Ribonucleoprotein Complex Is Required for Mitotic Spindle Assembly. Cell. 121(2); 223-234.

Keywords: exosome, mitosis, cytokinesis