The yeast endocytic protein Epsin 2 functions in a cell-division signaling pathway

The G-alpha Gpa1 directs septin localization in the mating projection of Saccharomyces cerevisiae through its Ubiquitination Domain and Endocytic Machinery

The yeast mating response uses a G-protein coupled receptor (GPCR), Ste2, to detect mating pheromone and initiate mating projection morphogenesis. The septin cytoskeleton plays a key role in the formation of the mating projection, forming structures at the base of the projection. Desensitization of the Gα, Gpa1, by the Regulator of G-protein Signaling (RGS), Sst2, is required for proper septin organization and morphogenesis. In cells where the Gα is hyperactive, septins are mislocalized to the site of polarity, and the cells are unable to track a pheromone gradient. We set out to identify the proteins that mediate Gα control of septins during the Saccharomyces cerevisiae mating response by making mutations to rescue septin localization in cells expressing the hyperactive Gα mutant gpa1G302S. We found that single deletions of the septin chaperone Gic1, the Cdc42 GAP Bem3, and the epsins Ent1 and Ent2 rescued the polar cap accumulation of septins in the hyperactive Gα. We created an agent-based model of vesicle trafficking that predicts how changes in endocytic cargo licensing alters localization of endocytosis that mirrors the septin localization we see experimentally. We hypothesized that hyperactive Gα may increase the rate of endocytosis of a pheromone responsive cargo, thereby altering where septins are localized. Both the GPCR and the Gα are known to be internalized by clathrin-mediated endocytosis during the pheromone response. Deletion of the GPCR C-terminus to block internalization partially rescued septin organization. However, deletion of the Gpa1 ubiquitination domain required for its endocytosis completely abrogated septin accumulation at the polarity site. Our data support a model where the location of endocytosis serves as a spatial mark for septin structure assembly and that desensitization of the Gα delays its endocytosis sufficiently that septins are placed peripheral to the site of Cdc42 polarity.

Endocytic proteins with prion-like domains form viscoelastic condensates that enable membrane remodeling

Membrane invagination and vesicle formation are key steps in endocytosis and cellular trafficking. Here, we show that endocytic coat proteins with prion-like domains (PLDs) form hemispherical puncta in the budding yeast, Saccharomyces cerevisiae These puncta have the hallmarks of biomolecular condensates and organize proteins at the membrane for actin-dependent endocytosis. They also enable membrane remodeling to drive actin-independent endocytosis. The puncta, which we refer to as endocytic condensates, form and dissolve reversibly in response to changes in temperature and solution conditions. We find that endocytic condensates are organized around dynamic protein-protein interaction networks, which involve interactions among PLDs with high glutamine contents. The endocytic coat protein Sla1 is at the hub of the protein-protein interaction network. Using active rheology, we inferred the material properties of endocytic condensates. These experiments show that endocytic condensates are akin to viscoelastic materials. We use these characterizations to estimate the interfacial tension between endocytic condensates and their surroundings. We then adapt the physics of contact mechanics, specifically modifications of Hertz theory, to develop a quantitative framework for describing how interfacial tensions among condensates, the membrane, and the cytosol can deform the plasma membrane to enable actin-independent endocytosis.

The Path towards Predicting Evolution as Illustrated in Yeast Cell Polarity

A bottom-up route towards predicting evolution relies on a deep understanding of the complex network that proteins form inside cells. In a rapidly expanding panorama of experimental possibilities, the most difficult question is how to conceptually approach the disentangling of such complex networks. These can exhibit varying degrees of hierarchy and modularity, which obfuscate certain protein functions that may prove pivotal for adaptation. Using the well-established polarity network in budding yeast as a case study, we first organize current literature to highlight protein entrenchments inside polarity. Following three examples, we see how alternating between experimental novelties and subsequent emerging design strategies can construct a layered understanding, potent enough to reveal evolutionary targets. We show that if you want to understand a cell’s evolutionary capacity, such as possible future evolutionary paths, seemingly unimportant proteins need to be mapped and studied. Finally, we generalize this research structure to be applicable to other systems of interest.

