Direct evidence for ligand-induced internalization of the yeast alpha-factor pheromone receptor

Optimization of the fluorogen-activating protein tag for quantitative protein trafficking and colocalization studies in S. cerevisiae

Spatial and temporal tracking of fluorescent proteins (FPs) in live cells permits visualization of proteome remodeling in response to extracellular cues. Historically, protein dynamics during trafficking have been visualized using constitutively active FPs fused to proteins of interest. While powerful, such FPs label all cellular pools of a protein, potentially masking the dynamics of select subpopulations. To help study protein subpopulations, bioconjugate tags, including the fluorogen activation proteins (FAPs), were developed. FAPs are comprised of two components: a single-chain antibody (SCA) fused to the protein of interest and a malachite-green (MG) derivative, which fluoresces only when bound to the SCA. Importantly, the MG derivatives can be either cell-permeant or -impermeant, thus permitting isolated detection of SCA-tagged proteins at the cell surface and facilitating quantitative endocytic measures. To expand FAP use in yeast, we optimized the SCA for yeast expression, created FAP-tagging plasmids, and generated FAP-tagged organelle markers. To demonstrate FAP efficacy, we coupled the SCA to the yeast G-protein coupled receptor Ste3. We measured Ste3 endocytic dynamics in response to pheromone and characterized cis- and trans-acting regulators of Ste3. Our work significantly expands FAP technology for varied applications in S. cerevisiae.

TOR signaling regulates GPCR levels on the plasma membrane and suppresses the Saccharomyces cerevisiae mating pathway

Saccharomyces cerevisiae respond to mating pheromone through the GPCRs Ste2 and Ste3, which promote growth of a mating projection in response to ligand binding. This commitment to mating is nutritionally and energetically taxing, and so we hypothesized that the cell may suppress mating signaling during starvation. We set out to investigate negative regulators of the mating pathway in nutritionally depleted environments. Here, we report that nutrient deprivation led to loss of Ste2 from the plasma membrane. Recapitulating this effect with nitrogen starvation led us to hypothesize that it was due to TORC1 signaling. Rapamycin inhibition of TORC1 impacted membrane levels of all yeast GPCRs. Inhibition of TORC1 also dampened mating pathway output. Deletion analysis revealed that TORC1 repression leads to α-arrestin-directed CME through TORC2-Ypk1 signaling. We then set out to determine whether major downstream effectors of the TOR complexes also downregulate pathway output during mating. We found that autophagy contributes to pathway downregulation through analysis of strains lacking ATG8 . We also show that Ypk1 significantly reduced pathway output. Thus, both autophagy machinery and TORC2-Ypk1 signaling serve as attenuators of pheromone signaling during mating. Altogether, we demonstrate that the stress-responsive TOR complexes coordinate GPCR endocytosis and reduce the magnitude of pheromone signaling, in ligand-independent and ligand-dependent contexts.

One Sentence Summary

TOR signaling regulates the localization of all Saccharomyces cerevisiae GPCRs during starvation and suppress the mating pathway in the presence and absence of ligand.

Dielectric Spectroscopy Based Detection of Specific and Nonspecific Cellular Mechanisms

Using radiofrequency dielectric spectroscopy, we have investigated the impact of the interaction between a G protein-coupled receptor (GPCR), the sterile2 α-factor receptor protein (Ste2), and its cognate agonist ligand, the α-factor pheromone, on the dielectric properties of the plasma membrane in living yeast cells (Saccharomyces cerevisiae). The dielectric properties of a cell suspension containing a saturating concentration of α-factor were measured over the frequency range 40Hz–110 MHz and compared to the behavior of a similarly prepared suspension of cells in the absence of α-factor. A spherical three-shell model was used to determine the electrical phase parameters for the yeast cells in both types of suspensions. The relative permittivity of the plasma membrane showed a significant increase after exposure to α-factor (by 0.06 ± 0.05). The equivalent experiment performed on yeast cells lacking the ability to express Ste2 showed no change in plasma membrane permittivity. Interestingly, a large change also occurred to the electrical properties of the cellular interior after the addition of α-factor to the cell suspending medium, whether or not the cells were expressing Ste2. We present a number of different complementary experiments performed on the yeast to support these dielectric data and interpret the results in terms of specific cellular reactions to the presence of α-factor.

Mnemons and the memorization of past signaling events

Current advances are raising our awareness of the diverse roles that protein condensation plays in the biology of cells. Particularly, findings in organisms as diverse as yeast and Drosophila suggest that cells may utilize protein condensation to establish long-lasting changes in cellular activities and thereby encode a memory of past signaling events. Proteins that oligomerize to confer such cellular memory have been termed 'mnemons'. In the forming of super-assemblies, mnemons change their function and modulate the influence that the affected protein originally had on cellular processes. Because mnemon assemblies are self-templating, they allow cells to retain the memory of past decisions over larger timescales. Here, we review the mechanisms behind the formation of cellular memory with an emphasis on mnemon-mediated memorization of past signaling events.

A Paradigm for Peptide Hormone-GPCR Analyses

Work from our laboratories over the last 35 years that has focused on Ste2p, a G protein-coupled receptor (GPCR), and its tridecapeptide ligand α-factor is reviewed. Our work utilized the yeast Saccharomyces cerevisiae as a model system for understanding peptide-GPCR interactions. It explored the structure and function of synthetic α-factor analogs and biosynthetic receptor domains, as well as designed mutations of Ste2p. The results and conclusions are described using the nuclear magnetic resonance interrogation of synthetic Ste2p transmembrane domains (TMs), the fluorescence interrogation of agonist and antagonist binding, the biochemical crosslinking of peptide analogs to Ste2p, and the phenotypes of receptor mutants. We identified the ligand-binding domain in Ste2p, the functional assemblies of TMs, unexpected and interesting ligand analogs; gained insights into the bound α-factor structure; and unraveled the function and structures of various Ste2p domains, including the N-terminus, TMs, loops connecting the TMs, and the C-terminus. Our studies showed interactions between specific residues of Ste2p in an active state, but not resting state, and the effect of ligand activation on the dimerization of Ste2p. We show that, using a battery of different biochemical and genetic approaches, deep insight can be gained into the structure and conformational dynamics of GPCR-peptide interactions in the absence of a crystal structure.

