MARK/Par1 Kinase Is Activated Downstream of NMDA Receptors through a PKA-Dependent Mechanism

The Par1 kinases, also known as microtubule affinity-regulating kinases (MARKs), are important for the establishment of cell polarity from worms to mammals. Dysregulation of these kinases has been implicated in autism, Alzheimer’s disease and cancer. Despite their important function in health and disease, it has been unclear how the activity of MARK/Par1 is regulated by signals from cell surface receptors. Here we show that MARK/Par1 is activated downstream of NMDA receptors in primary hippocampal neurons. Further, we show that this activation is dependent on protein kinase A (PKA), through the phosphorylation of Ser431 of Par4/LKB1, the major upstream kinase of MARK/Par1. Together, our data reveal a novel mechanism by which MARK/Par1 is activated at the neuronal synapse.     

Par6b Regulates the Dynamics of Apicobasal Polarity during Development of the Stratified Xenopus Epidermis

During early vertebrate development, epithelial cells establish and maintain apicobasal polarity, failure of which can cause developmental defects or cancer metastasis. This process has been mostly studied in simple epithelia that have only one layer of cells, but is poorly understood in stratified epithelia. In this paper we address the role of the polarity protein Partitioning defective-6 homolog beta (Par6b) in the developing stratified epidermis of Xenopus laevis. At the blastula stage, animal blastomeres divide perpendicularly to the apicobasal axis to generate partially polarized superficial cells and non-polarized deep cells. Both cell populations modify their apicobasal polarity during the gastrula stage, before differentiating into the superficial and deep layers of epidermis. Early differentiation of the epidermis is normal in Par6b-depleted embryos; however, epidermal cells dissociate and detach from embryos at the tailbud stage. Par6b-depleted epidermal cells exhibit a significant reduction in basolaterally localized E-cadherin. Examination of the apical marker Crumbs homolog 3 (Crb3) and the basolateral marker Lethal giant larvae 2 (Lgl2) after Par6b depletion reveals that Par6b cell-autonomously regulates the dynamics of apicobasal polarity in both superficial and deep epidermal layers. Par6b is required to maintain the “basolateral” state in both epidermal layers, which explains the reduction of basolateral adhesion complexes and epidermal cells shedding.     

Phosphorylation-dependent binding of 14-3-3 to the polarity protein Par3 regulates cell polarity in mammalian epithelia

The mammalian homologs of the C. elegans partitioning-defective (Par) proteins have been demonstrated to be necessary for establishment of cell polarity. In mammalian epithelia, the Par3/Par6/aPKC polarity complex is localized to the tight junction and regulates its formation and positioning with respect to basolateral and apical membrane domains. Here we demonstrate a previously undescribed phosphorylation-dependent interaction between a mammalian homolog of the C. elegans polarity protein Par5, 14-3-3, and the tight junction-associated protein Par3. We identify phosphorylated serine 144 as a site of 14-3-3 binding. Expression of a Par3 mutant that contains serine 144 mutated to alanine (S144A) results in defects in epithelial cell polarity. In addition, overexpression of 14-3-3zeta results in a severe disruption of polarity, whereas overexpression of a 14-3-3 mutant that is defective in binding to phosphoproteins has no effect on cell polarity. Together, these data suggest a novel, phosphorylation-dependent mechanism that regulates the function of the Par3/Par6/aPKC polarity complex through 14-3-3 binding.     

