The asymmetric segregation of cell fate determinants is an important mechanism for generating diverse cell fates. In the C. elegans zygote, MEX-5 is initially symmetrically distributed in the cytoplasm. During zygotic polarization, MEX-5 segregates to the anterior cytoplasm, leading to its asymmetric inheritance by the anterior daughter cell, AB (Schubert CM et al, 2000). The mechanisms that underlie zygotic MEX-5 asymmetry have not been determined. We find that MEX-5 segregation results from posterior to anterior redistribution. Using a photoconvertible Dendra:MEX-5 fusion protein, we find that MEX-5 segregation does not require new MEX-5 synthesis. Additionally, Dendra:MEX-5 levels do not decrease during segregation, indicating MEX-5 is not degraded during polarization. By photoconverting Dendra:MEX-5 subcellularly, we find important differences in the dynamics of MEX-5 in the anterior and posterior of the embryo. From the posterior, Dendra:MEX-5 diffuses throughout the embryo within 2 minutes before progressively accumulating in the anterior. In contrast, Dendra:MEX-5 moves more slowly from the anterior to the posterior, suggesting MEX-5 may be trapped or tethered in the anterior. Based on these data, we propose the differences in anterior and posterior MEX-5 mobility result in MEX-5 asymmetry. How is the difference between anterior and posterior MEX-5 mobility established? Our findings suggest the posterior kinase PAR-1 increases MEX-5 mobility in the posterior through phosphorylation. In
par-1 embryos, Dendra:MEX-5 mobility is low in both the anterior and the posterior. In vitro, recombinantly expressed PAR-1 can phosphorylate MBP:MEX-5, and this phosphorylation is significantly reduced for MBP:MEX-5 (S458A), a phosphorylation site previously shown to be required for MEX-5 asymmetry (J. Tenlen, E. Munro, J. Priess; 2005 International Worm Meeting). Similar to
par-1 embryos, Dendra:MEX-5 (S458A) mobility is low in both the anterior and posterior. Thus, PAR-1 phosphorylation of MEX-5 S458 may be required to increase MEX-5 mobility in the posterior. In
pkc-3 embryos, where PAR-1 mislocalizes to both the anterior and posterior, Dendra:MEX-5 mobility is high in both the anterior and posterior. This increased MEX-5 mobility requires the ability of MEX-5 to respond to PAR-1, because Dendra:MEX-5 (S458A) mobility remains low in
pkc-3 embryos. Taken together, these data suggest MEX-5 asymmetry results from differences in anterior and posterior mobility that are regulated by PAR-1 phosphorylation of MEX-5 S458 in the posterior.