The Na+ pump Ena1 is a yeast epsin-specific cargo requiring its ubiquitylation and phosphorylation sites for internalization

It is well known that in addition to its classical role in protein turnover, ubiquitylation is required for a variety of membrane protein sorting events. However, and despite substantial progress in the field, a long-standing question remains: given that all ubiquitin units are identical, how do different elements of the sorting machinery recognize their specific cargoes? Our results indicate that the yeast Na+ pump Ena1 is an epsin (Ent1 and Ent2 in yeast)-specific cargo and that its internalization requires K1090, which likely undergoes Art3-dependent ubiquitylation. In addition, an Ena1 serine and threonine (ST)-rich patch, proposed to be targeted for phosphorylation by casein kinases, was also required for its uptake. Interestingly, our data suggest that this phosphorylation was not needed for cargo ubiquitylation. Furthermore, epsin-mediated internalization of Ena1 required a specific spatial organization of the ST patch with respect to K1090 within the cytoplasmic tail of the pump. We hypothesize that ubiquitylation and phosphorylation of Ena1 are required for epsin-mediated internalization.

RhoGTPase-binding proteins, the exocyst complex and polarized vesicle trafficking

Cell polarity, the asymmetric distribution of proteins and lipids, is essential for a variety of cellular functions. One mechanism orchestrating cell polarity is polarized vesicle trafficking; whereby cargo loaded secretory vesicles are specifically transported to predetermined areas of the cell. The evolutionarily conserved exocyst complex and its small GTPase regulators play crucial roles in spatiotemporal control of polarized vesicle trafficking. In studies on neuronal membrane remodeling and synaptic plasticity, conserved mechanisms of exocyst regulation and cargo recycling during polarized vesicle trafficking are beginning to emerge as well. Recently, our lab demonstrated that RhoGTPase-binding proteins in both yeast (Bem3) and mammals (Ocrl1) are also required for the efficient traffic of secretory vesicles to sites of polarized growth and signaling. Together with our studies, we highlight the evolutionary conservation of the basic elements essential for polarized vesicle traffic across different cellular functions and model systems. In conclusion, we emphasize that studies on RhoGTPase-binding proteins in these processes should be included in the next level of investigation, for a more complete understanding of their hitherto unknown roles in polarized membrane traffic and exocyst regulation.

Bem3, a Cdc42 GTPase-activating protein, traffics to an intracellular compartment and recruits the secretory Rab GTPase Sec4 to endomembranes

Cell polarity is essential for many cellular functions including division and cell-fate determination. Although RhoGTPase signaling and vesicle trafficking are both required for the establishment of cell polarity, the mechanisms by which they are coordinated are unclear. Here, we demonstrate that the yeast RhoGAP (GTPase activating protein), Bem3, is targeted to sites of polarized growth by the endocytic and recycling pathways. Specifically, deletion of SLA2 or RCY1 led to mislocalization of Bem3 to depolarized puncta and accumulation in intracellular compartments, respectively. Bem3 partitioned between the plasma membrane and an intracellular membrane-bound compartment. These Bem3-positive structures were polarized towards sites of bud emergence and were mostly observed during the pre-mitotic phase of apical growth. Cell biological and biochemical approaches demonstrated that this intracellular Bem3 compartment contained markers for both the endocytic and secretory pathways, which were reminiscent of the Spitzenkörper present in the hyphal tips of growing fungi. Importantly, Bem3 was not a passive cargo, but recruited the secretory Rab protein, Sec4, to the Bem3-containing compartments. Moreover, Bem3 deletion resulted in less efficient localization of Sec4 to bud tips during early stages of bud emergence. Surprisingly, these effects of Bem3 on Sec4 were independent of its GAP activity, but depended on its ability to efficiently bind endomembranes. This work unveils unsuspected and important details of the relationship between vesicle traffic and elements of the cell polarity machinery: (1) Bem3, a cell polarity and peripherally associated membrane protein, relies on vesicle trafficking to maintain its proper localization; and (2) in turn, Bem3 influences secretory vesicle trafficking.