Magi Is Associated with the Par Complex and Functions Antagonistically with Bazooka to Regulate the Apical Polarity Complex

The mammalian MAGI proteins play important roles in the maintenance of adherens and tight junctions. The MAGI family of proteins contains modular domains such as WW and PDZ domains necessary for scaffolding of membrane receptors and intracellular signaling components. Loss of MAGI leads to reduced junction stability while overexpression of MAGI can lead to increased adhesion and stabilization of epithelial morphology. However, how Magi regulates junction assembly in epithelia is largely unknown. We investigated the single Drosophila homologue of Magi to study the in vivo role of Magi in epithelial development. Magi is localized at the adherens junction and forms a complex with the polarity proteins, Par3/Bazooka and aPKC. We generated a Magi null mutant and found that Magi null mutants were viable with no detectable morphological defects even though the Magi protein is highly conserved with vertebrate Magi homologues. However, overexpression of Magi resulted in the displacement of Baz/Par3 and aPKC and lead to an increase in the level of PIP3. Interestingly, we found that Magi and Baz functioned in an antagonistic manner to regulate the localization of the apical polarity complex. Maintaining the balance between the level of Magi and Baz is an important determinant of the levels and localization of apical polarity complex.     

哺乳动物神经发育中的Par极性复合体

哺乳动物的神经发育过程极其复杂, 其形态结构和机能变化受到严格的调控。细胞极性是哺乳动物神经发生中最基本的特征之一, 在其调控因素中, Par极性复合体是研究最多的蛋白质。神经发育过程中Par蛋白的分布与量呈现动态变化, 影响细胞连接建立、细胞极性形成、神经突触发生及神经元迁移, 也影响到神经前体细胞的命运。文章主要从胚胎新皮层神经前体细胞及体外培养神经元角度, 总结了近年在Par极性蛋白的细胞内分布、机能及作用机制方面的研究进展。     

The serine/threonine kinase Par1b regulates epithelial lumen polarity via IRSp53-mediated cell-ECM signaling

The serine/threonine kinase Par1b promotes cell-cell adhesion and determines the polarity of the luminal domain in epithelial cells. In this study, we demonstrate that Par1b also regulates cell-extracellular matrix (ECM) signaling in kidney-derived Madin-Darby canine kidney (MDCK) cells and identified the rho-guanosine triphosphatase adaptor and scaffolding protein IRSp53 as a Par1b substrate involved in this pathway. Par1b overexpression inhibits basal lamina formation, cell spreading, focal adhesion, stress fiber formation, and compaction, whereas Par1b depletion has the opposite effect. IRSp53 depletion mimics Par1b overexpression on cell-ECM signaling and lumen polarity but had no effect on adherens junction formation. Par1b directly phosphorylates IRSp53 on S366 in cell lysates and stimulates phosphorylation on S453/3/5 via an indirect mechanism. A Par1b phosphorylation-deficient IRSp53 mutant but not the wild-type protein efficiently rescues both the cell spreading and the lumen polarity defects in Par1b MDCK cells. Our data suggest a model in which Par1b phosphorylation prevents recruitment of IRSp53 effector proteins to its Src homology domain 3 by promoting 14-3-3 binding in the vicinity of that domain.     

The aPKC/Par3/Par6 Polarity Complex and Membrane Order Are Functionally Interdependent in Epithelia During Vertebrate Organogenesis

The differential distribution of lipids between apical and basolateral membranes is necessary for many epithelial cell functions, but how this characteristic membrane organization is integrated within the polarity network during ductal organ development is poorly understood. Here we quantified membrane order in the gut, kidney and liver ductal epithelia in zebrafish larvae at 3–11 days post fertilization (dpf) with Laurdan 2‐photon microscopy. We then applied a combination of Laurdan imaging, antisense knock‐down and analysis of polarity markers to understand the relationship between membrane order and apical‐basal polarity. We found a reciprocal relationship between membrane order and the cell polarity network. Reducing membrane condensation by exogenously added oxysterol or depletion of cholesterol reduced apical targeting of the polarity protein, aPKC. Conversely, using morpholino knock down in zebrafish, we found that membrane order was dependent upon the Crb3 and Par3 polarity protein expression in ductal epithelia. Hence our data suggest that the biophysical property of membrane lipid packing is a regulatory element in apical basal polarity.     

