Cortical flows powered by asymmetrical contraction transport PAR proteins to establish and maintain anterior-posterior polarity in the early C. elegans embryo

The C. elegans PAR proteins PAR-3, PAR-6, and PKC-3 are asymmetrically localized and have essential roles in cell polarity. We show that the one-cell C. elegans embryo contains a dynamic and contractile actomyosin network that appears to be destabilized near the point of sperm entry. This asymmetry initiates a flow of cortical nonmuscle myosin (NMY-2) and F-actin toward the opposite, future anterior, pole. PAR-3, PAR-6, and PKC-3, as well as non-PAR proteins that associate with the cytoskeleton, appear to be transported to the anterior by this cortical flow. In turn, PAR-3, PAR-6, and PKC-3 modulate cortical actomyosin dynamics and promote cortical flow. PAR-2, which localizes to the posterior cortex, inhibits NMY-2 from accumulating at the posterior cortex during flow, thus maintaining asymmetry by preventing inappropriate, posterior-directed flows. Similar actomyosin flows accompany the establishment of PAR asymmetries that form after the one-cell stage, suggesting that actomyosin-mediated cortical flows have a general role in PAR asymmetry.     

Depletion of the co-chaperone CDC-37 reveals two modes of PAR-6 cortical association in C. elegans embryos

PAR proteins play roles in the establishment and maintenance of polarity in many different cell types in metazoans. In C. elegans, polarity established in the one-cell embryo determines the anteroposterior axis of the developing animal and is essential to set the identities of the early blastomeres. PAR-1 and PAR-2 colocalize at the posterior cortex of the embryo. PAR-3, PAR-6 and PKC-3 (aPKC) colocalize at the anterior cortex of the embryo. A process of mutual exclusion maintains the anterior and posterior protein domains. We present results indicating that a homolog of the Hsp90 co-chaperone Cdc37 plays a role in dynamic interactions among the PAR proteins. We show that CDC-37 is required for the establishment phase of embryonic polarity; that CDC-37 reduction allows PAR-3-independent cortical accumulation of PAR-6 and PKC-3; and that CDC-37 is required for the mutual exclusion of the anterior and posterior group PAR proteins. Our results indicate that CDC-37 acts in part by maintaining PKC-3 levels and in part by influencing the activity or levels of other client proteins. Loss of the activities of these client proteins reveals that there are two sites for PAR-6 cortical association, one dependent on CDC-42 and not associated with PAR-3, and the other independent of CDC-42 and co-localizing with PAR-3. We propose that, in wild-type embryos, CDC-37-mediated inhibition of the CDC-42-dependent binding site and PAR-3-mediated release of this inhibition provide a key mechanism for the anterior accumulation of PAR-6.     

Modeling the establishment of PAR protein polarity in the one-cell C. elegans embryo

At the one-cell stage, the C. elegans embryo becomes polarized along the anterior-posterior axis. The PAR proteins form complementary anterior and posterior domains in a dynamic process driven by cytoskeletal rearrangement. Initially, the PAR proteins are uniformly distributed throughout the embryo. After a cue from fertilization, cortical actomyosin contracts toward the anterior pole. PAR-3/PAR-6/PKC-3 (the anterior PAR proteins) become restricted to the anterior cortex. PAR-1 and PAR-2 (the posterior PAR proteins) become enriched in the posterior cortical region. We present a mathematical model of this polarity establishment process, in which we take a novel approach to combine reaction-diffusion dynamics of the PAR proteins coupled to a simple model of actomyosin contraction. We show that known interactions between the PAR proteins are sufficient to explain many aspects of the observed cortical PAR dynamics in both wild-type and mutant embryos. However, cytoplasmic PAR protein polarity, which is vital for generating daughter cells with distinct molecular components, cannot be properly explained within such a framework. We therefore consider additional mechanisms that can reproduce the proper cytoplasmic polarity. In particular we predict that cytoskeletal asymmetry in the cytoplasm, in addition to the cortical actomyosin asymmetry, is a critical determinant of PAR protein localization.     