GPCR receptor phosphorylation and endocytosis are not necessary to switch polarized growth between internal cues during pheromone response in S. cerevisiae

Chemotactic/chemotropic cells follow accurately the direction of gradients of regulatory molecules. Many G-protein-coupled receptors (GPCR) function as chemoattractant receptors to guide polarized responses. In "a" mating type yeast, the GPCR Ste2 senses the α-cell's pheromone. Previously, phosphorylation and trafficking of this receptor have been implicated in the process of gradient sensing, where cells dynamically correct growth. Correction is often necessary since yeast have intrinsic polarity sites that interfere with a correct initial gradient decoding. We have recently showed that when actively dividing (not in G1) yeast are exposed to a uniform pheromone concentration, they initiate a pheromone-induced polarization next to the mother-daughter cytokinesis site. Then, they reorient their growth to the intrinsic polarity site. Here, to study if Ste2 phosphorylation and internalization are involved in this process, we generated receptor variants combining three types of mutated signals for the first time: phosphorylation, ubiquitylation and the NPFX1,2D Sla1-binding motif. We first characterized their effect on endocytosis and found that these processes regulate internalization in a more complex manner than previously shown. Interestingly, we showed that receptor phosphorylation can drive internalization independently of ubiquitylation and the NPFX1,2D motif. When tested in our assays, cells expressing either phosphorylation or endocytosis-deficient receptors were able to switch away from the cytokinesis site to find the intrinsic polarity site as efficiently as their WT counterparts. Thus, we conclude that these processes are not necessary for the reorientation of polarization.

Ratiometric GPCR signaling enables directional sensing in yeast

Accurate detection of extracellular chemical gradients is essential for many cellular behaviors. Gradient sensing is challenging for small cells, which can experience little difference in ligand concentrations on the up-gradient and down-gradient sides of the cell. Nevertheless, the tiny cells of the yeast Saccharomyces cerevisiae reliably decode gradients of extracellular pheromones to find their mates. By imaging the behavior of polarity factors and pheromone receptors, we quantified the accuracy of initial polarization during mating encounters. We found that cells bias the orientation of initial polarity up-gradient, even though they have unevenly distributed receptors. Uneven receptor density means that the gradient of ligand-bound receptors does not accurately reflect the external pheromone gradient. Nevertheless, yeast cells appear to avoid being misled by responding to the fraction of occupied receptors rather than simply the concentration of ligand-bound receptors. Such ratiometric sensing also serves to amplify the gradient of active G protein. However, this process is quite error-prone, and initial errors are corrected during a subsequent indecisive phase in which polarity clusters exhibit erratic mobile behavior.

Cells use surface receptors to decode spatial information from chemical gradients, but accurate decoding is hampered by small cell size and the presence of molecular noise. This study shows that yeast cells decode pheromone gradients by measuring the local ratio of bound to unbound receptors. This mechanism corrects for uneven receptor density at the surface and amplifies the gradient transmitted to downstream components.

Tracking yeast pheromone receptor Ste2 endocytosis using fluorogen-activating protein tagging

To observe internalization of the yeast pheromone receptor Ste2 by fluorescence microscopy in live cells in real time, we visualized only those molecules present at the cell surface at the time of agonist engagement (rather than the total cellular pool) by tagging this receptor at its N-terminus with an exocellular fluorogen-activating protein (FAP). A FAP is a single-chain antibody engineered to bind tightly a nonfluorescent, cell-impermeable dye (fluorogen), thereby generating a fluorescent complex. The utility of FAP tagging to study trafficking of integral membrane proteins in yeast, which possesses a cell wall, had not been examined previously. A diverse set of signal peptides and propeptide sequences were explored to maximize expression. Maintenance of the optimal FAP-Ste2 chimera intact required deletion of two, paralogous, glycosylphosphatidylinositol (GPI)-anchored extracellular aspartyl proteases (Yps1 and Mkc7). FAP-Ste2 exhibited a much brighter and distinct plasma membrane signal than Ste2-GFP or Ste2-mCherry yet behaved quite similarly. Using FAP-Ste2, new information was obtained about the mechanism of its internalization, including novel insights about the roles of the cargo-selective endocytic adaptors Ldb19/Art1, Rod1/Art4, and Rog3/Art7.

Interactions of α-Factor-1, a Yeast Pheromone, and Its Analogue with Copper(II) Ions and Low-Molecular-Weight Ligands Yield Very Stable Complexes

α-Factor-1 (WHWLQLKPGQPMY), a peptidic pheromone of Saccharomyces cerevisiae yeast, contains a XHX type copper(II) binding N-terminal site. Using a soluble analogue, WHWSKNR-amide, we demonstrated that the W(1)H(2)W(3) site alone binds copper(II) with a Kd value of 0.18 pM at pH 7.4 and also binds imidazole (Im) in a ternary complex (Kd of 1 mM at pH 7.4). This interaction boosts the ability of the peptide to sequester copper(II) depending on the Im concentration up to a subfemtomolar range, not available for any oligopeptidic system studied before. Therefore, α-factor-1 and other XHX-type peptides are likely copper(II) carriers in biological systems.

Effective Design of Multifunctional Peptides by Combining Compatible Functions

Multifunctionality is a common trait of many natural proteins and peptides, yet the rules to generate such multifunctionality remain unclear. We propose that the rules defining some protein/peptide functions are compatible. To explore this hypothesis, we trained a computational method to predict cell-penetrating peptides at the sequence level and learned that antimicrobial peptides and DNA-binding proteins are compatible with the rules of our predictor. Based on this finding, we expected that designing peptides for CPP activity may render AMP and DNA-binding activities. To test this prediction, we designed peptides that embedded two independent functional domains (nuclear localization and yeast pheromone activity), linked by optimizing their composition to fit the rules characterizing cell-penetrating peptides. These peptides presented effective cell penetration, DNA-binding, pheromone and antimicrobial activities, thus confirming the effectiveness of our computational approach to design multifunctional peptides with potential therapeutic uses. Our computational implementation is available at http://bis.ifc.unam.mx/en/software/dcf.

Most proteins and peptides in nature display multiple activities either by fusing different domains (with different activities) or by evolving multiple activities in a single domain. Understanding which activities may be combined to render multifunctional proteins remains an open question relevant to understanding the organization of living organisms and to improve the design of pharmacological peptides. To address this problem, we introduce the concept of compatible activities, that is, activities that may combine without losing any of these in a single polypeptide chain. To identify compatible activities in peptide sequences, we used a machine-learning approach and discovered that a penetrating activity should be compatible with DNA-binding and antimicrobial activities. To test if these activities may combine without any functional loss, we designed peptide sequences that harbor two independent activities (nuclear localization and pheromone) and experimentally showed that all our designed peptides display penetrability, pheromone, antimicrobial and DNA-binding activities, supporting the idea that multifunctionality may be achieved combining compatible activities.

Role of Polarized G Protein Signaling in Tracking Pheromone Gradients

Yeast cells track gradients of pheromones to locate mating partners. Intuition suggests that uniform distribution of pheromone receptors over the cell surface would yield optimal gradient sensing. However, yeast cells display polarized receptors. The benefit of such polarization was unknown. During gradient tracking, cell growth is directed by a patch of polarity regulators that wanders around the cortex. Patch movement is sensitive to pheromone dose, with wandering reduced on the up-gradient side of the cell, resulting in net growth in that direction. Mathematical modeling suggests that active receptors and associated G proteins lag behind the polarity patch and act as an effective drag on patch movement. In vivo, the polarity patch is trailed by a G protein-rich domain, and this polarized distribution of G proteins is required to constrain patch wandering. Our findings explain why G protein polarization is beneficial and illuminate a novel mechanism for gradient tracking.