Polarity protein Par3 controls B-cell receptor dynamics and antigen extraction at the immune synapse

B-cell receptor (BCR) engagement with surface-tethered antigens leads to the formation of an immune synapse, which facilitates antigen uptake for presentation to T-lymphocytes. Antigen internalization and processing rely on the early dynein-dependent transport of BCR–antigen microclusters to the synapse center, as well as on the later polarization of the microtubule-organizing center (MTOC). MTOC repositioning allows the release of proteases and the delivery of MHC class II molecules at the synapse. Whether and how these events are coordinated have not been addressed. Here we show that the ancestral polarity protein Par3 promotes BCR–antigen microcluster gathering, as well as MTOC polarization and lysosome exocytosis, at the synapse by facilitating local dynein recruitment. Par3 is also required for antigen presentation to T-lymphocytes. Par3 therefore emerges as a key molecule in the coupling of the early and late events needed for efficient extraction and processing of immobilized antigen by B-cells.     

Cell Polarity Factor Par3 Binds SPTLC1 and Modulates Monocyte Serine Palmitoyltransferase Activity and Chemotaxis

Elevated sphingolipids have been associated with increased cardiovascular disease. Conversely, atherosclerosis is reduced in mice by blocking de novo synthesis of sphingolipids catalyzed by serine palmitoyltransferase (SPT). The SPT enzyme is composed of the SPTLC1 and -2 subunits, and here we describe a novel protein-protein interaction between SPTLC1 and the PDZ protein Par3 (partitioning defective protein 3). Mammalian SPTLC1 orthologs have a highly conserved C terminus that conforms to a type II PDZ protein interaction motif, and by screening PDZ domain protein arrays with an SPTLC1 C-terminal peptide, we found it bound the third PDZ domain of Par3. Overlay and immunoprecipitation assays confirmed this interaction and indicate Par3 is able to associate with the SPTLC1/2 holoenzyme by binding the C-terminal SPTLC1 PDZ motif. The physiologic existence of the SPTLC1/2-Par3 complex was detected in mouse liver and macrophages, and short interfering RNA inhibition of Par3 in human THP-1 monocytes significantly reduced SPT activity and de novo ceramide synthesis by nearly 40%. Given monocyte recruitment into inflamed vessels is thought to promote atherosclerosis, and because Par3 and sphingolipids have been associated with polarized cell migration, we tested whether the ability of THP-1 monocytes to migrate toward MCP-1 (monocyte chemoattractant protein 1) depended upon Par3 and SPTLC1 expression. Knockdown of Par3 significantly reduced MCP1-induced chemotaxis of THP-1 monocytes, as did knockdown of SPTLC1, and this Par3 effect depended upon SPT activity and was blunted by ceramide treatment. In conclusion, protein arrays were used to identify a novel SPTLC1-Par3 interaction that associates with increased monocyte serine palmitoyltransferase activity and chemotaxis toward inflammatory signals.Sphingolipids are a structurally diverse class of lipids that play correspondingly diverse roles in membrane structure, cell proliferation, immune function, and skin physiology (1–4). De novo sphingolipid synthesis is initiated by serine palmitoyltransferase (SPT),² an enzyme that condenses serine and palmitoyl-CoA forming the biosynthetic intermediate 3-ketodihydrosphingosine that is subsequently converted to ceramide, sphingomyelin, and other sphingolipids (5). SPT is a heterodimer composed of the SPTLC1 and -2 subunits (6, 7), which may form higher order multimeric structures that can include a third subunit, SPTLC3 (8, 9). Both the SPTLC2 and -3 subunits are catalytically active and contain conserved lysines that act as Schiff bases during the condensation reaction (5, 8). In contrast, SPTLC1 does not contain the conserved catalytically active lysine, but is important for stabilizing