The par genes, defined by a group of maternal effect partitioning defective mutations, play an important role in the generation of polarity in the one-cell embryo of C. elegans. The phenotype of
par-6 embryos resembles that of
par-3 mutants in that they exhibit symmetric and synchronous early cleavages, mislocalize P granules, and misorient mitotic spindle in the AB blastomere. Because PAR-3, a protein containing 3 PDZ domains, is absent from the periphery of
par-6 blastomeres, it has been proposed that
par-6 is necessary to localize PAR-3 on the cortex or maintain the cortical localization of PAR-3. We have cloned
par-6 via germ line transformation and antisense RNA injection.
par-6 was mapped on LG I about 0.5 m.u. to the right of
unc-101. We were able to localize
par-6 further to a 200 kb region bordered by W05A3 and W02A11 cosmids with PCR-based deletion mapping. Cosmid R09B9 rescued
par-6 whereas the overlapping cosmid on the left, R02E6, and the overlapping cosmid on the right, T06G6, did not (Thanks to Dr. Alan Coulson for the cosmids). T06G6 and a largely overlapping cosmid of R09B9, T26E3, have been almost completely sequenced by the genome project. Two Genefinder-predicted open reading frames (ORF) were found on T26E3 but not on T06G6. Blast searches of these two ORFs revealed several EST cDNA clones. The injection of antisense RNA made from the cDNA clone of the second ORF phenocopied
par-6 mutant defects ( Thanks to Dr. Yuji Kohara for the cDNA clones). Based on the sequence of the ORF,
par-6 gene encodes a protein containing a PDZ domain and a putative transmembrane domain which is markered by a stretch of hybrophobic amino acids. PDZ domains have been known to be able to bind to PDZ domains and thus, mediate protein-protein interaction between PDZ-domain containing proteins. We propose that PAR-6 localizes PAR-3 protein to the periphery via PDZ/PDZ interaction. We are currently testing our hypothesis.