α-Arrestins participate in cargo selection for both clathrin-independent and clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is a well-studied mechanism to internalize plasma membrane proteins; however, to endocytose such cargo, most eukaryotic cells also use alternative clathrin-independent endocytic (CIE) pathways, which are less well characterized. The budding yeast Saccharomyces cerevisiae, a widely used model for studying CME, was recently shown to have a CIE pathway that requires the GTPase Rho1, the formin Bni1, and their regulators. Nevertheless, in both yeast and mammalian cells, the mechanisms underlying cargo selection in CME and CIE are only beginning to be understood. For CME in yeast, particular α-arrestins contribute to recognition of specific cargos and promote their ubiquitylation by recruiting the E3 ubiquitin protein ligase Rsp5. Here, we show that the same α-arrestin–cargo pairs promote internalization through the CIE pathway by interacting with CIE components. Notably, neither expression of Rsp5 nor its binding to α-arrestins is required for CIE. Thus, α-arrestins are important for cargo selection in both the CME and CIE pathways, but function by distinct mechanisms in each pathway.

Summary: In yeast, α-arrestins bind the Rho1 GTPase and regulate internalization of selective cargo through the clathrin-independent endocytic pathway.

Modulation of receptor dynamics by the regulator of G protein signaling Sst2

G protein–coupled receptor signaling is negatively regulated by both receptor internalization and regulator of G protein signaling (RGS) protein–stimulated inactivation of the G protein. The RGS protein can also positively regulate receptor signaling by binding to the receptor, thereby reducing receptor internalization.

G protein–coupled receptor (GPCR) signaling is fundamental to physiological processes such as vision, the immune response, and wound healing. In the budding yeast Saccharomyces cerevisiae, GPCRs detect and respond to gradients of pheromone during mating. After pheromone stimulation, the GPCR Ste2 is removed from the cell membrane, and new receptors are delivered to the growing edge. The regulator of G protein signaling (RGS) protein Sst2 acts by accelerating GTP hydrolysis and facilitating pathway desensitization. Sst2 is also known to interact with the receptor Ste2. Here we show that Sst2 is required for proper receptor recovery at the growing edge of pheromone-stimulated cells. Mathematical modeling suggested pheromone-induced synthesis of Sst2 together with its interaction with the receptor function to reestablish a receptor pool at the site of polarized growth. To validate the model, we used targeted genetic perturbations to selectively disrupt key properties of Sst2 and its induction by pheromone. Together our results reveal that a regulator of G protein signaling can also regulate the G protein–coupled receptor. Whereas Sst2 negatively regulates G protein signaling, it acts in a positive manner to promote receptor retention at the growing edge.

Spatiotemporal dynamics of membrane remodeling and fusion proteins during endocytic transport

Endosomal sorting requires consecutive steps of membrane remodeling and fusion in the course of endosomal maturation. Tracing of cargo relative to machinery reveals similar temporal localization of ESCRT and endosomal fusion machinery, which precedes the retromer complex. However, blocking fusion with the vacuole does not impair maturation.

Organelles of the endolysosomal system undergo multiple fission and fusion events to combine sorting of selected proteins to the vacuole with endosomal recycling. This sorting requires a consecutive remodeling of the organelle surface in the course of endosomal maturation. Here we dissect the remodeling and fusion machinery on endosomes during the process of endocytosis. We traced selected GFP-tagged endosomal proteins relative to exogenously added fluorescently labeled α-factor on its way from the plasma membrane to the vacuole. Our data reveal that the machinery of endosomal fusion and ESCRT proteins has similar temporal localization on endosomes, whereas they precede the retromer cargo recognition complex. Neither deletion of retromer nor the fusion machinery with the vacuole affects this maturation process, although the kinetics seems to be delayed due to ESCRT deletion. Of importance, in strains lacking the active Rab7-like Ypt7 or the vacuolar SNARE fusion machinery, α-factor still proceeds to late endosomes with the same kinetics. This indicates that endosomal maturation is mainly controlled by the early endosomal fusion and remodeling machinery but not the downstream Rab Ypt7 or the SNARE machinery. Our data thus provide important further understanding of endosomal biogenesis in the context of cargo sorting.

The yeast Arf-GAP Glo3p is required for the endocytic recycling of cell surface proteins

Small GTP-binding proteins of the Ras superfamily play diverse roles in intracellular trafficking. Among them, the Rab, Arf, and Rho families function in successive steps of vesicle transport, in forming vesicles from donor membranes, directing vesicle trafficking toward target membranes and docking vesicles onto target membranes. These proteins act as molecular switches that are controlled by a cycle of GTP binding and hydrolysis regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). In this study we explored the role of GAPs in the regulation of the endocytic pathway using fluorescently labeled yeast mating pheromone α-factor. Among 25 non-essential GAP mutants, we found that deletion of the GLO3 gene, encoding Arf-GAP protein, caused defective internalization of fluorescently labeled α-factor. Quantitative analysis revealed that glo3Δ cells show defective α-factor binding to the cell surface. Interestingly, Ste2p, the α-factor receptor, was mis-localized from the plasma membrane to the vacuole in glo3Δ cells. Domain deletion mutants of Glo3p revealed that a GAP-independent function, as well as the GAP activity, of Glo3p is important for both α-factor binding and Ste2p localization at the cell surface. Additionally, we found that deletion of the GLO3 gene affects the size and number of Arf1p-residing Golgi compartments and causes a defect in transport from the TGN to the plasma membrane. Furthermore, we demonstrated that glo3Δ cells were defective in the late endosome-to-TGN transport pathway, but not in the early endosome-to-TGN transport pathway. These findings suggest novel roles for Arf-GAP Glo3p in endocytic recycling of cell surface proteins.

Interaction among Saccharomyces cerevisiae pheromone receptors during endocytosis

This study investigates endocytosis of Saccharomyces cerevisiae α-factor receptor and the role that receptor oligomerization plays in this process. α-factor receptor contains signal sequences in the cytoplasmic C-terminal domain that are essential for ligand-mediated endocytosis. In an endocytosis complementation assay, we found that oligomeric complexes of the receptor undergo ligand-mediated endocytosis when the α-factor binding site and the endocytosis signal sequences are located in different receptors. Both in vitro and in vivo assays suggested that ligand-induced conformational changes in one Ste2 subunit do not affect neighboring subunits. Therefore, recognition of the endocytosis signal sequence and recognition of the ligand-induced conformational change are likely to be two independent events.