the SPTLC2 subunit and anchoring the SPT holoenyzme on the cytosolic face of the endoplasmic reticulum (10, 11).Expression and regulation of the SPTLC1/2 holoenyzme are of interest because its activity controls de novo synthesis of sphingomyelin, and increased plasma levels of this sphingolipid have been correlated with an increased incidence of cardiovascular disease in humans (12, 13). Conversely, inhibition of SPT activity with myriocin, a fungal metabolite, strongly inhibits atherosclerotic development in ApoE^−/− mice (14–18). Moreover, the increased atherosclerosis seen in ApoE^−/− mice has been associated with a post-translational increase in liver SPT activity (19). How SPT activity and sphingolipids may act to promote the progression of atherosclerosis is unclear, but the data do suggest analysis of factors that regulate SPT activity should provide mechanistic insight into the link between de novo sphingolipid synthesis and atherosclerosis. In this regard we have found that SPTLC1 can interact with the ABCA1 transporter and inhibit its ability to transfer cholesterol to apoA-I, a mechanism that would be expected to promote atherosclerosis (20). Thus, along with playing a direct role in the synthesis of sphingolipids, SPTLC1 may also have evolved as an SPT subunit whose function is to regulate SPT activity in response to the cellular demand for sphingolipids and other membrane constituents such as cholesterol. To play such a role, SPTLC1 may engage additional protein-protein interactions that integrate input from signaling pathways and allow SPTLC1 to modulate SPT activity in response to altered demand for sphingolipids.Here we have explored this hypothesis by first conducting a protein array screen for SPTLC1 interacting factors. Consistent with the potential to engage cellular factors in protein-protein interactions, sequence alignment of the SPTLC1 C terminus indicates it has been strongly conserved in mammals as a type II PDZ domain binding motif. Moreover, because topology studies indicate the SPTLC1 C terminus resides in the cytoplasm where it could be bound by PDZ proteins, we used protein arrays spotted with 123 PDZ domains from 73 different proteins to screen for interactions with the SPTLC1 C terminus. This screen indicated the SPTLC1 C terminus directly interacts with the third PDZ domain of PARD3 (partitioning defective protein 3). PARD3, also known as Par3, is a scaffolding factor that recruits signaling molecules, including atypical protein kinase C and Cdc42 into multiprotein complexes that regulate formation of membrane microdomains required for apical/basal polarity and for directed cell migration (21–25). Mutation analysis confirmed the SPTLC1-Par3 interaction depended upon the SPTLC1 C-terminal PDZ motif, and immunoprecipitation assays indicate Par3 is able to associate with the SPTLC1/2 holoenyzme by binding the SPTLC1 C-terminal PDZ motif. The Par-3 interaction with the SPTLC1/2 holoenyzme was detected in the liver, a major site of SPT activity and sphingolipid synthesis. Given SPT activity is proatheroslerotic, and because we have also detected SPTLC1 expression in macrophages, a cell type that plays a central role in the progression of atherosclerosis, we tested and found that Par3 was expressed and interacted with the SPT holoenyzme in primary mouse macrophages. Significantly, loss of Par3 expression in human THP-1 monocyte macrophages reduced SPT activity and inhibited their ability to migrate toward MCP-1 (monocyte chemotactic protein-1). Likewise, shRNA suppression of SPTLC1 reduced monocyte migration toward MCP-1, as did myriocin inhibition of SPT activity, an effect that was blunted by loss of Par3 expression. In aggregate, our work has identified a novel protein-protein interaction between SPTLC1 and Par3 that is associated with an increase in SPT activity and the promotion of polarized cell migration in response to an inflammatory signal.     

