Clathrin facilitates the internalization of seven transmembrane segment receptors for mating pheromones in yeast

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.

Rational Design of Surface-State Controlled Multicolor Cross-Linked Carbon Dots with Distinct Photoluminescence and Cellular Uptake Properties

We disclose for the first time a facile synthetic methodology for the preparation of multicolor carbon dots (CDs) from a single source barring any chromatographic separations. This was achieved via sequential intraparticle cross-linking of surface abundant carboxylic acid groups on the CDs synthesized from a precursor to control their photoluminescence (PL) spectra as well as affect their degree of cellular internalization in cancer cells. The change in PL spectra with sequential cross-linking was projected by theoretical density functional theory (DFT) studies and validated by multiple characterization tools such as X-ray photoelectron spectroscopy (XPS), PL spectroscopy, ninhydrin assay, etc. The variation in cellular internalization of these cross-linked CDs was demonstrated using inhibitor assays, confocal microscopy, and flow cytometry. We supplemented our findings with high-resolution dark-field imaging to visualize and confirm the colocalization of these CDs into distinct intracellular compartments. Finally, to prove the surface-state controlled PL mechanisms of these cross-linked CDs, we fabricated a triple-channel sensor array for the identification of different analytes including metal ions and biologically relevant proteins.

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.

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.

Evolution of a G protein-coupled receptor response by mutations in regulatory network interactions

All cellular functions depend on the concerted action of multiple proteins organized in complex networks. To understand how selection acts on protein networks, we used the yeast mating receptor Ste2, a pheromone-activated G protein-coupled receptor, as a model system. In Saccharomyces cerevisiae, Ste2 is a hub in a network of interactions controlling both signal transduction and signal suppression. Through laboratory evolution, we obtained 21 mutant receptors sensitive to the pheromone of a related yeast species and investigated the molecular mechanisms behind this newfound sensitivity. While some mutants show enhanced binding affinity to the foreign pheromone, others only display weakened interactions with the network's negative regulators. Importantly, the latter changes have a limited impact on overall pathway regulation, despite their considerable effect on sensitivity. Our results demonstrate that a new receptor–ligand pair can evolve through network-altering mutations independently of receptor–ligand binding, and suggest a potential role for such mutations in disease.

Co-evolution of a new receptor-ligand pair will affect the downstream signal transduction network. Here, the authors use experimental evolution of yeast mating receptor Ste2 to show the effect of enhanced binding affinity and weakened interactions with the network's negative regulators on protein evolution.

Clathrin modulates vesicle scission, but not invagination shape, in yeast endocytosis

In a previous paper (Picco et al., 2015), the dynamic architecture of the protein machinery during clathrin-mediated endocytosis was visualized using a new live imaging and particle tracking method. Here, by combining this approach with correlative light and electron microscopy, we address the role of clathrin in this process. During endocytosis, clathrin forms a cage-like coat around the membrane and associated protein components. There is growing evidence that clathrin does not determine the membrane morphology of the invagination but rather modulates the progression of endocytosis. We investigate how the deletion of clathrin heavy chain impairs the dynamics and the morphology of the endocytic membrane in budding yeast. Our results show that clathrin is not required for elongating or shaping the endocytic membrane invagination. Instead, we find that clathrin contributes to the regularity of vesicle scission and thereby to controlling vesicle size.

DOI:http://dx.doi.org/10.7554/eLife.16036.001

Analysis of random PCR-originated mutants of the yeast Ste2 and Ste3 receptors

The G protein-coupled receptors Ste2 and Ste3 bind α- and a-factor, respectively, in Saccharomyces cerevisiae. These receptors share a similar conformation, with seven transmembrane segments, three intracellular loops, a C-terminus tail, and three extracellular loops. However, the amino acid sequences of these two receptors bear no resemblance to each other. Coincidently the two ligands, α- and a-factor, have different sequences. Both receptors activate the same G protein. To identify amino acid residues that are important for signal transduction, the STE2 and STE3 genes were mutagenized by a random PCR-based method. Mutant receptors were analyzed in MATα cells mutated in the ITC1 gene, whose product represses transcription of a-specific genes in MATα. Expression of STE2 or STE3 in these cells results in autocrine activation of the mating pathway, since this strain produces the Ste2 receptor in addition to its specific ligand, α-factor. It also produces a-factor in addition to its specific receptor, Ste3. Therefore, this strain provides a convenient model to analyze mutants of both receptors in the same background. Many hyperactive mutations were found in STE3, whereas none was detected in STE2. This result is consistent with the different strategies that the two genes have adopted to be expressed.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

