The C. elegans zygote becomes polarized shortly after fertilization by the sperm-donated microtubule-organizing center (MTOC), which induces PAR polarity on the cortex. How the MTOC communicates with the cortex is not known. Possible signals include proteins that accumulate on the MTOC''s pericentriolar material (PCM) and the microtubules themselves (Motegi and Seydoux, 2007 for review). We have obtained evidence for a direct role for microtubules by studying PAR-2. PAR-2 is a RING finger domain protein that accumulates on the cortex nearest the MTOC during polarization (Boyd et al, 1996). PAR-2 also accumulates on the MTOC itself, most prominently during mitosis. We have found that PAR-2 has a strong affinity for microtubules in vitro (KDapp<0.2 mM). This affinity depends on three separate microtubule-binding domains, which overlap with the domain in PAR-2 required for localization to the cortex (Hao et al., 2006). Mutations in one of the domains reduce PAR-2 affinity for microtubules by 5 fold. When expressed in zygotes depleted of endogenous PAR-2, this PAR-2 mutant was defective in localizing to MTOCs, but was able to localize to the posterior cortex and rescue the
par-2 embryonic lethality as efficiently as wild-type, suggesting that microtubule binding is not essential under normal conditions. The microtubule-binding defective mutant, however, was not able to localize to the cortex in zygotes that lack or delay PCM assembly or have reduced cortical actin contractility. In those zygotes, wild-type PAR-2 can still accumulate on the cortex nearest the MTOC, but PAR-2 defective in microtubule binding cannot and remains in the cytoplasm. We conclude that microtubule binding contributes to PAR-2 localization under conditions when the MTOC signal is compromised. Along with previous findings, these data suggest that polarity establishment in the zygote depends on two redundant mechanisms: a first mechanism dependent on efficient PCM assembly and actin contractility, and a second mechanism dependent on microtubules and PAR-2. Boyd, L., Guo, S., Levitan, D., Stinchcomb, D.T., Kemphues, K.J. (1996). Development 122, 3075-3084. Hao, Y., Boyd, L., Seydoux, G. (2006). Developmental Cell 10, 199-208. Motegi, F., Seydoux, G. (2007). JCB 179, 367-369.