Scrib复合物和Par复合物——从细胞极性调控到肿瘤发生

上皮细胞和组织发生癌变时,通常会伴随细胞极性丧失和组织结构紊乱的现象。然而,极性丧失对肿瘤发生的贡献却一直存在争论。随着控制上皮细胞顶-底极性分子机制的逐步揭示,人们发现,这一极性机制与肿瘤发生联系紧密。上皮细胞顶-底极性的确立主要依赖于顶膜区Par复合物与底侧膜区Scrib复合物之间的拮抗。当Scrib复合物活性下调或Par复合物活性上调时,两复合物间的这种相互制约的平衡就会被打破,从而导致肿瘤发生。本文主要综述Scrib复合物和Par复合物如何参与上皮细胞顶-底极性的建立,以及两者间的互作与肿瘤发生的关系。     

Proteome identification of binding-partners interacting with cell polarity protein Par3 in Jurkat cells

The evolutionarily conserved cell polarity protein Par3, a scaffold-like PDZreontaining protein, plays a critical role in the establishment and maintenance of epithelial cell polarity. Although the role of Par3 in establishing cell polarity in epithelial cells has been intensively explored, the function of Par3 in hematopoietic cells remains elusive. To address this issue, we generated GST-fusion proteins of Par3 PDZ domains. By combiningthe GST-pull-down approach with liquid chromatography-tandem mass spectrometry, we identified 10 potential novel binding proteins of PDZ domains of Par3 in Jurkat cells (a T-cell line). The interaction of Par3 with three proteins—nuclear transport protein importin-α4 and proteasome activators PA28β and PA28γ—was confirmed using in vitro binding assay, co-immunoprecipitation assay and immunofluorescence microscopy. Our results have the potential to uncover novel functions of the cell polarity protein Par3 in blood cells.     

Nucleotide exchange factor ECT2 interacts with the polarity protein complex Par6/Par3/protein kinase Czeta (PKCzeta) and regulates PKCzeta activity

Regulation of cell polarity is an important biological event that governs diverse cell functions such as localization of embryonic determinants and establishment of tissue and organ architecture. The Rho family GTPases and the polarity complex Par6/Par3/atypical protein kinase C (PKC) play a key role in the signaling pathway, but the molecules that regulate upstream signaling are still not known. Here we identified the guanine nucleotide exchange factor ECT2 as an activator of the polarity complex. ECT2 interacted with Par6 as well as Par3 and PKCzeta. Coexpression of Par6 and ECT2 efficiently activated Cdc42 in vivo. Overexpression of ECT2 also stimulated the PKCzeta activity, whereas dominant-negative ECT2 inhibited the increase in PKCzeta activity stimulated by Par6. ECT2 localization was detected at sites of cell-cell contact as well as in the nucleus of MDCK cells. The expression and localization of ECT2 were regulated by calcium, which is a critical regulator of cell-cell adhesion. Together, these results suggest that ECT2 regulates the polarity complex Par6/Par3/PKCzeta and possibly plays a role in epithelial cell polarity.     

The cell-polarity protein Par6 links Par3 and atypical protein kinase C to Cdc42

PAR (partitioning-defective) proteins, which were first identified in the nematode Caenorhabditis elegans, are essential for asymmetric cell division and polarized growth, whereas Cdc42 mediates establishment of cell polarity. Here we describe an unexpected link between these two systems. We have identified a family of mammalian Par6 proteins that are similar to the C. elegans PDZ-domain protein PAR-6. Par6 forms a complex with Cdc42-GTP, with a human homologue of the multi-PDZ protein PAR-3 and with the regulatory domains of atypical protein kinase C (PKC) proteins. This assembly is implicated in the formation of normal tight junctions at epithelial cell-cell contacts. Thus, Par6 is a key adaptor that links Cdc42 and atypical PKCs to Par3.     