The C. elegans homolog of Drosophila Lethal giant larvae functions redundantly with PAR-2 to maintain polarity in the early embryo

Polarity is essential for generating cell diversity. The one-cell C. elegans embryo serves as a model for studying the establishment and maintenance of polarity. In the early embryo, a myosin II-dependent contraction of the cortical meshwork asymmetrically distributes the highly conserved PDZ proteins PAR-3 and PAR-6, as well as an atypical protein kinase C (PKC-3), to the anterior. The RING-finger protein PAR-2 becomes enriched on the posterior cortex and prevents these three proteins from returning to the posterior. In addition to the PAR proteins, other proteins are required for polarity in many metazoans. One example is the conserved Drosophila tumor-suppressor protein Lethal giant larvae (Lgl). In Drosophila and mammals, Lgl contributes to the maintenance of cell polarity and plays a role in asymmetric cell division. We have found that the C. elegans homolog of Lgl, LGL-1, has a role in polarity but is not essential. It localizes asymmetrically to the posterior of the early embryo in a PKC-3-dependent manner, and functions redundantly with PAR-2 to maintain polarity. Furthermore, overexpression of LGL-1 is sufficient to rescue loss of PAR-2 function. LGL-1 negatively regulates the accumulation of myosin (NMY-2) on the posterior cortex, representing a possible mechanism by which LGL-1 might contribute to polarity maintenance.     

CDC-42 and RHO-1 coordinate acto-myosin contractility and PAR protein localization during polarity establishment in C. elegans embryos

In C. elegans one-cell embryos, polarity is conventionally defined along the anteroposterior axis by the segregation of partitioning-defective (PAR) proteins into anterior (PAR-3, PAR-6) and posterior (PAR-1, PAR-2) cortical domains. The establishment of PAR asymmetry is coupled with acto-myosin cytoskeleton rearrangements. The small GTPases RHO-1 and CDC-42 are key players in cytoskeletal remodeling and cell polarity in a number of different systems. We investigated the roles of these two GTPases and the RhoGEF ECT-2 in polarity establishment in C. elegans embryos. We show that CDC-42 is required to remove PAR-2 from the cortex at the end of meiosis and to localize PAR-6 to the cortex. By contrast, RHO-1 activity is required to facilitate the segregation of CDC-42 and PAR-6 to the anterior. Loss of RHO-1 activity causes defects in the early organization of the myosin cytoskeleton but does not inhibit segregation of myosin to the anterior. We therefore propose that RHO-1 couples the polarization of the acto-myosin cytoskeleton with the proper segregation of CDC-42, which, in turn, localizes PAR-6 to the anterior cortex.     

Polarization of the C. elegans zygote proceeds via distinct establishment and maintenance phases

Polarization of the C. elegans zygote along the anterior-posterior axis depends on cortically enriched (PAR) and cytoplasmic (MEX-5/6) proteins, which function together to localize determinants (e.g. PIE-1) in response to a polarizing cue associated with the sperm asters. Using time-lapse microscopy and GFP fusions, we have analyzed the localization dynamics of PAR-2, PAR-6, MEX-5, MEX-6 and PIE-1 in wild-type and mutant embryos. These studies reveal that polarization involves two genetically and temporally distinct phases. During the first phase (establishment), the sperm asters at one end of the embryo exclude the PAR-3/PAR-6/PKC3 complex from the nearby cortex, allowing the ring finger protein PAR-2 to accumulate in an expanding 'posterior' domain. Onset of the establishment phase involves the non-muscle myosin NMY-2 and the 14-3-3 protein PAR-5. The kinase PAR-1 and the CCCH finger proteins MEX-5 and MEX-6 also function during the establishment phase in a feedback loop to regulate growth of the posterior domain. The second phase begins after pronuclear meeting, when the sperm asters begin to invade the anterior. During this phase (maintenance), PAR-2 maintains anterior-posterior polarity by excluding the PAR-3/PAR-6/PKC3 complex from the posterior. These findings provide a model for how PAR and MEX proteins convert a transient asymmetry into a stably polarized axis.     

MEX-5 asymmetry in one-cell C. elegans embryos requires PAR-4- and PAR-1-dependent phosphorylation

Anteroposterior polarity in early C. elegans embryos is required for the specification of somatic and germline lineages, and is initiated by a sperm-induced reorganization of the cortical cytoskeleton and PAR polarity proteins. Through mechanisms that are not understood, the kinases PAR-1 and PAR-4, and other PAR proteins cause the cytoplasmic zinc finger protein MEX-5 to accumulate asymmetrically in the anterior half of the one-cell embryo. We show that MEX-5 asymmetry requires neither vectorial transport to the anterior, nor protein degradation in the posterior. MEX-5 has a restricted mobility before fertilization and in the anterior of one-cell embryos. However, MEX-5 mobility in the posterior increases as asymmetry develops, presumably allowing accumulation in the anterior. The MEX-5 zinc fingers and a small, C-terminal domain are essential for asymmetry; the zinc fingers restrict MEX-5 mobility, and the C-terminal domain is required for the increase in posterior mobility. We show that a crucial residue in the C-terminus, Ser 458, is phosphorylated in vivo. PAR-1 and PAR-4 kinase activities are required for the phosphorylation of S458, providing a link between PAR polarity proteins and the cytoplasmic asymmetry of MEX-5.     

