PARsing Embryonic Polarity

est effects are the par genes (partitioning defective). Mutations in these genes lead to similar but somewhat mutant embryos have cleavage pattern defects and al-Current understanding of the way in which embryonic terations in the fates of the founder cells that can be polarity is established relies heavily on studies of mater-attributed to the mislocalization of some or all of the nal effect lethal mutants in D. melanogaster and C. ele-fate-determining proteins. Molecular analyses of the par gans (St. Johnston and Nü sslein-Volhard, 1992; Rose genes have revealed that they encode proteins with and Kemphues, 1998). Although the analysis in worms functional domains that could act in intracellular signal-began in earnest about a decade after the explosion of ing. These include two proteins with serine/threonine information from flies, we now know enough about both kinase domains (PAR-1 and PAR-4), two proteins with systems to make comparison meaningful (Bowerman, PDZ domains (PAR-3 and PAR-6), and a protein with a 1995, 1998), and to ask whether there are conserved " ring finger " zinc binding domain (PAR-2). Four of the mechanisms used for establishing embryonic polarity. PAR proteins are asymmetrically distributed in asym-Thus far, the single common feature is translational re-metrically dividing cells of the early germline lineage (P 0 , pression, which has been shown to localize important P 1 , P 2 , P 3). PAR-1 and PAR-2 become enriched at the fate regulators in both systems (see Evans et al., 1994). posterior periphery of the zygote and PAR-3 and PAR-6 Now, however, in this issue of Cell, Shulman and col-become enriched at the anterior periphery. Another pro-leagues (2000) report an analysis in D. melanogaster of tein, PKC-3, an atypical protein kinase C, colocalizes the first molecule to play an important and perhaps with PAR-3 and PAR-6 and has a similar loss-of-function conserved role in both animals, PAR-1. phenotype. The current model is that PAR proteins act Establishing Embryonic Polarity in C. elegans in the one-cell embryo to interpret the polarity cue pro-Embryogenesis in C. elegans begins with a series of five vided by the sperm, mediating the local changes in cyto-asymmetric cleavages that create six founder blasto-plasm that establish the A/P axis. meres, each of which produces a clone of cells with a Much remains to be understood about how the PAR distinct behavior and set of cell fates (Figure 1A). Al-proteins function. Perhaps the most significant gap in …