Characterization of plant Aurora kinases during mitosis

The Aurora kinase family is a well-characterized serine/threonine protein kinase family that regulates different processes of mitotic events. Although functions of animal and yeast Aurora kinases have been analyzed, plant aurora kinases were not identified and characterized. We identified three Aurora kinase orthologs in Arabidopsis thaliana and designated these as AtAUR1, AtAUR2, and AtAUR3. These AtAURs could phosphorylate serine 10 in histone H3, in vitro. Dynamic analyses of GFP-fused AtAUR proteins revealed that AtAUR1 and AtAUR2 localized at the nuclear membrane in interphase and located in mitotic spindles during cell division. AtAUR1 also localized in the cell plates. AtAUR3 showed dot-like distribution on condensed chromosomes at prophase and then localized at the metaphase plate. At late anaphase, AtAUR3 is evenly localized on chromosomes. The localization of AtAUR3 during mitosis is very similar to that of phosphorylated histone H3. Interestingly, an overexpression of AtAUR3 induces disassembly of spindle microtubules and alteration of orientation of cell division. Our results indicate that plant Aurora kinases have different characters from that of Aurora kinases of other eukaryotes.†These authors equally contributed to this work     

Par proteins and neuronal polarity

A hallmark of neurons is their ability to polarize with dendrite and axon specification to allow the proper flow of information through the nervous system. Over the past decade, extensive research has been performed in an attempt to understand the molecular and cellular machinery mediating this neuronal polarization process. It has become evident that many of the critical regulators involved in establishing neuronal polarity are evolutionarily conserved proteins that had previously been implicated in controlling the polarization of other cell types. At the forefront of this research are the partition defective (Par) proteins. In this review,we will provide a commentary on the progress of work regarding the central importance of Parproteins in the establishment of neuronal polarity.     

Dynamic regulation of GEF-H1 localization at microtubules by Par1b/MARK2

Par1b/MARK2 is a serine/threonine kinase that plays key roles in the development of cell polarity, but its precise mechanism of action remains unknown. Here we report that GEF-H1, a guanine nucleotide exchange factor for Rho-family small GTPases, is a novel substrate for Par1b. GEF-H1 directly associates with microtubules via its N-terminal C1 domain, which is known to regulate the activity of GEF-H1. Ectopically expressed GEF-H1 markedly promotes stabilization of microtubules, resulting in acetylation of microtubules. We find that Par1b phosphorylates GEF-H1 at three serine residues conserved in vertebrates and releases GEF-H1 from microtubules, which abrogates stabilization and acetylation of microtubules induced by GEF-H1 overexpression. The alanine mutant for the three phosphorylation sites (3SA) of GEF-H1 strongly induces stabilization and acetylation of microtubules, which was resistant to Par1b. Time-lapse imaging analyses reveal that GFP-fused GEF-H1 dynamically moved on microtubules from one protrusion to another, whereas the 3SA mutant was static. These data suggest that Par1b-phosphorylation regulates turnover of GEF-H1 localization by regulating its interaction with microtubules, which may contribute to cell polarization.     

Neuronal Polarity

The assembly of functional neuronal networks in the developing animal relies on the polarization of neurons, i.e., the formation of a single axon and multiple dendrites. Breaking the symmetry of neurons depends on cytoskeletal rearrangements. In particular, axon specification requires local dynamic instability of actin and stabilization of microtubules. The polarized cytoskeleton also provides the basis for selective trafficking and retention of cellular components in the future somatodendritic or axonal compartments. Hence, these mechanisms are not only essential to achieve neuronal polarization, but also to maintain it. Different extracellular and intracellular signals converge on the regulation of the cytoskeleton. Most notably, Rho GTPases, PI3K, Ena/VASP, cofilin and SAD kinases are major intracellular regulators of neuronal polarity. Analyzing polarity signals under physiological conditions will provide a better understanding of how neurons can be induced to repolarize under pathological conditions, i.e., to regenerate their axons after central nervous system (CNS) injury.One ambitious aim in cellular biology is to unravel the molecular mechanisms driving cellular asymmetry and polarization. The polarity of neurons is particularly dramatic as neurons undergo complex morphological rearrangements to assemble into neuronal circuits and propagate signals. They start as round neuronal spheres, gradually adopting a complex morphology by forming one long axon and several shorter dendrites to eventually connect to other neurons via synapses. Neuronal compartments segregate into molecularly and functionally distinct zones. For example, signal input takes place at the postsynaptic densities where a chemical signal elicits electric postsynaptic potentials. These potentials are integrated along the dendritic tree and cell body to trigger an action potential arising at the axon hillock and propagating further along the axon. At their terminals, the electrical signal is reconverted into a chemical signal by the release of synaptic vesicles containing neurotransmitter.Neurons maintain their polarity throughout life by different intracellular mechanisms and molecular signals. During the last decade, cell biological and molecular approaches helped to discover many of the molecules and signaling mechanisms regulating neuronal polarity (Yoshimura et al. 2006; Arimura and Kaibuchi 2007; Witte and Bradke 2008). The aim of this article is to summarize the current knowledge and principles of breaking neuronal symmetry to generate functional neurons, and to discuss the future challenges in the field. The article covers two different topics: intrinsic mechanisms that govern symmetry breaking in the absence of external cues (in vitro systems) and the role of extracellular signaling in the establishment of neuronal polarity in vivo.     