Myosin and the PAR proteins polarize microfilament-dependent forces that shape and position mitotic spindles in Caenorhabditis elegans

In Caenorhabditis elegans, the partitioning proteins (PARs), microfilaments (MFs), dynein, dynactin, and a nonmuscle myosin II all localize to the cortex of early embryonic cells. Both the PARs and the actomyosin cytoskeleton are required to polarize the anterior-posterior (a-p) body axis in one-cell zygotes, but it remains unknown how MFs influence embryonic polarity. Here we show that MFs are required for the cortical localization of PAR-2 and PAR-3. Furthermore, we show that PAR polarity regulates MF-dependent cortical forces applied to astral microtubules (MTs). These forces, which appear to be mediated by dynein and dynactin, produce changes in the shape and orientation of mitotic spindles. Unlike MFs, dynein, and dynactin, myosin II is not required for the production of these forces. Instead, myosin influences embryonic polarity by limiting PAR-3 to the anterior cortex. This in turn produces asymmetry in the forces applied to MTs at each pole and allows PAR-2 to accumulate in the posterior cortex of a one-cell zygote and maintain asymmetry.     

CDC-42 controls early cell polarity and spindle orientation in C. elegans.

BACKGROUND: Generation of asymmetry in the one-cell embryo of C. elegans establishes the anterior--posterior axis (A-P), and is necessary for the proper identity of early blastomeres. Conserved PAR proteins are asymmetrically distributed and are required for the generation of this early asymmetry. The small G protein Cdc42 is a key regulator of polarity in other systems, and recently it has been shown to interact with the mammalian homolog of PAR-6. The function of Cdc42 in C. elegans had not yet been investigated, however. RESULTS: Here, we show that C. elegans cdc-42 plays an essential role in the polarity of the one-cell embryo and the proper localization of PAR proteins. Inhibition of cdc-42 using RNA interference results in embryos with a phenotype that is nearly identical to par-3, par-6, and pkc-3 mutants, and asymmetric localization of these and other PAR proteins is lost. We further show that C. elegans CDC-42 physically interacts with PAR-6 in a yeast two-hybrid system, consistent with data on the interaction of human homologs. CONCLUSIONS: Our results show that CDC-42 acts in concert with the PAR proteins to control the polarity of the C. elegans embryo, and provide evidence that the interaction of CDC-42 and the PAR-3/PAR-6/PKC-3 complex has been evolutionarily conserved as a functional unit.     

PAR-3 and PAR-1 inhibit LET-99 localization to generate a cortical band important for spindle positioning in Caenorhabditis elegans embryos

The conserved PAR proteins are localized in asymmetric cortical domains and are required for the polarized localization of cell fate determinants in many organisms. In Caenorhabditis elegans embryos, LET-99 and G protein signaling act downstream of the PARs to regulate spindle positioning and ensure asymmetric division. PAR-3 and PAR-2 localize LET-99 to a posterior cortical band through an unknown mechanism. Here we report that LET-99 asymmetry depends on cortically localized PAR-1 and PAR-4 but not on cytoplasmic polarity effectors. In par-1 and par-4 embryos, LET-99 accumulates at the entire posterior cortex, but remains at low levels at the anterior cortex occupied by PAR-3. Further, PAR-3 and PAR-1 have graded cortical distributions with the highest levels at the anterior and posterior poles, respectively, and the lowest levels of these proteins correlate with high LET-99 accumulation. These results suggest that PAR-3 and PAR-1 inhibit the localization of LET-99 to generate a band pattern. In addition, PAR-1 kinase activity is required for the inhibition of LET-99 localization, and PAR-1 associates with LET-99. Finally, examination of par-1 embryos suggests that the banded pattern of LET-99 is critical for normal posterior spindle displacement and to prevent spindle misorientation caused by cell shape constraints.     