A super-assembly of Whi3 encodes memory of deceptive encounters by single cells during yeast courtship

Cellular behavior is frequently influenced by the cell's history, indicating that single cells may memorize past events. We report that budding yeast permanently escape pheromone-induced cell-cycle arrest when experiencing a deceptive mating attempt, i.e., not reaching their putative partner within reasonable time. This acquired behavior depends on super-assembly and inactivation of the G1/S inhibitor Whi3, which liberates the G1 cyclin Cln3 from translational inhibition. Super-assembly of Whi3 is a slow response to pheromone, driven by polyQ and polyN domains, counteracted by Hsp70, and stable over generations. Unlike prion aggregates, Whi3 super-assemblies are not inherited mitotically but segregate to the mother cell. We propose that such polyQ- and polyN-based elements, termed here mnemons, act as cellular memory devices to encode previous environmental conditions.

The proteolytic landscape of the yeast vacuole

The vacuole in the yeast Saccharomyces cerevisiae plays a number of essential roles, and to provide some of these required functions the vacuole harbors at least seven distinct proteases. These proteases exhibit a range of activities and different classifications, and they follow unique paths to arrive at their ultimate, common destination in the cell. This review will first summarize the major functions of the yeast vacuole and delineate how proteins are targeted to this organelle. We will then describe the specific trafficking itineraries and activities of the characterized vacuolar proteases, and outline select features of a new member of this protease ensemble. Finally, we will entertain the question of why so many proteases evolved and reside in the vacuole, and what future research challenges exist in the field.

Vps1 in the late endosome-to-vacuole traffic

Vacuolar protein sorting 1 (Vps1), the yeast homolog to human dynamin, is a GTP hydrolyzing protein, which plays an important role in protein sorting and targeting between the Golgi and late endosomal compartments. In this study, we assessed the functional significance of Vps1 in the membrane traffic towards the vacuole. We show here that vps1 delta cells accumulated FM4-64 to a greater extent than wild-type (WT))cells, suggesting slower endocytic degradation traffic toward the vacuole. In addition, we observed that two endosome-to-vacuole traffic markers, DsRed-FYVE and Ste2-GFP, were highly accumulated in Vps1-deficient cells, further supporting Vps1's implication in efficient trafficking of endocytosed materials to the vacuole. Noteworthy, a simultaneous imaging analysis in conjunction with FM4-64 pulse-chase experiment further revealed that Vps1 plays a role in late endosome to the vacuole transport. Consistently, our subcellular localization analysis showed that Vps1 is present at the late endosome. The hyperaccumulation of endosomal intermediates in the vps1 mutant cells appears to be caused by the disruption of integrity of HOPS tethering complexes, manifested by mislocalization of Vps39 to the cytoplasm. Finally, we postulate that Vps1 functions together with the Endosomal Sorting Complex Required for Transport (ESCRT) complex at the late endosomal compartments, based on the observation that the double mutants, in which VPS1 along with singular ESCRT I, II and III genes have been disrupted, exhibited synthetic lethality. Together, we propose that Vps1 is required for correct and efficient trafficking from the late endosomal compartments to the vacuole.

Disentangling signaling gradients generated by equivalent sources

Yeast cells approach a mating partner by polarizing along a gradient of mating pheromones that are secreted by cells of the opposite mating type. The Bar1 protease is secreted by a-cells and, paradoxically, degrades the α-factor pheromones which are produced by cells of the opposite mating type and trigger mating in a-cells. This degradation may assist in the recovery from pheromone signaling but has also been shown to play a positive role in mating. Previous studies suggested that widely diffusing protease can bias the pheromone gradient towards the closest secreting cell. Here, we show that restricting the Bar1 protease to the secreting cell itself, preventing its wide diffusion, facilitates discrimination between equivalent mating partners. This may be mostly relevant during spore germination, where most mating events occur in nature.

Fungal mating pheromones: choreographing the dating game

Pheromones are ubiquitous from bacteria to mammals - a testament to their importance in regulating inter-cellular communication. In fungal species, they play a critical role in choreographing interactions between mating partners during the program of sexual reproduction. Here, we describe how fungal pheromones are synthesized, their interactions with G protein-coupled receptors, and the signals propagated by this interaction, using Saccharomyces cerevisiae as a reference point. Divergence from this model system is compared amongst the ascomycetes and basidiomycetes, which reveals the wealth of information that has been gleaned from studying pheromone-driven processes across a wide spectrum of the fungal kingdom.

How a mycoparasite employs g-protein signaling: using the example of trichoderma

Mycoparasitic Trichoderma spp. act as potent biocontrol agents against a number of plant pathogenic fungi, whereupon the mycoparasitic attack includes host recognition followed by infection structure formation and secretion of lytic enzymes and antifungal metabolites leading to the host's death. Host-derived signals are suggested to be recognized by receptors located on the mycoparasite's cell surface eliciting an internal signal transduction cascade which results in the transcription of mycoparasitism-relevant genes. Heterotrimeric G proteins of fungi transmit signals originating from G-protein-coupled receptors mainly to the cAMP and the MAP kinase pathways resulting in regulation of downstream effectors. Components of the G-protein signaling machinery such as Gα subunits and G-protein-coupled receptors were recently shown to play crucial roles in Trichoderma mycoparasitism as they govern processes such as the production of extracellular cell wall lytic enzymes, the secretion of antifungal metabolites, and the formation of infection structures.

Nonsense-mediated mRNA decay maintains translational fidelity by limiting magnesium uptake

Inactivation of the yeast nonsense-mediated mRNA decay (NMD) pathway stabilizes nonsense mRNAs and promotes readthrough of premature translation termination codons. Although the latter phenotype is thought to reflect a direct role of NMD factors in translation termination, its mechanism is unknown. Here we show that the reduced termination efficiency of NMD-deficient cells is attributable to increased expression of the magnesium transporter Alr1p and the resulting effects of elevated Mg(2+) levels on termination fidelity. Alr1p levels increase because an upstream ORF in ALR1 mRNA targets the transcript for NMD. Our results demonstrate that NMD, at least in yeast, controls Mg(2+) homeostasis and, consequently, translational fidelity.

Increased accuracy of ligand sensing by receptor internalization

Many types of cells can sense external ligand concentrations with cell-surface receptors at extremely high accuracy. Interestingly, ligand-bound receptors are often internalized, a process also known as receptor-mediated endocytosis. While internalization is involved in a vast number of important functions for the life of a cell, it was recently also suggested to increase the accuracy of sensing ligand as the overcounting of the same ligand molecules is reduced. Here we show, by extending simple ligand-receptor models to out-of-equilibrium thermodynamics, that internalization increases the accuracy with which cells can measure ligand concentrations in the external environment. Comparison with experimental rates of real receptors demonstrates that our model has indeed biological significance.

Maximum likelihood and the single receptor

The accuracy by which biological cells sense chemical concentration is ultimately limited by the random arrival of particles at the receptors by diffusion. This fundamental physical limit is generally considered to be the Berg-Purcell limit [Biophys. J. 20, 193 (1977)]. Here we derive a lower limit by applying maximum likelihood to the time series of receptor occupancy. The increased accuracy stems from solely considering the unoccupied time intervals--disregarding the occupied time intervals as these do not contain any information about the external particle concentration, and only decrease the accuracy of the concentration estimate. Receptors which minimize the bound time intervals achieve the highest possible accuracy. We discuss how a cell could implement such an optimal sensing strategy by absorbing or degrading bound particles.