Specific α-arrestins negatively regulate Saccharomyces cerevisiae pheromone response by down-modulating the G-protein-coupled receptor Ste2

G-protein-coupled receptors (GPCRs) are integral membrane proteins that initiate responses to extracellular stimuli by mediating ligand-dependent activation of cognate heterotrimeric G proteins. In yeast, occupancy of GPCR Ste2 by peptide pheromone α-factor initiates signaling by releasing a stimulatory Gβγ complex (Ste4-Ste18) from its inhibitory Gα subunit (Gpa1). Prolonged pathway stimulation is detrimental, and feedback mechanisms have evolved that act at the receptor level to limit the duration of signaling and stimulate recovery from pheromone-induced G1 arrest, including upregulation of the expression of an α-factor-degrading protease (Bar1), a regulator of G-protein signaling protein (Sst2) that stimulates Gpa1-GTP hydrolysis, and Gpa1 itself. Ste2 is also downregulated by endocytosis, both constitutive and ligand induced. Ste2 internalization requires its phosphorylation and subsequent ubiquitinylation by membrane-localized protein kinases (Yck1 and Yck2) and a ubiquitin ligase (Rsp5). Here, we demonstrate that three different members of the α-arrestin family (Ldb19/Art1, Rod1/Art4, and Rog3/Art7) contribute to Ste2 desensitization and internalization, and they do so by discrete mechanisms. We provide genetic and biochemical evidence that Ldb19 and Rod1 recruit Rsp5 to Ste2 via PPXY motifs in their C-terminal regions; in contrast, the arrestin fold domain at the N terminus of Rog3 is sufficient to promote adaptation. Finally, we show that Rod1 function requires calcineurin-dependent dephosphorylation.

Endocytic accessory factors and regulation of clathrin-mediated endocytosis

Up to 60 different proteins are recruited to the site of clathrin-mediated endocytosis in an ordered sequence. These accessory proteins have roles during all the different stages of clathrin-mediated endocytosis. First, they participate in the initiation of the endocytic event, thereby determining when and where endocytic vesicles are made; later they are involved in the maturation of the clathrin coat, recruitment of specific cargo molecules, bending of the membrane, and finally in scission and uncoating of the nascent vesicle. In addition, many of the accessory components are involved in regulating and coupling the actin cytoskeleton to the endocytic membrane. We will discuss the different accessory components and their various roles. Most of the data comes from studies performed with cultured mammalian cells or yeast cells. The process of endocytosis is well conserved between these different organisms, but there are also many interesting differences that may shed light on the mechanistic principles of endocytosis.

Imaging and modeling the dynamics of clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) plays a central role in cellular homeostasis and is mediated by clathrin-coated pits (CCPs). Live-cell imaging has revealed a remarkable heterogeneity in CCP assembly kinetics, which can be used as an intrinsic source of mechanistic information on CCP regulation but also poses several major problems for unbiased analysis of CME dynamics. The backbone of unveiling the molecular control of CME is an imaging-based inventory of the full diversity of individual CCP behaviors, which requires detection and tracking of structural fiduciaries and regulatory proteins with an accuracy of >99.9%, despite very low signals. This level of confidence can only be achieved by combining appropriate imaging modalities with self-diagnostic computational algorithms for image analysis and data mining.

Clathrin-independent pathways of endocytosis

There are many pathways of endocytosis at the cell surface that apparently operate at the same time. With the advent of new molecular genetic and imaging tools, an understanding of the different ways by which a cell may endocytose cargo is increasing by leaps and bounds. In this review we explore pathways of endocytosis that occur in the absence of clathrin. These are referred to as clathrin-independent endocytosis (CIE). Here we primarily focus on those pathways that function at the small scale in which some have distinct coats (caveolae) and others function in the absence of specific coated intermediates. We follow the trafficking itineraries of the material endocytosed by these pathways and finally discuss the functional roles that these pathways play in cell and tissue physiology. It is likely that these pathways will play key roles in the regulation of plasma membrane area and tension and also control the availability of membrane during cell migration.

