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During the development of multicellular organisms, cellular diversity is often achieved through asymmetric cell divisions that produce two daughter cells having different developmental potentials. Prior to an asymmetric cell division, cellular components segregate to opposite ends of the cell defining an axis of polarity. The mitotic spindle rotationally aligns along this axis of polarity, thereby ensuring that the cleavage plane is positioned such that segregated components end up in individual daughter cells. Here we report our characterization of a novel gene required for spindle alignment in Caenorhabditis elegans. During the first mitosis in spd-3(oj35) embryos the spindle failed to align along the anterior/posterior axis, leading to abnormal cleavage configurations, spd-3(oj35) embryos had additional defects reminiscent of dynein/dynactin loss-of-function possibly caused by the mislocalization of dynactin. Surprisingly, we found that SPD-3 :: GFP localized to mitochondria. Consistent with this localization, spd-3(oj35) worms exhibited slow growth and increased ATP concentrations, which are phenotypes similar to those described for other mitochondrial mutants in C. elegans. To our knowledge, SPD-3 is the first example of a link between mitochondria and spindle alignment in C. elegans.

Cytokinesis is the essential process that partitions cellular contents into daughter cells. To identify and characterize cytokinesis proteins rapidly, we used a functional proteomic and comparative genomic strategy. Midbodies were isolated from mammalian cells, proteins were identified by multidimensional protein identification technology (MudPIT), and protein function was assessed in Caenorhabditis elegans. Of 172 homologs disrupted by RNA interference, 58% displayed defects in cleavage furrow formation or completion, or germline cytokinesis. Functional dissection of the midbody demonstrated the importance of lipid rafts and vesicle trafficking pathways in cytokinesis, and the utilization of common membrane cytoskeletal components in diverse morphogenetic events in the cleavage furrow, the germline, and neurons. [ABSTRACT FROM AUTHOR]

In many organisms, the dynein-dynactin complex is required for the alignment of the mitotic spindle onto the axis of polarity of a cell undergoing asymmetric cell division. How this complex transduces polarity cues, either intrinsic or extrinsic, and rotationally aligns the spindle accordingly is not well understood. The Caenorhabditis elegans blastomere P2 polarizes the neighboring EMS blastomere, which causes the EMS spindle to rotationally align along the defined axis of polarity via two redundant signaling pathways: Wnt and Src. Here, we describe how components of the dynactin complex became locally enriched at the P2-EMS border prior to and during rotational alignment of their spindles. Wnt and Src signaling were required for both localized dynactin enrichment, and for rotational alignment of the P2 and EMS spindles. Depleting the trimeric G-protein subunit Gα did not abolish dynactin accumulation to the P2-EMS border, yet both EMS and P2 spindles failed to rotationally align, indicating that Gα might act to regulate dynein/dynactin motor activity. By RNAi of a weak dnc-1(ts) allele, we showed that dynactin activity was required at least for EMS spindle rotational alignment. [ABSTRACT FROM AUTHOR]

In many organisms, the dynein-dynactin complex is required for the alignment of the mitotic spindle onto the axis of polarity of a cell undergoing asymmetric cell division. How this complex transduces polarity cues, either intrinsic or extrinsic, and rotationally aligns the spindle accordingly is not well understood. The Caenorhabditis elegans blastomere P2 polarizes the neighboring EMS blastomere, which causes the EMS spindle to rotationally align along the defined axis of polarity via two redundant signaling pathways: Wnt and Src. Here, we describe how components of the dynactin complex became locally enriched at the P2-EMS border prior to and during rotational alignment of their spindles. Wnt and Src signaling were required for both localized dynactin enrichment, and for rotational alignment of the P2 and EMS spindles. Depleting the trimeric G-protein subunit G alpha did not abolish dynactin accumulation to the P2-EMS border, yet both EMS and P2 spindles failed to rotationally align, indicating that G alpha might act to regulate dynein/dynactin motor activity. By RNAi of a weak dnc-1(ts) allele, we showed that dynactin activity was required at least for EMS spindle rotational alignment.