The PAR-aPKC system: lessons in polarity

Ten years ago, par-1 and par-3 were cloned as two of the six par genes essential for the asymmetric division of the Caenorhabditis elegans zygote. PAR-1 is a protein kinase, whereas PAR-3 is a PDZ-domain-containing scaffold protein. Work over the past decade has shown that they are part of an evolutionarily conserved PAR-aPKC system involved in cell polarity in various biological contexts. Recent progress has illustrated the common principle that the PAR-aPKC system is the molecular machinery that converts initial polarity cues in the establishment of complementary membrane domains along the polarity axis. In most cases, this is achieved by mutually antagonistic interactions between the aPKC-PAR-3-PAR6 complex and PAR-1 or PAR2 located opposite. However, accumulating evidence has also revealed that mechanisms by which the asymmetrically localized components of the PAR-aPKC system are linked with other cellular machinery for developing polarity are divergent depending on the cell type.