PAR-3 and PAR-6 are required to polarize the C. elegans zygote and early embryonic cells. In flies and vertebrates, homologs of PAR-3 and PAR-6 function in the early stages of epithelial cell polarization. Recently it was shown that removal of C. elegans PAR-3 by RNAi disrupts the polarity of a small number of epithelial cells in the larval spermatheca (Aono et al., 2004). Because it has not been possible to remove PAR function at earlier stages of development, it is unclear if PAR proteins play a more general role in polarization of different types of epithelial cells. Many epithelial cells first form in the embryo and can display marked asymmetries. For example, nuclei become positioned near the apical surface of intestinal epithelial cells, and rows of microfilaments needed for the process of embryonic elongation become positioned at the apical surface of hypodermal (skin) cells. Antibodies raised against PAR-3 and PAR-6 stain the apical surface of embryonic epithelial cells, suggesting that PAR-3 and PAR-6 may function in polarizing these cells. However, existing mutations in
par-3 or
par-6 do not cause obvious embryonic defects in epithelial cell polarity. We have obtained a putative null allele of
par-6 (generously provided by the National Bioresource Project, Dr. Shohei Mitani) that causes a more severe recessive larval lethal phenotype and can be rescued by a wild-type
par-6::gfp transgene. Because perduring maternal protein could mask an even earlier role for
par-6 in embryonic epithelia, we engineered a maternally expressed transgene encoding a tagged PAR-6 that is degraded in early embryonic cells. Using this transgene in combination with the
par-6 null allele, we can obtain embryos that lack detectable PAR-6 prior to formation of the first embryonic epithelia. Surprisingly, intestinal nuclei still become asymmetrically localized in these PAR-depleted embryos. However, embryonic elongation fails and internal cells can be extruded through the hypodermis, suggesting defects in hypodermal cell polarization. We are currently investigating the cell biological basis of these defects.