Clathrin-mediated endocytosis in budding yeast

Clathrin-mediated endocytosis in the budding yeast Saccharomyces cerevisiae involves the ordered recruitment, activity and disassembly of nearly 60 proteins at distinct sites on the plasma membrane. Two-color live-cell fluorescence microscopy has proven to be invaluable for in vivo analysis of endocytic proteins: identifying new components, determining the order of protein arrival and dissociation, and revealing even very subtle mutant phenotypes. Yeast genetics and functional genomics facilitate identification of complex interaction networks between endocytic proteins and their regulators. Quantitative datasets produced by these various analyses have made theoretical modeling possible. Here, we discuss recent findings on budding yeast endocytosis that have advanced our knowledge of how -60 endocytic proteins are recruited, perform their functions, are regulated by lipid and protein modifications, and are disassembled, all with remarkable regularity.

Characterization of a temperature-sensitive vertebrate clathrin heavy chain mutant as a tool to study clathrin-dependent events in vivo

Clathrin and clathrin-dependent events are evolutionary conserved although it is believed that there are differences in the requirement for clathrin in yeast and higher vertebrates. Clathrin is a long-lived protein and thus, with clathrin knockdowns only long-term consequences of clathrin depletion can be studied. Here, we characterize the first vertebrate temperature-sensitive clathrin heavy chain mutant as a tool to investigate responses to rapid clathrin inactivation in higher eukaryotes. Although we created this mutant using a clathrin cryo-electron microscopy model and a yeast temperature-sensitive mutant as a guide, the resulting temperature-sensitive clathrin showed an altered phenotype compared to the corresponding yeast temperature-sensitive clathrin. First, it seemed to form stable triskelions at the non-permissive temperature although endocytosis was impaired under these conditions. Secondly, as a likely consequence of the stable triskelions at the non-permissive temperature, clathrin also localized correctly to its target membranes. Thirdly, we did not observe missorting of the lysosomal enzyme beta-glucuronidase which could indicate that the temperature-sensitive clathrin is still operating at the non-permissive temperature at the Golgi or, that, like in yeast, more than one TGN trafficking pathway exists. Fourthly, in contrast to yeast, actin does not appear to actively compensate in general endocytosis. Thus, there seem to be differences between vertebrates and yeast which can be studied in further detail with this newly created tool.

Clathrin functions in the absence of the terminal domain binding site for adaptor-associated clathrin-box motifs

Clathrin is involved in vesicle formation in the trans-Golgi network (TGN)/endosomal system and during endocytosis. Clathrin recruitment to membranes is mediated by the clathrin heavy chain (HC) N-terminal domain (TD), which forms a seven-bladed beta-propeller. TD binds membrane-associated adaptors, which have short peptide motifs, either the clathrin-box (CBM) and/or the W-box; however, the importance of the TD binding sites for these motifs has not been tested in vivo. We investigated the importance of the TD in clathrin function by generating 1) mutations in the yeast HC gene (CHC1) to disrupt the binding sites for the CBM and W-box (chc1-box), and 2) four TD-specific temperature-sensitive alleles of CHC1. We found that TD is important for the retention of resident TGN enzymes and endocytosis of alpha-factor; however, the known adaptor binding sites are not necessary, because chc1-box caused little to no effect on trafficking pathways involving clathrin. The Chc1-box TD was able to interact with the endocytic adaptor Ent2 in a CBM-dependent manner, and HCs encoded by chc1-box formed clathrin-coated vesicles. These data suggest that additional or alternative binding sites exist on the TD propeller to help facilitate the recruitment of clathrin to sites of vesicle formation.

