in c.elegans the first embryonic cleavage is asymmetric, producing blastomeres of different sizes and developmental potential.
par-3 is a maternally expressed gene that is required for several aspects of embryonic polarity. in embryos from homozygous
par-3 mothers, the first cleavage is equal, p granules fail to localize, and the fates of the daughter blastomeres (ab and
p1) are altered. at second cleavage, the reorientation of the spindle apparatus that normally takes place only in
p1 takes place in the ab as well. we cloned and sequenced
par-3. the transcript is 4,893 nucleotides and encodes a novel protein of 138 kd. using antibodies raised against a portion of the protein, we determined that
par-3 is localized to the anterior periphery of the one-cell stage embryo. at the two-cell stage, the protein is found all around the periphery of ab, but is restricted to the anterior periphery of
p1. in
p2 and
p3, this pattern is repeated, with
par-3 protein being restricted to the side of the cell destined to give rise to the somatic daughter. the staining in somatic cells persists for several divisions. based on analysis of the distribution of the
par-3 protein in various par mutants, we propose that
par-3 plays two roles in establishing the axis of polarity in cells of the germline lineage. first,
par-3 is required to restrict the location of
par-1 and
par-2 to the posterior. second,
par-3 contributes to regulation of spindle orientation by promoting a strong interaction between the cortex and astral microtubules.