The metaphase-to-anaphase transition is a critical juncture during mitosis and meiosis. Studies in our lab have identified the
emb-30 gene as being required for this crucial cell-cycle transition. Previous genetic and phenotypic analysis of 16 chemically induced mutations has revealed that
emb-30 is required for the metaphase-to-anaphase transition in many, if not all, cells in C. elegans [1]. Germline-specific
emb-30 alleles block the meiotic divisions. Oocyte meiotic maturation, fertilization, and spindle assembly all occur normally, but the oocytes do not progress to anaphase of meiosis I. Analysis of a temperature-sensitive allele revealed that
emb-30 function is required for germline mitosis as well as meiosis. The most severe class of alleles, class I, cause zygotic sterility with germline and somatic defects that are consistent with a mitotic role for
emb-30 in somatic lineages. Analysis of the vulval lineages in
emb-30 class I alleles suggest that mitosis is lengthened and eventually arrested, when maternally contributed
emb-30 becomes limiting. When the maternal load of
emb-30 contributed to class I mutants is further reduced, class I homozygotes arrest earlier during embryogenesis and larval development with many cells blocked at metaphase of mitosis. This suggests that
emb-30 function is required in many or all cells for the metaphase-to-anaphase transition. Consistent with this role, positional cloning of
emb-30 has shown that it encodes the likely C. elegans ortholog of APC4, a component of the anaphase promoting complex/cyclosome in yeast and vertebrates. Metaphase arrest in
emb-30 mutants likely reflects an essential role in M-phase progression, and is not a secondary consequence of spindle assembly checkpoint activation [1]. In addition, a reduction in
emb-30 activity can suppress the lethality and sterility caused by a null mutation in
mdf-1, a component of the spindle assembly checkpoint machinery [2]. This suggests that the essential role of the spindle assembly checkpoint in C. elegans is not in the process of chromosome segregation per se, but rather in delaying anaphase onset until all chromosomes are properly attached to the spindle. In order to identify other genes that function with
emb-30 to promote anaphase onset, we have screened for mutations that phenocopy zygotic sterile
emb-30 class I alleles. So far, after screening 6,000 haploid genomes, six mutations have been isolated. Four of these mutations fail to complement
evl-22(
ar104) which was isolated in a screen for sterile mutations with vulval defects [3]. In addition, like
emb-30 class I mutants, germ cells in the distal mitotic zone in these animals block in mitosis as revealed by phosphohistone H3 staining. Future work will be focused on the genetic identification of new genes required for the metaphase-to-anaphase transition and on characterizing the localization of EMB-30 using antibodies and a rescuing GFP reporter. Progress on these fronts will be reported. 1. Furuta, T., Tuck, S., Kirchner, J., Koch, B., Auty, R., Kitagawa, R., Rose, A. M., and Greenstein, D. 2000. Molecular Biology of the Cell 11: 1401-1419. 2. Kitagawa, R. and Rose, A.M. 1999. Nature Cell Biology 1: 514-521. 3. Seydoux, G., Savage, C., and Greenwald, I. 1993. Developmental Biology 157: 423-36.