Crystallographic analysis of the ENTH domain from yeast epsin Ent2 that induces a cell division phenotype

Epsins are eukaryotic, endocytic adaptor proteins primarily involved in the early steps of clathrin mediated endocytosis. Two epsins exist in Saccharomyces cerevisiae, Ent1 and Ent2, with single epsin knockouts being viable, while the double knockout is not. These proteins contain a highly conserved Epsin N-terminal homology (ENTH) domain that is essential for cell viability. In addition, overexpression of the ENTH domain of Ent2 (ENTH2) was shown to play a role in cell division by interacting with the septin organizing, Cdc42 GTPase activating protein, Bem3, leading to increased cytokinesis failure. In contrast, overexpression of the ENTH domain of Ent1 (ENTH1) does not affect cytokinesis, despite being 75% identical to ENTH2. An ENTH2(N112D, S114E, E118Q) mutant that switches residues in loop 7 to those found correspondingly in ENTH1 was incapable of inducing the cytokinesis phenotype. In order to better understand the role of loop 7 in the ENTH2-induced phenotype at a molecular level, X-ray crystallography was used to elucidate the structures of yeast ENTH2(WT) and ENTH2(DEQ). Our results indicate that mutations did not affect the conformation of loop 7, but rather introduce an increased negative charge on a potential interaction interface. Morphological analysis of cells overexpressing ENTH2 loop 7 mutants showed that the cytokinesis failure phenotype was abolished by the single mutants N112D, E118Q, and to a lesser extent by S114E. Taken together, our results indicate that the interaction surface that contains loop 7 and the specific nature of these residues are crucial for ENTH2 involvement in cytokinesis. This research provides insight into a molecular mechanism by which ENTH2, but not ENTH1, overexpression in yeast leads to cell division defects. Structural data of WT and mutant ENTH2 domains along with in vivo phenotypic analysis of ENTH2 overexpressing cells indicate that the biochemical nature of three loop 7 residues is crucial for its role in cytokinesis.

Endocytic adaptor protein epsin is elevated in prostate cancer and required for cancer progression

Epsins have an important role in mediating clathrin-mediated endocytosis of ubiquitinated cell surface receptors. The potential role for epsins in tumorigenesis and cancer metastasis by regulating intracellular signaling pathways has largely not been explored. Epsins are reportedly upregulated in several types of cancer including human skin, lung, and canine mammary cancers. However, whether their expression is elevated in prostate cancer is unknown. In this study, we investigated the potential role of epsins in prostate tumorigenesis using the wild type or epsin-deficient human prostate cancer cells, LNCaP, in a human xenograft model, and the spontaneous TRAMP mouse model in wild type or epsin-deficient background. Here, we reported that the expression of epsins 1 and 2 is upregulated in both human and mouse prostate cancer cells and cancerous tissues. Consistent with upregulation of epsins in prostate tumors, we discovered that depletion of epsins impaired tumor growth in both the human LNCaP xenograft and the TRAMP mouse prostate. Furthermore, epsin depletion significantly prolonged survival in the TRAMP mouse model. In summary, our findings suggest that epsins may act as oncogenic proteins to promote prostate tumorigenesis and that depletion or inhibition of epsins may provide a novel therapeutic target for future prostate cancer therapies.