Copper transport activity of yeast Ctr1 is down-regulated via its C terminus in response to excess copper

Copper is an essential yet toxic trace element. The Ctr1 family of proteins plays a critical role for copper uptake in eukaryotes. However, the mechanisms of action of Ctr1 are largely unknown. Our previous data demonstrated that copper transport induces conformational changes in the cytosolic C terminus of the yeast Saccharomyces cerevisiae Ctr1. To define the physiological significance of this molecular event and gain better insights into the mechanism of Ctr1-mediated copper uptake, we have characterized the functional roles of the Ctr1 C terminus. A Ctr1 mutant lacking the entire C-terminal cytosolic tail is functional in high affinity copper uptake; however, yeast cells expressing this mutant are extremely sensitive to excess copper. Toxic copper uptake is not attributed to elevated expression or distinct subcellular localization of this mutant as compared with wild type Ctr1. Further characterization of the function of Ctr1 containing deletions or site-directed mutations at the C terminus indicates a structural role for the C terminus in controlling Ctr1 activities. In response to excess copper, Ctr1-mediated copper transport is rapidly blocked in a C terminus-dependent mechanism associated with direct binding of copper. We propose that conformational changes in the cytosolic tail of yeast Ctr1 by copper sensing within this domain lead to the inhibition of Ctr1-mediated copper transport. These data suggest a new regulatory mechanism by which yeast cells maintain homeostatic copper acquisition.

Clathrin is important for normal actin dynamics and progression of Sla2p-containing patches during endocytosis in yeast

Clathrin is a major vesicle coat protein involved in receptor-mediated endocytosis. In yeast and higher eukaryotes, clathrin is recruited to the plasma membrane during the early stage of endocytosis along with clathrin-associated adaptors. As coated pits undergo maturation, a burst of actin polymerization accompanies and helps drive vesicle internalization. Here, we investigate the dynamics of clathrin relative to the early endocytic patch protein Sla2p. We find that clathrin is recruited to the cortex prior to Sla2p. In the absence of clathrin, normal numbers of Sla2p patches form, but many do not internalize or are dramatically delayed in completion of endocytosis. Patches that do internalize receive Sla1p late, which is followed by Abp1, which appears near the end of Sla2p lifetime. In addition, clathrin mutants develop actin comet tails, suggesting an important function in actin patch organization/dynamics. Similar to its mammalian counterparts, the light chain (LC) subunit of yeast clathrin interacts directly with the coiled-coil domain of Sla2p. A mutant of Sla2p that no longer interacts with LC (sla2Delta376-573) results in delayed progression of endocytic patches and aberrant actin dynamics. These data demonstrate an important role for clathrin in organization and progression of early endocytic patches to the late stages of endocytosis.

Composite synthetic lethal identification of membrane traffic inhibitors

Small molecule inhibitors provide powerful tools to characterize highly dynamic and complex eukaryotic cell pathways such as those mediating membrane traffic. However, a lack of easy and generalizable assays has constrained identification of novel inhibitors despite availability of diverse chemical libraries. Here, we report a facile growth-based strategy in yeast to screen for pathway-specific inhibitors. The approach uses well characterized synthetic genetic growth defects to guide design of cells genetically sensitized for inhibition of chosen pathways. With this strategy, we identified a family of piperazinyl phenylethanone compounds as inhibitors of traffic between the trans-Golgi network (TGN) and endosomes that depends on the clathrin adaptor complex AP-1. The compounds did not significantly alter other trafficking pathways involving the TGN or endosomes, indicating specificity. Compound treatment also altered localization of AP-1 in mammalian cells. These previously uncharacterized inhibitors will be useful for future studies of clathrin-mediated transport in yeast, and potentially in other organisms. Furthermore, the easily automated technology should be adaptable for identification of inhibitors of other cellular processes.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

In Vivo Dynamics of Clathrin and Its Adaptor-Dependent Recruitment to the Actin-Based Endocytic Machinery in Yeast

Clathrin-mediated transport is a major pathway for endocytosis. However, in yeast, where cortical actin patches are essential for endocytosis, plasma membrane-associated clathrin has never been observed. Using live cell imaging, we demonstrate cortical clathrin in association with the actin-based endocytic machinery in yeast. Fluorescently tagged clathrin is found in highly mobile internal trans-Golgi/endosomal structures and in smaller cortical patches. Total internal reflection fluorescence microscopy showed that cortical patches are likely endocytic sites, as clathrin is recruited prior to a burst of intensity of the actin patch/endocytic marker, Abp1. Clathrin also accumulates at the cortex with internalizing alpha factor receptor, Ste2p. Cortical clathrin localizes with epsins Ent1/2p and AP180s, and its recruitment to the surface is dependent upon these adaptors. In contrast, Sla2p, End3p, Pan1p, and a dynamic actin cytoskeleton are not required for clathrin assembly or exchange but are required for the mobility, maturation, and/or turnover of clathrin-containing endocytic structures.