Interaction of the deubiquitinating enzyme Ubp2 and the e3 ligase Rsp5 is required for transporter/receptor sorting in the multivesicular body pathway

Protein ubiquitination is essential for many events linked to intracellular protein trafficking. We sought to elucidate the possible involvement of the S. cerevisiae deubiquitinating enzyme Ubp2 in transporter and receptor trafficking after we (this study) and others established that affinity purified Ubp2 interacts stably with the E3 ubiquitin ligase Rsp5 and the (ubiquitin associated) UBA domain containing protein Rup1. UBP2 interacts genetically with RSP5, while Rup1 facilitates the tethering of Ubp2 to Rsp5 via a PPPSY motif. Using the uracil permease Fur4 as a model reporter system, we establish a role for Ubp2 in membrane protein turnover. Similar to hypomorphic rsp5 alleles, cells deleted for UBP2 exhibited a temporal stabilization of Fur4 at the plasma membrane, indicative of perturbed protein trafficking. This defect was ubiquitin dependent, as a Fur4 N-terminal ubiquitin fusion construct bypassed the block and restored sorting in the mutant. Moreover, the defect was absent in conditions where recycling was absent, implicating Ubp2 in sorting at the multivesicular body. Taken together, our data suggest a previously overlooked role for Ubp2 as a positive regulator of Rsp5-mediated membrane protein trafficking subsequent to endocytosis.

A Candida albicans-specific region of the alpha-pheromone receptor plays a selective role in the white cell pheromone response

Candida albicans strains homozygous at the mating type locus can switch from white to opaque, and must do so to mate. Opaque cells then secrete mating pheromones that stimulate opaque cells of opposite mating type to undergo mating. These same pheromones stimulate mating-incompetent white cells to become cohesive and adhesive, and enhance white cell biofilm development, a pathogenic trait. Stimulation is mediated through the same receptor, G protein complex and mitogen-activated protein kinase pathway. Here we present evidence that a C. albicans-specific 55-amino-acid region of the first intracellular loop, IC1, of the alpha-pheromone receptor Ste2p, is required for the alpha-pheromone response of white cells, but not that of opaque cells. This represents a unique regulatory configuration in which activation of a common pathway by the same ligand, the same receptor and the same signal transduction pathway is dependent on a unique region of an intracellular loop of the common receptor in one of the two responding phenotypes.

Magnificent seven: roles of G protein-coupled receptors in extracellular sensing in fungi

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors and are responsible for transducing extracellular signals into intracellular responses that involve complex intracellular-signaling networks. This review highlights recent research advances in fungal GPCRs, including classification, extracellular sensing, and G protein-signaling regulation. The involvement of GPCRs in pheromone and nutrient sensing has been studied extensively over the past decade. Following recent advances in fungal genome sequencing projects, a panoply of GPCR candidates has been revealed and some have been documented to play key roles sensing diverse extracellular signals, such as pheromones, sugars, amino acids, nitrogen sources, and even photons. Identification and deorphanization of additional putative GPCRs may require the development of new research tools. Here, we compare research on GPCRs in fungi with information derived from mammalian systems to provide a useful road map on how to better understand ligand-GPCR-G protein interactions in general. We also emphasize the utility of yeast as a discovery tool for systemic studies of GPCRs from other organisms.

Distinct roles for two Galpha-Gbeta interfaces in cell polarity control by a yeast heterotrimeric G protein

Saccharomyces cerevisiae mating pheromones trigger dissociation of a heterotrimeric G protein (Galphabetagamma) into Galpha-guanosine triphosphate (GTP) and Gbetagamma. The Gbetagamma dimer regulates both mitogen-activated protein (MAP) kinase cascade signaling and cell polarization. Here, by independently activating the MAP kinase pathway, we studied the polarity role of Gbetagamma in isolation from its signaling role. MAP kinase signaling alone could induce cell asymmetry but not directional growth. Surprisingly, active Gbetagamma, either alone or with Galpha-GTP, could not organize a persistent polarization axis. Instead, following pheromone gradients (chemotropism) or directional growth without pheromone gradients (de novo polarization) required an intact receptor-Galphabetagamma module and GTP hydrolysis by Galpha. Our results indicate that chemoattractant-induced cell polarization requires continuous receptor-Galphabetagamma communication but not modulation of MAP kinase signaling. To explore regulation of Gbetagamma by Galpha, we mutated Gbeta residues in two structurally distinct Galpha-Gbeta binding interfaces. Polarity control was disrupted only by mutations in the N-terminal interface, and not the Switch interface. Incorporation of these mutations into a Gbeta-Galpha fusion protein, which enforces subunit proximity, revealed that Switch interface dissociation regulates signaling, whereas the N-terminal interface may govern receptor-Galphabetagamma coupling. These findings raise the possibility that the Galphabetagamma heterotrimer can function in a partially dissociated state, tethered by the N-terminal interface.

Identification of an N-acetylglucosamine transporter that mediates hyphal induction in Candida albicans

The sugar N-acetylglucosamine (GlcNAc) plays an important role in nutrient sensing and cellular regulation in a wide range of organisms from bacteria to humans. In the fungal pathogen Candida albicans, GlcNAc induces a morphological transition from budding to hyphal growth. Proteomic comparison of plasma membrane proteins from buds and from hyphae induced by GlcNAc identified a novel hyphal protein (Ngt1) with similarity to the major facilitator superfamily of transporters. An Ngt1-GFP fusion was detected in the plasma membrane after induction with GlcNAc, but not other related sugars. Ngt1-GFP was also induced by macrophage phagocytosis, suggesting a role for the GlcNAc response in signaling entry into phagolysosomes. NGT1 is needed for efficient GlcNAc uptake and for the ability to induce hyphae at low GlcNAc concentrations. High concentrations of GlcNAc could bypass the need for NGT1 to induce hyphae, indicating that elevated intracellular levels of GlcNAc induce hyphal formation. Expression of NGT1 in Saccharomyces cerevisiae promoted GlcNAc uptake, indicating that Ngt1 acts directly as a GlcNAc transporter. Transport mediated by Ngt1 was specific, as other sugars could not compete for the uptake of GlcNAc. Thus, Ngt1 represents the first eukaryotic GlcNAc transporter to be discovered. The presence of NGT1 homologues in the genome sequences of a wide range of eukaryotes from yeast to mammals suggests that they may also function in the cellular processes regulated by GlcNAc, including those that underlie important diseases such as cancer and diabetes.