Hepatocyte polarity establishment and apical lumen formation are organized by Par3, Cdc42, and aPKC in conjunction with Lgl

Hepatocytes differ from columnar epithelial cells by their multipolar organization, which follows the initial formation of central lumen-sharing clusters of polarized cells as observed during liver development and regeneration. The molecular mechanism for hepatocyte polarity establishment, however, has been comparatively less studied than those for other epithelial cell types. Here, we show that the tight junction protein Par3 organizes hepatocyte polarization via cooperating with the small GTPase Cdc42 to target atypical protein kinase C (aPKC) to a cortical site near the center of cell–cell contacts. In 3D Matrigel culture of human hepatocytic HepG2 cells, which mimics a process of liver development and regeneration, depletion of Par3, Cdc42, or aPKC results in an impaired establishment of apicobasolateral polarity and a loss of subsequent apical lumen formation. The aPKC activity is also required for bile canalicular (apical) elongation in mouse primary hepatocytes. The lateral membrane-associated proteins Lgl1 and Lgl2, major substrates of aPKC, seem to be dispensable for hepatocyte polarity establishment because Lgl-depleted HepG2 cells are able to form a single apical lumen in 3D culture. On the other hand, Lgl depletion leads to lateral invasion of aPKC, and overexpression of Lgl1 or Lgl2 prevents apical lumen formation, indicating that they maintain proper lateral integrity. Thus, hepatocyte polarity establishment and apical lumen formation are organized by Par3, Cdc42, and aPKC; Par3 cooperates with Cdc42 to recruit aPKC, which plays a crucial role in apical membrane development and regulation of the lateral maintainer Lgl.     

The Rac-GAP Bcr is a novel regulator of the Par complex that controls cell polarity

Cell polarization is essential for many biological processes, including directed cell migration, and loss of polarity contributes to pathological conditions such as cancer. The Par complex (Par3, Par6, and PKCζ) controls cell polarity in part by recruiting the Rac-specific guanine nucleotide exchange factor T-lymphoma invasion and metastasis 1 (Tiam1) to specialized cellular sites, where Tiam1 promotes local Rac1 activation and cytoskeletal remodeling. However, the mechanisms that restrict Par-Tiam1 complex activity to the leading edge to maintain cell polarity during migration remain unclear. We identify the Rac-specific GTPase-activating protein (GAP) breakpoint cluster region protein (Bcr) as a novel regulator of the Par-Tiam1 complex. We show that Bcr interacts with members of the Par complex and inhibits both Rac1 and PKCζ signaling. Loss of Bcr results in faster, more random migration and striking polarity defects in astrocytes. These polarity defects are rescued by reducing PKCζ activity or by expressing full-length Bcr, but not an N-terminal deletion mutant or the homologous Rac-GAP, Abr, both of which fail to associate with the Par complex. These results demonstrate that Bcr is an integral member of the Par-Tiam1 complex that controls polarized cell migration by locally restricting both Rac1 and PKCζ function.