Ubiquitin and endocytic internalization in yeast and animal cells

Endocytosis is involved in a wide variety of cellular processes, and the internalization step of endocytosis has been extensively studied in both lower and higher eukaryotic cells. Studies in mammalian cells have described several endocytic pathways, with the main emphasis on clathrin-dependent endocytosis. Genetic studies in yeast have underlined the critical role of actin and actin-binding proteins, lipid modification, and the ubiquitin conjugation system. The combined results of studies of endocytosis in higher and lower eukaryotic cells reveal an interesting interplay in the two systems, including a crucial role for ubiquitin-associated events. The ubiquitylation of yeast cell-surface proteins clearly acts as a signal triggering their internalization. Mammalian cells display variations on the common theme of ubiquitin-linked endocytosis, according to the cell-surface protein considered. Many plasma membrane channels, transporters and receptors undergo cell-surface ubiquitylation, required for the internalization or later endocytic steps of some cell-surface proteins, whereas for others, internalization involves interaction with the ubiquitin conjugation system or with ancillary proteins, which are themselves ubiquitylated. Epsins and Eps15 (or Eps15 homologs), are commonly involved in the process of endocytosis in all eukaryotes, their critical role in this process stemming from their capacity to bind ubiquitin, and to undergo ubiquitylation.

The yeast dynamin-related GTPase Vps1p functions in the organization of the actin cytoskeleton via interaction with Sla1p

Recent studies have suggested that the function of the large GTPase dynamin in endocytosis in mammalian cells may comprise a modulation of actin cytoskeleton. The role of dynamin in actin cytoskeleton organization in the yeast Saccharomyces cerevisiae has remained undefined. In this report, we found that one of the yeast dynamin-related proteins, Vps1p, is required for normal actin cytoskeleton organization. At both permissive and non-permissive temperatures, the vps1 mutants exhibited various degrees of phenotypes commonly associated with actin cytoskeleton defects: depolarized and aggregated actin structures, hypersensitivity to the actin cytoskeleton toxin latrunculin-A, randomized bud site selection and chitin deposition, and impaired efficiency in the internalization of membrane receptors. Over-expression of the GTPase mutants of vps1 also led to actin abnormalities. Consistent with these actin-related defects, Vps1p was found to interact physically, and partially co-localize, with the actin-regulatory protein Sla1p. The normal cellular localization of Sla1p required Vps1p and could be altered by over-expression of a region of Vps1p that was involved in the interaction with Sla1p. The same region also promoted mis-sorting of the vacuolar protein carboxypeptidase Y upon over-expression. These findings suggest that the functions of the dynamin-related protein Vps1p in actin cytoskeleton dynamics and vacuolar protein sorting are probably related to each other.

The Rsp5 Ubiquitin Ligase Binds to and Ubiquitinates Members of the Yeast CIN85-Endophilin Complex, Sla1-Rvs167

Sla1 and Rvs167 are yeast proteins required for receptor internalization and organization of the actin cytoskeleton. Here we provide evidence that Sla1 and Rvs167 are orthologues of the mammalian CIN85 and endophilin proteins, respectively, which are required for ligand-stimulated growth factor receptor internalization. Sla1 is similar in domain structure to CIN85 and binds directly to the endophilin-like Rvs167. Akin to CIN85, Sla1 interacts with synaptojanins and a ubiquitin ligase that regulates endocytosis. This ubiquitin ligase, Rsp5, binds directly to both Sla1 and Rvs167. The interaction between Rsp5 and Rvs167 is mediated through Rsp5 WW domains and PXY motifs in the central Gly-Pro-Ala-rich domain of Rvs167. Rvs167 PXY motifs are required for Rsp5-dependent monoubiquitination of Rvs167 on Lys481 in the Src homology 3 (SH3) domain. Mutation of Lys481 --> Arg causes cells to grow slowly on medium containing 1 M NaCl, although this phenotype is not due to the defect in ubiquitination caused by the K481R mutation. We propose that Rsp5 interaction with Sla1-Rvs167 promotes Rvs167 ubiquitination and regulates activity of this protein complex. Rvs167 ubiquitination is not required for general function of Rvs167, but may control specific Rvs167 SH3 domain-protein interactions or negatively regulate SH3 domain activity.