Opaque cells signal white cells to form biofilms in Candida albicans

Upon homozygosis from a/alpha to a/a or alpha/alpha, Candida albicans must still switch from the 'white' to 'opaque' phenotype to mate. It was, therefore, surprising to discover that pheromone selectively upregulated mating-associated genes in mating-incompetent white cells without causing G1 arrest or shmoo formation. White cells, like opaque cells, possess pheromone receptors, although their distribution and redistribution upon pheromone treatment differ between the two cell types. In speculating about the possible role of the white cell pheromone response, it is hypothesized that in overlapping white a/a and alpha/alpha populations in nature, rare opaque cells, through the release of pheromone, signal majority white cells of opposite mating type to form a biofilm that facilitates mating. In support of this hypothesis, it is demonstrated that pheromone induces cohesiveness between white cells, minority opaque cells increase two-fold the thickness of majority white cell biofilms, and majority white cell biofilms facilitate minority opaque cell chemotropism. These results reveal a novel form of communication between switch phenotypes, analogous to the inductive events during embryogenesis in higher eukaryotes.

Identification of functional domains of Mid1, a stretch-activated channel component, necessary for localization to the plasma membrane and Ca2+ permeation

The Saccharomyces cerevisiae MID1 gene product (Mid1) is a stretch-activated Ca(2+)-permeable channel component required for Ca2+ influx and the maintenance of viability of cells exposed to the mating pheromone, alpha-factor. It is composed of 548-amino-acid (aa) residues with four hydrophobic segments, H1 (aa 2-22), H2 (aa 92-111), H3 (aa 337-356) and H4 (aa 366-388). It also has 16 putative N-glycosylation sites. In this study, sequentially truncated Mid1 proteins conjugated with GFP were expressed in S. cerevisiae cells. The truncated protein containing the region from H1 to H3 (Mid1(1-360)-GFP) localized normally in the plasma and endoplasmic reticulum (ER) membranes and complemented the low viability and Ca(2+)-uptake activity of the mid1 mutant, whereas Mid1(1-133)-GFP containing the region from H1 to H2 did not. Mid1(Delta3-22)-GFP lacking the H1 region failed to localize in the plasma membrane. Membrane fractionation showed that Mid1(1-22)-GFP containing only H1 localized in the plasma membrane in the presence of alpha-factor, suggesting that H1 is a signal sequence responsible for the alpha-factor-induced Mid1 delivery to the plasma membrane. The region from H1 to H3 is required for the localization of Mid1 in the plasma and ER membranes. Finally, trafficking of Mid1-GFP to the plasma membrane was dependent on the N-glycosylation of Mid1 and the transporter protein Sec12.

Pathogenic mutations inactivate parkin by distinct mechanisms

Loss of parkin function is the major cause of autosomal recessive Parkinson's disease (ARPD). A wide variety of parkin mutations have been identified in patients; however, the pathophysiological mechanisms leading to the inactivation of mutant parkin are poorly understood. In this study we characterized pathogenic C- and N-terminal parkin mutants and found distinct pathways of parkin inactivation. Deletion of the C terminus abrogated the association of parkin with cellular membranes and induced rapid misfolding and aggregation. Four N-terminal missense mutations, located within the ubiquitin-like domain (UBL), decrease the stability of parkin; as a consequence, these mutants are rapidly degraded by the proteasome. Furthermore, we present evidence that a smaller parkin species of 42 kDa, which is present in extracts prepared from human brain and cultured cells, originates from an internal start site and lacks the N-terminal UBL domain.

Subcellular localization and oligomeric structure of the yeast putative stretch-activated Ca2+ channel component Mid1

The yeast Mid1 protein with an apparent molecular mass of 100 kDa is required for Ca2+ influx stimulated by the mating pheromone and by a capacitative calcium entrylike mechanism acting in response to Ca2+ depletion from the endoplasmic reticulum (ER) and functions as a stretch-activated Ca2+ -permeable channel when expressed in mammalian cells. Our previous work with protease protection experiments has indicated that Mid1 is present in the plasma membrane. In this study, we examined a possible intracellular localization of this protein by indirect fluorescence microscopy and found that Mid1 is present in the ER membrane as well as the plasma membrane. Intracellular fluorescence images for Mid1 were the same as those for the ER marker protein Sec71 but quite different from those of the Golgi protein Ypt1. The results were confirmed by membrane fractionation using Angiografin density gradient analysis. We also investigated the oligomeric structures and protein levels of Mid1 and found that Mid1 forms a 200-kDa oligomer by disulfide bonding. The protein level and modification of Mid1 in the plasma membrane and the ER membrane were unchanged by the mating pheromone. These findings provide new insight into the function of Mid1 in relation to localization, modification, and activation mechanisms.

The C-terminal globular domain of the prion protein is necessary and sufficient for import into the endoplasmic reticulum

The mammalian prion protein (PrP) is composed of an unstructured flexible N-terminal region and a C-terminal globular domain. We examined the import of PrP into the endoplasmic reticulum (ER) of neuronal cells and show that information present in the C-terminal globular domain is required for ER import of the N terminus. N-terminal fragments of PrP, devoid of structural domains located in the C terminus, remained in the cytosol with an uncleaved signal peptide and were rapidly degraded by the proteasome. Conversely, the separate C-terminal domain of PrP, comprising the highly ordered helix 2-loop-helix 3 motif, was entirely imported into the ER. As a consequence, two PrP mutants linked to inherited prion disease in humans, PrP-W145Stop and PrP-Q160Stop, were partially retained in the cytosol. The cytosolic fraction was characterized by an uncleaved N-terminal signal peptide and was degraded by the proteasome. Our study identified a new regulatory element in the C-terminal globular domain of PrP necessary and sufficient to promote import of PrP into the ER.

Post-translational import of the prion protein into the endoplasmic reticulum interferes with cell viability: a critical role for the putative transmembrane domain

Aberrant folding of the mammalian prion protein (PrP) is linked to prion diseases in humans and animals. We show that during post-translational targeting of PrP to the endoplasmic reticulum (ER) the putative transmembrane domain induces misfolding of PrP in the cytosol and interferes with its import into the ER. Unglycosylated and misfolded PrP with an uncleaved N-terminal signal sequence associates with ER membranes, and, moreover, decreases cell viability. PrP expressed in the cytosol, lacking the N-terminal ER targeting sequence, also adopts a misfolded conformation; however, this has no adverse effect on cell growth. PrP processing, productive ER import, and cellular viability can be restored either by deleting the putative transmembrane domain or by using a N-terminal signal sequence specific for co-translational ER import. Our study reveals that the putative transmembrane domain features in the formation of misfolded PrP conformers and indicates that post-translational targeting of PrP to the ER can decrease cell viability.