Epsin Family of Endocytic Adaptor Proteins as Oncogenic Regulators of Cancer Progression

Tumor angiogenesis, tumor cell proliferation, and tumor cell migration result from an accumulation of oncogenic mutations that alter protein expression and the regulation of various signaling cascades. Epsins, a small family of clathrin-mediated endocytic adaptor proteins, are reportedly upregulated in a variety of cancers. Importantly, loss of epsins protects against tumorigenesis, thus supporting an oncogenic role for epsins in cancer. Although a clear relationship between epsins and cancer has evolved, the importance of this relationship with regards to cancer progression and anti-cancer therapies remains unclear. In this review, we summarize epsins’ role as endocytic adaptors that modulate VEGF and Notch signaling through the regulated internalization of VEGFR2 and trans-endocytosis of Notch receptors. As both VEGF and Notch signaling have significant implications in angiogenesis, we focus on the newly identified role for epsins in tumor angiogenesis. In addition to epsins’ canonical role in receptor-mediated endocytosis, and the resulting downstream signaling regulation, we discuss the non-canonical role of epsins as regulators of small GTPases and the implications this has on tumor cell proliferation and invasion. Given epsins’ identified roles in tumor angiogenesis, tumor cell proliferation, and tumor cell invasion, we predict that the investigative links between epsins and cancer will provide new insights into the importance of endocytic adaptors and their potential use as future therapeutic targets.

Role of Scd5, a protein phosphatase-1 targeting protein, in phosphoregulation of Sla1 during endocytosis

Phosphorylation regulates assembly and disassembly of proteins during endocytosis. In yeast, Prk1 and Ark1 phosphorylate factors after vesicle internalization leading to coat disassembly. Scd5, a protein phosphatase-1 (PP1)-targeting subunit, is proposed to regulate dephosphorylation of Prk1/Ark1 substrates to promote new rounds of endocytosis. In this study we analyzed scd5-PP1Δ2, a mutation causing impaired PP1 binding. scd5-PP1Δ2 caused hyperphosphorylation of several Prk1 endocytic targets. Live-cell imaging of 15 endocytic components in scd5-PP1Δ2 revealed that most factors arriving before the invagination/actin phase of endocytosis had delayed lifetimes. Severely affected were early factors and Sla2 (Hip1R homolog), whose lifetime was extended nearly fourfold. In contrast, the lifetime of Sla1, a Prk1 target, was extended less than twofold, but its cortical recruitment was significantly reduced. Delayed Sla2 dynamics caused by scd5-PP1Δ2 were suppressed by SLA1 overexpression. This was dependent on the LxxQxTG repeats (SR) of Sla1, which are phosphorylated by Prk1 and bind Pan1, another Prk1 target, in the dephosphorylated state. Without the SR, Sla1ΔSR was still recruited to the cell surface, but was less concentrated in cortical patches than Pan1. sla1ΔSR severely impaired endocytic progression, but this was partially suppressed by overexpression of LAS17, suggesting that without the SR region the SH3 region of Sla1 causes constitutive negative regulation of Las17 (WASp). These results demonstrate that Scd5/PP1 is important for recycling Prk1 targets to initiate new rounds of endocytosis and provide new mechanistic information on the role of the Sla1 SR domain in regulating progression to the invagination/actin phase of endocytosis.

Cell polarization and cytokinesis in budding yeast

Asymmetric cell division, which includes cell polarization and cytokinesis, is essential for generating cell diversity during development. The budding yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, and has thus served as an attractive model for unraveling the general principles of eukaryotic cell polarization and cytokinesis. Polarity development requires G-protein signaling, cytoskeletal polarization, and exocytosis, whereas cytokinesis requires concerted actions of a contractile actomyosin ring and targeted membrane deposition. In this chapter, we discuss the mechanics and spatial control of polarity development and cytokinesis, emphasizing the key concepts, mechanisms, and emerging questions in the field.

The epsin protein family: coordinators of endocytosis and signaling

The epsins are a conserved family of endocytic adaptors essential for cell viability in yeast and for embryo development in higher eukaryotes. Epsins function as adaptors by recognizing ubiquitinated cargo and as endocytic accessory proteins by contributing to endocytic network stability/regulation and membrane bending. Importantly, epsins play a critical role in signaling by contributing to epidermal growth factor receptor downregulation and the activation of notch and RhoGTPase pathways. In this review, we present an overview of the epsins and emphasize their functional importance as coordinators of endocytosis and signaling.