A Semi-Dominant Mutation in the General Splicing Factor SF3a66 Causes Anterior-Posterior Axis Reversal in One-Cell Stage C. elegans Embryos

Establishment of anterior-posterior polarity in one-cell stage Caenorhabditis elegans embryos depends in part on astral microtubules. As the zygote enters mitosis, these microtubules promote the establishment of a posterior pole by binding to and protecting a cytoplasmic pool of the posterior polarity protein PAR-2 from phosphorylation by the cortically localized anterior polarity protein PKC-3. Prior to activation of the sperm aster, the oocyte Meiosis I and II spindles assemble and function, usually at the future anterior pole, but these meiotic spindle microtubules fail to establish posterior polarity through PAR-2. Here we show that a semi-dominant mutation in the general splicing factor SF3a66 can lead to a reversed axis of AP polarity that depends on PAR-2 and possibly on close proximity of oocyte meiotic spindles with the cell cortex. One possible explanation is that reduced levels of PKC-3, due to a general splicing defect, can result in axis reversal due to a failure to prevent oocyte meiotic spindle microtubules from interfering with AP axis formation.     

Bazooka and PAR-6 are required with PAR-1 for the maintenance of oocyte fate in Drosophila

The anterior-posterior axis of C. elegans is defined by the asymmetric division of the one-cell zygote, and this is controlled by the PAR proteins, including PAR-3 and PAR-6, which form a complex at the anterior of the cell, and PAR-1, which localizes at the posterior [1-4]. PAR-1 plays a similar role in axis formation in Drosophila: the protein localizes to the posterior of the oocyte and is necessary for the localization of the posterior and germline determinants [5, 6]. PAR-1 has recently been shown to have an earlier function in oogenesis, where it is required for the maintenance of oocyte fate and the posterior localization of oocyte-specific markers [7, 8]. Here, we show that the homologs of PAR-3 (Bazooka) and PAR-6 are also required to maintain oocyte fate. Germline clones of mutants in either gene give rise to egg chambers that develop 16 nurse cells and no oocyte. Furthermore, oocyte-specific factors, such as Orb protein and the centrosomes, still localize to one cell but fail to move from the anterior to the posterior cortex. Thus, PAR-1, Bazooka, and PAR-6 are required for the earliest polarity in the oocyte, providing the first example in Drosophila where the three homologs function in the same process. Although these PAR proteins therefore seem to play a conserved role in early anterior-posterior polarity in C. elegans and Drosophila, the relationships between them are different, as the localization of PAR-1 does not require Bazooka or PAR-6 in Drosophila, as it does in the worm.     

The Caenorhabditis elegans par-5 gene encodes a 14-3-3 protein required for cellular asymmetry in the early embryo.

The establishment of anterior-posterior polarity in the Caenorhabditis elegans embryo requires the activity of the maternally expressed par genes. We report the identification and analysis of a new par gene, par-5. We show that par-5 is required for asynchrony and asymmetry in the first embryonic cell divisions, normal pseudocleavage, normal cleavage spindle orientation at the two-cell stage, and localization of P granules and MEX-5 during the first and subsequent cell cycles. Furthermore, par-5 activity is required in the first cell cycle for the asymmetric cortical localization of PAR-1 and PAR-2 to the posterior, and PAR-3, PAR-6, and PKC-3 to the anterior. When PAR-5 is reduced by mutation or by RNA interference, these proteins spread around the cortex of the one-cell embryo and partially overlap. We have shown by sequence analysis of par-5 mutants and by RNA interference that the par-5 gene is the same as the ftt-1 gene, and encodes a 14-3-3 protein. The PAR-5 14-3-3 protein is present in gonads, oocytes, and early embryos, but is not asymmetrically distributed. Our analysis indicates that the par-5 14-3-3 gene plays a crucial role in the early events leading to polarization of the C. elegans zygote.     

PAR proteins regulate microtubule dynamics at the cell cortex in C. elegans

BACKGROUND: The PAR proteins are known to be localized asymmetrically in polarized C. elegans, Drosophila, and human cells and to participate in several cellular processes, including asymmetric cell division and spindle orientation. Although astral microtubules are known to play roles in these processes, their behavior during these events remains poorly understood. RESULTS: We have developed a method that makes it possible to examine the residence time of individual astral microtubules at the cell cortex of developing embryos. Using this method, we found that microtubules are more dynamic at the posterior cortex of the C. elegans embryo compared to the anterior cortex during spindle displacement. We further observed that this asymmetry depends on the PAR-3 protein and heterotrimeric G protein signaling, and that the PAR-2 protein affects microtubule dynamics by restricting PAR-3 activity to the anterior of the embryo. CONCLUSIONS: These results indicate that PAR proteins function to regulate microtubule dynamics at the cortex during microtubule-dependent cellular processes.     