C5a receptor oligomerization. II. Fluorescence resonance energy transfer studies of a human G protein-coupled receptor expressed in yeast

Recent studies demonstrate that members of the superfamily of G protein-coupled receptors (GPCRs) form oligomers both in vitro and in vivo. The mechanisms by which GPCRs oligomerize and the roles of accessory proteins in this process are not well understood. We used disulfide-trapping experiments to show that C5a receptors, expressed in mammalian cells, reside in membranes as oligomers (Klco, J. M., Lassere, T. B., and Baranski, T. J. (2003) J. Biol. Chem. 278, 35345-35353). To begin to address how C5a receptors form oligomers, we now use fluorescence resonance energy transfer experiments on human C5a receptors expressed in the lower eukaryote Saccharomyces cerevisiae. C5a receptors tagged with variants of the green fluorescent protein display energy transfer in intact yeast, demonstrating that mammalian accessory proteins are not required for C5a receptor oligomerization. In both intact yeast cells and membrane preparations, agonist does not affect FRET efficiency, and little energy transfer is observed between the C5a receptor and a co-expressed yeast pheromone receptor (encoded by STE2), indicating that C5a receptor oligomerization is both receptor-specific and constitutive. FRET studies performed on fractionated membranes demonstrate similar levels of energy transfer between tagged C5a receptors in endoplasmic reticulum compared with plasma membrane, and urea washing of membranes has little effect on the extent of energy transfer. The oligomerization of C5a receptors expressed in yeast displays characteristics similar to those observed for other GPCRs studied in mammalian cells. This model system should prove useful for further studies to define mechanisms of oligomerization of mammalian GPCRs.

An essential subfamily of Drs2p-related P-type ATPases is required for protein trafficking between Golgi complex and endosomal/vacuolar system

The Saccharomyces cerevisiae genome contains five genes encoding P-type ATPases that are potential aminophospholipid translocases (APTs): DRS2, NEO1, and three uncharacterized open reading frames that we have named DNF1, DNF2, and DNF3 for DRS2/NEO1 family. NEO1 is the only essential gene in APT family and seems to be functionally distinct from the DRS2/DNF genes. The drs2Delta dnf1Delta dnf2Delta dnf3Delta quadruple mutant is inviable, although any one member of this group can maintain viability, indicating that there is a substantial functional overlap between the encoded proteins. We have previously implicated Drs2p in clathrin function at the trans-Golgi network. In this study, we constructed strains carrying all possible viable combinations of null alleles from this group and analyzed them for defects in protein transport. The drs2Delta dnf1Delta mutant grows slowly, massively accumulates intracellular membranes, and exhibits a substantial defect in the transport of alkaline phosphatase to the vacuole. Transport of carboxypeptidase Y to the vacuole is also perturbed, but to a lesser extent. In addition, the dnf1Delta dnf2Delta dnf3Delta mutant exhibits a defect in recycling of GFP-Snc1p in the early endocytic-late secretory pathways. Drs2p and Dnf3p colocalize with the trans-Golgi network marker Kex2p, whereas Dnf1p and Dnf2p seem to localize to the plasma membrane and late exocytic or early endocytic membranes. We propose that eukaryotes express multiple APT subfamily members to facilitate protein transport in multiple pathways.

Regulation of G protein-initiated signal transduction in yeast: paradigms and principles

All cells have the capacity to evoke appropriate and measured responses to signal molecules (such as peptide hormones), environmental changes, and other external stimuli. Tremendous progress has been made in identifying the proteins that mediate cellular response to such signals and in elucidating how events at the cell surface are linked to subsequent biochemical changes in the cytoplasm and nucleus. An emerging area of investigation concerns how signaling components are assembled and regulated (both spatially and temporally), so as to control properly the specificity and intensity of a given signaling pathway. A related question under intensive study is how the action of an individual signaling pathway is integrated with (or insulated from) other pathways to constitute larger networks that control overall cell behavior appropriately. This review describes the signal transduction pathway used by budding yeast (Saccharomyces cerevisiae) to respond to its peptide mating pheromones. This pathway is comprised by receptors, a heterotrimeric G protein, and a protein kinase cascade all remarkably similar to counterparts in multicellular organisms. The primary focus of this review, however, is recent advances that have been made, using primarily genetic methods, in identifying molecules responsible for regulation of the action of the components of this signaling pathway. Just as many of the constituent proteins of this pathway and their interrelationships were first identified in yeast, the functions of some of these regulators have clearly been conserved in metazoans, and others will likely serve as additional models for molecules that carry out analogous roles in higher organisms.

The cytoplasmic end of transmembrane domain 3 regulates the activity of the Saccharomyces cerevisiae G-protein-coupled alpha-factor receptor

The binding of alpha-factor to its receptor (Ste2p) activates a G-protein-signaling pathway leading to conjugation of MATa cells of the budding yeast S. cerevisiae. We conducted a genetic screen to identify constitutively activating mutations in the N-terminal region of the alpha-factor receptor that includes transmembrane domains 1-5. This approach identified 12 unique constitutively activating mutations, the strongest of which affected polar residues at the cytoplasmic ends of transmembrane domains 2 and 3 (Asn84 and Gln149, respectively) that are conserved in the alpha-factor receptors of divergent yeast species. Targeted mutagenesis, in combination with molecular modeling studies, suggested that Gln149 is oriented toward the core of the transmembrane helix bundle where it may be involved in mediating an interaction with Asn84. These residues appear to play specific roles in maintaining the inactive conformation of the protein since a variety of mutations at either position cause constitutive receptor signaling. Interestingly, the activity of many mammalian G-protein-coupled receptors is also regulated by conserved polar residues (the E/DRY motif) at the cytoplasmic end of transmembrane domain 3. Altogether, the results of this study suggest a conserved role for the cytoplasmic end of transmembrane domain 3 in regulating the activity of divergent G-protein-coupled receptors.

Identification of residues of the Saccharomyces cerevisiae G protein-coupled receptor contributing to alpha-factor pheromone binding

The Saccharomyces cerevisiae pheromone, alpha-factor (WHWLQLKPGQPMY), and Ste2p, its G protein-coupled receptor, were studied as a model for peptide ligand-receptor interaction. The affinities and activities of various synthetic position-10 alpha-factor analogs with Ste2p expressing mutations at residues Ser47 and Thr48 were investigated. All mutant receptors were expressed at a similar level in the cytoplasmic membrane, and their efficacies of signal transduction were similar to that of the wild-type receptor. Mutant receptors differed in binding affinity (Kd) and potency (EC50) for gene induction by alpha-factor. One mutant receptor (S47K,T48K) had dramatically reduced affinity and activity for [Lys10]- and [Orn10]alpha-factor, whereas the affinity for Saccharomyces kluyveri alpha-factor (WHWLSFSKGEPMY) was increased over 20-fold compared with that of wild-type receptor. In contrast, the affinity of [Lys10]- and [Orn10]alpha-factor was increased greatly in a S47E,T48E mutant receptor, whereas the binding of the S. kluyveri alpha-factor was abolished. The affinity of [Lys10]- and [Orn10]alpha-factor for the S47E,T48E receptor dropped 4-6-fold in the presence of 1 m NaCl, whereas the affinity of alpha-factor was not affected by this treatment. These results demonstrate that when bound to its receptor the 10th residue (Gln) of the S. cerevisiae alpha-factor is adjacent to Ser47 and Thr48 residues in the receptor and that the 10th residue of alpha-factors from two Saccharomyces species is responsible for the ligand selectivity to their cognate receptors. Based on these data, we have developed a two-dimensional model of alpha-factor binding to its receptor.

