The spindle apparatus is critical for chromosome segregation as well as cleavage-plane determination. During development, regulated positioning of the spindle is used in asymmetric divisions that produce daughter cells with different sizes, cytoplasmic determinants and developmental fates. Previously, we have shown that
lin-5 is required for chromosome segregation and spindle positioning. LIN-5 localizes to the meiotic and mitotic spindle as well as the cell cortex. During mitotic divisions, LIN-5 appears to act together with the G protein regulators GPR-1/GPR-2 and GOA-1/GPA-16 G<font face=symbol>a</font> subunits in the generation of forces that pull on the spindle asters. The role of LIN-5 in meiosis and the relative contributions of LIN-5 at the cell cortex versus spindle asters are still poorly understood. We have identified novel candidate partners of LIN-5 through immunoprecipitation from embryonic lysates followed by mass spectrometry and yeast two-hybrid assays. One of these proteins is related to human ASPM (Abnormal SPindle-like, Microcephaly-associated) and named ASPM-1. We found that
aspm-1 knockdown by RNAi greatly reduces the localization of LIN-5 at the meiotic spindle and mitotic spindle asters, but not at the cortex. While defects in meiosis and mitotic chromosome segregation were apparent, the cleavage plane of the early cell divisions remained unaltered. Animals homozygous for the
aspm-1(
ok1208) deletion develop into sterile adults, with defects in the postembryonic lineages that are limited when compared to
lin-5(0) mutants. Interestingly, the
ok1208 phenotype is stronger at reduced temperatures, which may point to a function associated with microtubule dynamics.
lin-5 and
aspm-1 RNAi embryos show partially overlapping defects.
aspm-1 is involved in meiotic spindle formation while
lin-5 is not. Time-lapse recordings showed that positioning of the meiotic spindle is especially disturbed in
aspm-1(RNAi) embryos. As in
lin-5(RNAi) embryos, such embryos show defects in rotation of the meiotic spindle which prevent polar body extrusion. Thus, ASPM-1 appears to localize and function with LIN-5 at the meiotic spindle and mitotic spindle asters. In contrast, cortical LIN-5 is likely required in the generation of forces that pull on the asters. We have also identified a potential novel partner for cortical LIN-5, and will present our latest results at the meeting.