Asymmetrically distributed C. elegans homologs of AGS3/PINS control spindle position in the early embryo

BACKGROUND: Spindle positioning during an asymmetric cell division is of fundamental importance to ensure correct size of daughter cells and segregation of determinants. In the C. elegans embryo, the first spindle is asymmetrically positioned, and this asymmetry is controlled redundantly by two heterotrimeric Galpha subunits, GOA-1 and GPA-16. The Galpha subunits act downstream of the PAR polarity proteins, which control the relative pulling forces acting on the poles. How these heterotrimeric G proteins are regulated and how they control spindle position is still unknown. RESULTS: Here we show that the Galpha subunits are regulated by a receptor-independent mechanism. RNAi depletion of gpr-1 and gpr-2, homologs of mammalian AGS3 and Drosophila PINS (receptor-independent G protein regulators), results in a phenotype identical to that of embryos depleted of both GPA-16 and GOA-1; the first cleavage is symmetric, but polarity is not affected. The loss of spindle asymmetry after RNAi of gpr-1 and gpr-2 appears to be the result of weakened pulling forces acting on the poles. The GPR protein(s) localize around the cortex of one-cell embryos and are enriched at the posterior. Thus, asymmetric G protein regulation could explain the posterior displacement of the spindle. Posterior enrichment is abolished in the absence of the PAR polarity proteins PAR-2 or PAR-3. In addition, LIN-5, a coiled-coil protein also required for spindle positioning, binds to and is required for cortical association of the GPR protein(s). Finally, we show that the GPR domain of GPR-1 and GPR-2 behaves as a GDP dissociation inhibitor for GOA-1, and its activity is thus similar to that of mammalian AGS3. CONCLUSIONS: Our results suggest that GPR-1 and/or GPR-2 control an asymmetry in forces exerted on the spindle poles by asymmetrically modulating the activity of the heterotrimeric G protein in response to a signal from the PAR proteins.     

Microtubules induce self-organization of polarized PAR domains in Caenorhabditis elegans zygotes

A hallmark of polarized cells is the segregation of the PAR polarity regulators into asymmetric domains at the cell cortex. Antagonistic interactions involving two conserved kinases, atypical protein kinase C (aPKC) and PAR-1, have been implicated in polarity maintenance, but the mechanisms that initiate the formation of asymmetric PAR domains are not understood. Here, we describe one pathway used by the sperm-donated centrosome to polarize the PAR proteins in Caenorhabditis elegans zygotes. Before polarization, cortical aPKC excludes PAR-1 kinase and its binding partner PAR-2 by phosphorylation. During symmetry breaking, microtubules nucleated by the centrosome locally protect PAR-2 from phosphorylation by aPKC, allowing PAR-2 and PAR-1 to access the cortex nearest the centrosome. Cortical PAR-1 phosphorylates PAR-3, causing the PAR-3-aPKC complex to leave the cortex. Our findings illustrate how microtubules, independently of actin dynamics, stimulate the self-organization of PAR proteins by providing local protection against a global barrier imposed by aPKC.     

C. elegans Brat homologs regulate PAR protein-dependent polarity and asymmetric cell division

The evolutionary conserved PAR proteins control polarization and asymmetric division in many organisms. Recent work in Caenorhabditis elegans demonstrated that nos-3 and fbf-1/2 can suppress par-2(it5ts) lethality, suggesting that they participate in cell polarity by regulating the function of the anterior PAR-3/PAR-6/PKC-3 proteins. In Drosophila embryos, Nanos and Pumilio are homologous to NOS-3 and FBF-1/2 respectively and control cell polarity by forming a complex with the tumor suppressor Brat to inhibit Hunchback mRNA translation. In this study, we investigated the possibility that Brat could control cell polarity and asymmetric cell division in C. elegans. We found that disrupting four of the five C. elegans Brat homologs (Cebrats) individually results in suppression of par-2(it5ts) lethality, indicating that these genes are involved in embryonic polarity. Two of the Cebrats, ncl-1 and nhl-2, partially restore the localization of PAR proteins at the cortex. While mutations in the four Cebrat genes do not severely impair polarity, they display polarity-associated defects. Surprisingly, these defects are absent from nos-3 mutants. Similarly, while nos-3 controls PAR-6 protein levels, this is not the case for any of the Cebrats. Our results, together with results from Drosophila, indicate that Brat family members function in generating cellular asymmetries and suggest that, in contrast to Drosophila embryos, the C. elegans homologs of Brat and Nanos could participate in embryonic polarity via distinct mechanisms.     