Cellular requirements for the small molecule forchlorfenuron to stabilize the septin cytoskeleton

The septins are filament-forming, GTP-binding proteins that are conserved from yeast to humans. Septins assemble into higher-order structures such as rings, bars, and gauzes with diverse functions including serving as membrane diffusion barriers and scaffolds for cell signaling. The basis for septin filament polymerization and the rules governing septin polymer dynamics are presently not well understood. Pharmacological agents are essential tools in studying such properties of the actin and microtubule cytoskeletons however there are only limited reports of a drug specific to the septin cytoskeleton. Forchlorfenuron (FCF) is a synthetic plant cytokinin used in agriculture which has been shown to alter septin organization in yeast and mammalian tissue culture cells. Here we assess cellular requirements and properties of septin-based structures induced by FCF. Treatment of the filamentous fungus Ashbya gossypii with FCF leads to assembly of extensive septin fibers throughout hyphae which is rapidly reversed upon removal of the drug. These fibers do not exchange or add septin subunits after assembly, indicating that FCF suppresses normal septin dynamics and stabilizes the polymers. While FCF-induced septin fibers do not co-localize to actin or microtubules, a polarized F-actin cytoskeleton is likely required for the assembly of drug-induced septin fibers. Thus, FCF is a potent inducer of septin polymerization and acts as a reversible stabilizer of extended septin polymers. This drug will be a powerful tool for studying mechanisms of septin polymerization and function, particularly in cell types where molecular analyses are complicated by the presence of multiple isoforms and limited genetics.

The epsin family of endocytic adaptors promotes fibrosarcoma migration and invasion

Abnormalities in the process of endocytosis are classically linked to malignant transformation through the deficient down-regulation of signaling receptors. The present study describes a non-classical mechanism that does not require internalization by which endocytic proteins affect cell migration and basement membrane invasion. Specifically, we found that the endocytic adaptor epsin binds and regulates the biological properties of the signaling molecule RalBP1 (Ral-binding protein 1). Epsin interacted with the N terminus of RalBP1 via its characteristic epsin N-terminal homology (ENTH) domain. A combination of siRNA-mediated knock-down and transfection of siRNA-resistant constructs in fibrosarcoma cells demonstrated that impairment of the epsin-RalBP1 interaction led to cell migration and basement membrane invasion defects. We found the ENTH domain was necessary and sufficient to sustain normal cell migration and invasion. Because all the epsin endocytic motifs reside in the C-terminal part of the molecule, these results suggest that this novel regulatory circuit does not require endocytosis. In addition, cells depleted of epsin-RalBP1 complex displayed deficient activation of Rac1 and Arf6 suggesting a signaling function for this novel interaction. Further, overexpression of either epsin or RalBP1 enhanced migration and invasion of fibrosarcoma cells. Collectively, our results indicate that epsin regulates RalBP1 function in Rac1- and Arf6-dependent pathways to ultimately affect cell migration and invasion. We propose that the observed up-regulation of both epsin and RalBP1 in certain cancers contributes to their invasive characteristics.

Analysis of the development of a morphological phenotype as a function of protein concentration in budding yeast

Gene deletion and protein overexpression are common methods for studying functions of proteins. In this article, we describe a protocol for analysis of phenotype development as a function of protein concentration at population and single-cell levels in Saccharomyces cerevisiae. Although this protocol is based on the overexpression of a protein, it can easily be adapted for morphological phenotypes dependent on suppression of protein expression. Our lab is interested in studying the signaling properties of the endocytic adaptor protein epsin. To that purpose we used a dominant negative approach in which we over-expressed the conserved Epsin N-Terminal Homology (ENTH) domain in order to interfere with the functions of endogenous epsin-2 (Ent2 or YLR206W). We observed that overexpression of the ENTH domain of Ent2 (ENTH2) in wild type cells led to a cell division defect that is dependent on the mislocalization of a family of scaffolding proteins, septins.