Actin assembly and endocytosis: from yeast to mammals

Internalization of receptors, lipids, pathogens, and other cargo at the plasma membrane involves several different pathways and requires coordinated interactions between a variety of protein and lipid molecules. The actin cytoskeleton is an integral part of the cell cortex, and there is growing evidence that F-actin plays a direct role in these endocytic events. Genetic studies in yeast have firmly established a functional connection between actin and endocytosis. Identification of several proteins that may function at the interface between actin and the endocytic machinery has provided further evidence for this association in both yeast and mammalian cells. Several of these proteins are directly involved in regulating actin assembly and could thus harness forces produced during actin polymerization to facilitate specific steps in the endocytic process. Recent microscopy studies in mammalian cells provide powerful evidence that localized recruitment and polymerization of actin occurs at endocytic sites. In this review, we focus on progress made in elucidating the functions of the actin cytoskeleton in endocytosis.

The Sla2p Talin Domain Plays a Role in Endocytosis in Saccharomyces cerevisiae

Clathrin-binding adaptors play critical roles for endocytosis in multicellular organisms, but their roles in budding yeast have remained unclear. To address this question, we created a quadruple mutant yeast strain lacking the genes encoding the candidate clathrin adaptors Yap1801p, Yap1802p, and Ent2p and containing a truncated version of Ent1p, Ent1ΔCBMp, missing its clathrin-binding motif. This strain was viable and competent for endocytosis, suggesting the existence of other redundant adaptor-like factors. To identify these factors, we mutagenized the quadruple clathrin adaptor mutant strain and selected cells that were viable in the presence of full-length Ent1p, but inviable with only Ent1ΔCBMp; these strains were named Rcb (requires clathrin binding). One mutant strain, rcb432, contained a mutation in SLA2 that resulted in lower levels of a truncated protein lacking the F-actin binding talin homology domain. Analyses of this sla2 mutant showed that the talin homology domain is required for endocytosis at elevated temperature, that SLA2 exhibits genetic interactions with both ENT1 and ENT2, and that the clathrin adaptors and Sla2p together regulate the actin cytoskeleton and revealed conditions under which Yap1801p and Yap1802p contribute to viability. Together, our data support the view that Sla2p is an adaptor that links actin to clathrin and endocytosis.

Endocytosis and the cytoskeleton

In this review we describe the potential roles of the actin cytoskeleton in receptor-mediated endocytosis in mammalian cells and summarize the efforts of recent years in establishing a relationship between these two cellular functions. With molecules such as dynamin, syndapin, HIP1R, Abp1, synaptojanin, N-WASP, intersectin, and cortactin a set of molecular links is now available and it is likely that their further characterization will reveal the basic principles of a functional interconnection between the membrane cytoskeleton and the vesicle-budding machinery. We will therefore discuss proteins involved in endocytic clathrin coat formation and accessory factors to control and regulate coated vesicle formation but we will also focus on actin cytoskeletal components such as the Arp2/3 complex, spectrin, profilin, and motor proteins involved in actin dynamics and organization. Additionally, we will discuss how phosphoinositides, such as PI(4,5)P2, small GTPases thought to control the actin cytoskeleton, such as Rho, Rac, and Cdc42, or membrane trafficking, such as Rab GTPases and ARF proteins, and different kinases may participate in the functional connection of actin and endocytosis. We will compare the concepts and different molecular mechanisms involved in mammalian cells with yeast as well as with specialized cells, such as epithelial cells and neurons, because different model organisms often offer complementary advantages for further studies in this thriving field of current cell biological research.