aPKC phosphorylates NuMA-related LIN-5 to position the mitotic spindle during asymmetric division

The position of the mitotic spindle controls the plane of cell cleavage and determines whether polarized cells divide symmetrically or asymmetrically. In animals, an evolutionarily conserved pathway of LIN-5 (homologues: Mud and NuMA), GPR-1/2 (homologues: Pins, LGN, AGS-3) and Gα mediates spindle positioning, and acts downstream of the conserved PAR-3-PAR-6-aPKC polarity complex. However, molecular interactions between polarity proteins and LIN-5-GPR-Gα remain to be identified. Here we describe a quantitative mass spectrometry approach for in vivo identification of protein kinase substrates. Applying this strategy to Caenorhabditis elegans embryos, we found that depletion of the polarity kinase PKC-3 results in markedly decreased levels of phosphorylation of a cluster of four LIN-5 serine residues. These residues are directly phosphorylated by PKC-3 in vitro. Phospho-LIN-5 co-localizes with PKC-3 at the anterior cell cortex and temporally coincides with a switch from anterior- to posterior-directed spindle movements in the one-cell embryo. LIN-5 mutations that prevent phosphorylation increase the extent of anterior-directed spindle movements, whereas phosphomimetic mutations decrease spindle migration. Our results indicate that anterior-located PKC-3 inhibits cortical microtubule pulling forces through direct phosphorylation of LIN-5. This molecular interaction between polarity and spindle-positioning proteins may be used broadly in cell cleavage plane determination.     

Coupling between cytoplasmic concentration gradients through local control of protein mobility in the Caenorhabditis elegans zygote

Cell polarity is characterized by the asymmetric distribution of factors at the cell cortex and in the cytoplasm. Although mechanisms that establish cortical asymmetries have been characterized, less is known about how persistent cytoplasmic asymmetries are generated. During the asymmetric division of the Caenorhabditis elegans zygote, the PAR proteins orchestrate the segregation of the cytoplasmic RNA-binding proteins MEX-5/6 to the anterior cytoplasm and PIE-1, POS-1, and MEX-1 to the posterior cytoplasm. In this study, we find that MEX-5/6 control the segregation of GFP::PIE-1, GFP::POS-1, and GFP::MEX-1 by locally increasing their mobility in the anterior cytoplasm. Remarkably, PIE-1, POS-1, and MEX-1 form gradients with distinct strengths, which correlates with differences in their responsiveness to MEX-5/6. We show that MEX-5/6 act downstream of the polarity regulators PAR-1 and PAR-3 and in a concentration-dependent manner to increase the mobility of GFP::PIE-1. These findings suggest that the MEX-5/6 concentration gradients are directly coupled to the establishment of posterior-rich PIE-1, POS-1, and MEX-1 concentration gradients via the formation of anterior-fast, posterior-slow mobility gradients.     

A genomewide screen for suppressors of par-2 uncovers potential regulators of PAR protein-dependent cell polarity in Caenorhabditis elegans

The PAR proteins play an essential role in establishing and maintaining cell polarity. While their function is conserved across species, little is known about their regulators and effectors. Here we report the identification of 13 potential components of the C. elegans PAR polarity pathway, identified in an RNAi-based, systematic screen to find suppressors of par-2(it5ts) lethality. Most of these genes are conserved in other species. Phenotypic analysis of double-mutant animals revealed that some of the suppressors can suppress lethality associated with the strong loss-of-function allele par-2(lw32), indicating that they might impinge on the PAR pathway independently of the PAR-2 protein. One of these is the gene nos-3, which encodes a homolog of Drosophila Nanos. We find that nos-3 suppresses most of the phenotypes associated with loss of par-2 function, including early cell division defects and maternal-effect sterility. Strikingly, while PAR-1 activity was essential in nos-3; par-2 double mutants, its asymmetric localization at the posterior cortex was not restored, suggesting that the function of PAR-1 is independent of its cortical localization. Taken together, our results identify conserved components that regulate PAR protein function and also suggest a role for NOS-3 in PAR protein-dependent cell polarity.