Secretion for cytokinesis

The actomyosin contractile ring involved in separating a dividing cell in two only gets so far. The job is finished, according to work from Adam Gromley, Stephen Doxsey (University of Massachusetts Medical Center, Worcester, MA), and colleagues, by a burst of secretory vesicle fusion.Doxsey's group was looking for a function for a vertebrate centrosomal protein called centriolin. A defect in cytokinesis was not what they expected to find when they knocked down centriolin function, but there it was. “We saw a thin wisp of cytoplasm retained between [daughter] cells,” says Doxsey.

In wild-type cells, this normally transient wisp harbored a ring of centriolin, which then recruited several components of the secretory pathway, including the exocyst. Later, SNARE proteins followed.

Unlike the actomyosin ring, the centriolin/SNARE ring did not constrict during cytokinesis. Instead, the authors saw, secretory vesicles from one of the daughter cells moved to the ring, piling up on that side. After accumulating briefly, the vesicles apparently fused in a rapid burst. The daughter cells then split apart on the vesicle side of the ring, leaving the cell on the opposite side with an intact lingering ring, similar to the bud scar of yeast. As in centriolin mutants, the cells remained linked when vesicle fusion was impaired.

The triggers for vesicle transport and for fusion are unknown. “It's clearly highly regulated,” says Doxsey. “But what the cell is monitoring—DNA, centrosomes, or something else—is still unclear.” The group is especially keen to determine how the asymmetric vesicle secretion is generated. They hypothesize that differences in the centrioles—one daughter gets the original and the other a copy—might be involved.

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