Multiple roles for Rsp5p-dependent ubiquitination at the internalization step of endocytosis

Ubiquitination of integral plasma membrane proteins triggers their rapid internalization into the endocytic pathway. The yeast ubiquitin ligase Rsp5p, a homologue of mammalian Nedd4 and Itch, is required for the ubiquitination and subsequent internalization of multiple plasma membrane proteins, including the alpha-factor receptor (Ste2p). Here we demonstrate that Rsp5p plays multiple roles at the internalization step of endocytosis. Temperature-sensitive rsp5 mutant cells were defective in the internalization of alpha-factor by a Ste2p-ubiquitin chimera, a receptor that does not require post-translational ubiquitination. Similarly, a modified version of Ste2p bearing a NPFXD linear peptide sequence as its only internalization signal was not internalized in rsp5 cells. Internalization of these variant receptors was dependent on the catalytic cysteine residue of Rsp5p and on ubiquitin-conjugating enzymes that bind Rsp5p. Thus, a Rsp5p-dependent ubiquitination event is required for internalization mediated by ubiquitin-dependent and -independent endocytosis signals. Constitutive Ste2p-ubiquitin internalization and fluid-phase endocytosis also required active ubiquitination machinery, including Rsp5p. These observations indicate that Rsp5p-dependent ubiquitination of a trans-acting protein component of the endocytosis machinery is required for the internalization step of endocytosis.

Molecular requirements for the internalisation step of endocytosis: insights from yeast

Molecular genetic studies of endocytosis using the unicellular eukaryote Saccharomyces cerevisiae (budding yeast) have led to the identification of many cellular components, both proteins and lipids, required for this process. While initially, many of these requirements (e.g. for actin, various actin-associated proteins, the ubiquitin conjugation system, and for ergosterol and sphingolipids) appeared to differ from known requirements for endocytosis in higher eukaryotes (e.g. clathrin, AP-2, dynamin), it now seems that endocytosis in higher and lower eukaryotes share many requirements. Often, what were initially identified as actin cytoskeleton-associated proteins in S. cerevisiae, are now revealing themselves as clathrin-coated pit- and vesicle-associated proteins in higher eukaryotes. So rather than delineating two endocytic pathways, one actin-based and one clathrin-based, the combined studies on higher and lower eukaryotes are revealing interesting interplay in both systems between the actin cytoskeleton, clathrin coats, and lipids in the formation of endocytic vesicles at the plasma membrane. Recent results from the yeast system show that the Arp2/3p complex, Wiskott-Aldrich syndrome protein (WASP), and WASP-interacting protein (WIP), proteins involved in the nucleation step of actin filament assembly, play a major role in the formation of endocytic vesicles. This discovery suggests models whereby endocytic vesicles may be actively pushed from the plasma membrane and into the cell by newly forming and rapidly extending actin filaments.

The human papillomavirus type 16 E5 protein alters vacuolar H(+)-ATPase function and stability in Saccharomyces cerevisiae

The human papillomavirus 16 (HPV-16) E5 oncoprotein is a small integral membrane protein that binds to the 16-kDa subunit of the vacuolar H(+)-ATPase (v-ATPase). Conservation within the family of v-ATPases prompted us to look to Saccharomyces cerevisiae as a potential model organism for E5 study. The E5 open reading frame, driven by a galactose-inducible promoter, was integrated into the yeast genome, and the resulting strain demonstrated a nearly complete growth arrest at neutral pH, consistent with defects associated with yeast v-ATPase mutants. Furthermore, this strain demonstrated a severe reduction in pH-dependent and v-ATPase-dependent vacuolar localization of fluorescent markers. Overexpression of the yeast 16-kDa subunit homolog partially suppressed E5-associated growth defects. E5 expression was correlated with a disassociation of the integral (V(o)) and peripheral (V(i)) v-ATPase sub-complexes, as well as a dramatic reduction of the steady-state levels of one mature V(o) subunit and the concomitant accumulation of its major proteolytic fragment, with unchanged levels of two V(i) subunits. Similar analyses of selected E5 mutants in yeast demonstrated a correlation between E5 biology and v-ATPase disruption. Our observations suggest that wild-type HPV-16 E5 acts during the assembly of the v-ATPase to inhibit, either directly or indirectly, V(o) stability and complex formation.

Domains of the Rsp5 ubiquitin-protein ligase required for receptor-mediated and fluid-phase endocytosis

Yeast Rsp5p and its mammalian homologue, Nedd4, are hect domain ubiquitin-protein ligases (E3s) required for the ubiquitin-dependent endocytosis of plasma membrane proteins. Because ubiquitination is sufficient to induce internalization, E3-mediated ubiquitination is a key regulatory event in plasma membrane protein endocytosis. Rsp5p is an essential, multidomain protein containing an amino-terminal C2 domain, three WW protein-protein interaction domains, and a carboxy-terminal hect domain that carries E3 activity. In this study, we demonstrate that Rsp5p is peripherally associated with membranes and provide evidence that Rsp5p functions as part of a multimeric protein complex. We define the function of Rsp5p and its domains in the ubiquitin-dependent internalization of the yeast alpha-factor receptor, Ste2p. Temperature-sensitive rsp5 mutants were unable to ubiquitinate or to internalize Ste2p at the nonpermissive temperature. Deletion of the entire C2 domain had no effect on alpha-factor internalization; however, point mutations in any of the three WW domains impaired both receptor ubiquitination and internalization. These observations indicate that the WW domains play a role in the important regulatory event of selecting phosphorylated proteins as endocytic cargo. In addition, mutations in the C2 and WW1 domains had more severe defects on transport of fluid-phase markers to the vacuole than on receptor internalization, suggesting that Rsp5p functions at multiple steps in the endocytic pathway.

The yeast endocytic membrane transport system

Progress has been made recently in visualizing the structures and organelles responsible for endocytic membrane traffic from the cell surface to the lysosome-like vacuole in Saccharomyces cerevisiae. This, together with the recent discovery of several new membrane trafficking pathways connecting these organelles, has led to a quantum leap in our understanding of the S. cerevisiae endocytic pathway. We now know that although the cortical actin cytoskeleton is required for the internalization step of endocytosis, the internalization event occurs at furrow-like invaginations of the plasma membrane, which are distinct from cortical actin patches. Internalized material is taken into the cell in the form of small (30-50 nm diameter) vesicles and delivered to tubulo-vesicular early endosomes at the cell periphery. Subsequently, the internalized material arrives in multivesicular late endosomes adjacent to the vacuole. Recent microscopy evidence suggests that transfer from late endosomes to the vacuole may involve direct fusion of late endosomes with the vacuole. The visualization of the S. cerevisiae endocytic pathway has revealed similarities to endocytic pathways visualized in higher eukaryotes.