Asymmetric cell division depends on coordinating the position of the mitotic spindle with the axis of cellular polarity. We provide evidence that LET-99 is a link between polarity cues and the downstream machinery that determines spindle positioning in C. elegans embryos. In let-99 one-cell embryos, the nuclear-centrosome complex exhibits a hyperactive oscillation that is dynein dependent, instead of the normal anteriorly directed migration and rotation of the nuclear-centrosome complex. Furthermore, at anaphase in let-99 embryos the spindle poles do not show the characteristic asymmetric movements typical of wild type animals. LET-99 is a DEP domain protein that is asymmetrically enriched in a band that encircles P lineage cells. The LET-99 localization pattern is dependent on PAR polarity cues and correlates with nuclear rotation and anaphase spindle pole movements in wild-type embryos, as well as with changes in these movements in par mutant embryos. In particular, LET-99 is uniformly localized in one-cell par-3 embryos at the time of nuclear rotation. Rotation fails in spherical par-3 embryos in which the eggshell has been removed, but rotation occurs normally in spherical wild-type embryos. The latter results indicate that nuclear rotation in intact par-3 embryos is dictated by the geometry of the oblong egg and are consistent with the model that the LET-99 band is important for rotation in wild-type embryos. Together, the data indicate that LET-99 acts downstream of PAR-3 and PAR-2 to determine spindle positioning, potentially through the asymmetric regulation of forces on the spindle.
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