Required for nuclear membrane fusion during karyogamy. During metaphase, localizes near the mitotic spindle region, and its localization shifts to the chromosomes as they reach the end of the spindle. During karyomere fusion, detected in prominent puncta, mainly at karyomere-karyomere interfaces corresponding to putative fusion sites. Embryos arrest development shortly after the midblastula transition. During cleavage (2-, 64-, and 1,000-cell-stage embryos) all blastomeres of mutant embryos contain morphologically abnormal nuclei that appeared fragmented in formation of multiple micronuclei. Has a similar domain organization and 27% homology to yeast kar5.