In a screen for temperature-sensitive embryonic lethal mutations, we isolated a mutant called
or198ts that has several defects in early C. elegans embryos. In
or198ts mutant embryos, the first mitotic spindle often is delayed in aligning along the anterior-posterior axis, and shortly after the spindle begins to elongate, it is no longer clearly visible by DIC videomicroscopy. In about half of
or198ts mutant embryos, the first attempt at cytokinesis fails. In embryos in which cytokinesis succeeds, we observe penetrant defects in nuclear positioning and in mitotic spindle orientation at the two cell stage. In addition, in interphase partial ectopic cleavage furrows may form; there is extra pinching and blebbing at the cell membranes, and the cells are misshapen.
or198ts is on LGIII at approximately --0.8 m.u., and corresponds to the Genefinder locus F11H8.1. We have identified a single base lesion in the mutant that converts a valine to alanine at amino acid position 269. F11H8.1 encodes an E1-like activating enzyme with around 30% amino acid identity to its yeast and human homologs. E1-like activating enzymes are involved in conjugating ubiquitin-related proteins (e.g. SUMO-1/Smt3p) to a small subset of other cellular proteins in a manner similar, although not identical to ubiquitination. In contrast to ubiquitin, which targets proteins for degradation, post-translational modification by ubiquitin-like proteins may regulate the activity and intracellular localization of the proteins they modify. In both yeast and mammals, Smt3p/SUMO modifies multiple proteins. Perhaps the pleiotropic defects observed in
or198ts embryos reflect multiple targets of this post-translational modification. To our knowledge, this is the first example of a ubiquitin-like modification being important for cell division processes in multicellular eukaryotes.