Vps10p cycles between the TGN and the late endosome via the plasma membrane in clathrin mutants

Clathrin-coated vesicles mediate the transport of the soluble vacuolar protein CPY from the TGN to the endosomal/prevacuolar compartment. Surprisingly, CPY sorting is not affected in clathrin deletion mutant cells. Here, we have investigated the clathrin-independent pathway that allows CPY transport to the vacuole. We find that CPY transport is mediated by the endosome and requires normal trafficking of its sorting receptor, Vps10p, the steady state distribution of which is not altered in chc1 cells. In contrast, Vps10p accumulates at the cell surface in a chc1/end3 double mutant, suggesting that Vps10p is rerouted to the cell surface in the absence of clathrin. We used a chimeric protein containing the first 50 amino acids of CPY fused to a green fluorescent protein (CPY-GFP) to mimic CPY transport in chc1. In the absence of clathrin, CPY-GFP resides in the lumen of the vacuole as in wild-type cells. However, in chc1/sec6 double mutants, CPY-GFP is present in internal structures, possibly endosomal membranes, that do not colocalize with the vacuole. We propose that Vps10p must be transported to and retrieved from the plasma membrane to mediate CPY sorting to the vacuole in the absence of clathrin-coated vesicles. In this circumstance, precursor CPY may be captured by retrieved Vps10p in an early or late endosome, rather than as it normally is in the trans-Golgi, and delivered to the vacuole by the normal VPS gene-dependent process. Once relieved of cargo protein, Vps10p would be recycled to the trans-Golgi and then to the cell surface for further rounds of sorting.

Scd5p and clathrin function are important for cortical actin organization, endocytosis, and localization of sla2p in yeast

SCD5 was identified as a multicopy suppressor of clathrin HC-deficient yeast. SCD5 is essential, but an scd5-Delta338 mutant, expressing Scd5p with a C-terminal truncation of 338 amino acids, is temperature sensitive for growth. Further studies here demonstrate that scd5-Delta338 affects receptor-mediated and fluid-phase endocytosis and normal actin organization. The scd5-Delta338 mutant contains larger and depolarized cortical actin patches and a prevalence of G-actin bars. scd5-Delta338 also displays synthetic negative genetic interactions with mutations in several other proteins important for cortical actin organization and endocytosis. Moreover, Scd5p colocalizes with cortical actin. Analysis has revealed that clathrin-deficient yeast also have a major defect in cortical actin organization and accumulate G-actin. Overexpression of SCD5 partially suppresses the actin defect of clathrin mutants, whereas combining scd5-Delta338 with a clathrin mutation exacerbates the actin and endocytic phenotypes. Both Scd5p and yeast clathrin physically associate with Sla2p, a homologue of the mammalian huntingtin interacting protein HIP1 and the related HIP1R. Furthermore, Sla2p localization at the cell cortex is dependent on Scd5p and clathrin function. Therefore, Scd5p and clathrin are important for actin organization and endocytosis, and Sla2p may provide a critical link between clathrin and the actin cytoskeleton in yeast, similar to HIP1(R) in animal cells.

Sla1p serves as the targeting signal recognition factor for NPFX(1,2)D-mediated endocytosis

Efficient endocytosis requires cytoplasmic domain targeting signals that specify incorporation of cargo into endocytic vesicles. Adaptor proteins play a central role in cargo collection by linking targeting signals to the endocytic machinery. We have characterized NPFX(1,2) (NPFX[1,2]D) targeting signals and identified the actin-associated protein Sla1p as the adaptor for NPFX(1,2)D-mediated endocytosis in Saccharomyces cerevisiae. 11 amino acids encompassing an NPFX(1,2)D sequence were sufficient to direct uptake of a truncated form of the pheromone receptor Ste2p. In this context, endocytic targeting activity was not sustained by conservative substitutions of the phenylalanine or aspartate. An NPFX1,2D-related sequence was identified in native Ste2p that functions redundantly with ubiquitin-based endocytic signals. A two-hybrid interaction screen for NPFX(1,2)D-interacting proteins yielded SLA1, but no genes encoding Eps15 homology (EH) domains, protein modules known to recognize NPF peptides. Furthermore, EH domains did not recognize an NPFX(1,2)D signal when directly tested by two-hybrid analysis. SLA1 disruption severely inhibited NPFX(1,2)D-mediated endocytosis, but only marginally affected ubiquitin-directed uptake. NPFX(1,2)D-dependent internalization required a conserved domain of Sla1p, SLA1 homology domain, which selectively bound an NPFX(1,2)D-containing fusion protein in vitro. Thus, through a novel NPF-binding domain, Sla1p serves as an endocytic targeting signal adaptor, providing a means to couple cargo with clathrin- and actin-based endocytic machineries.

A subset of yeast vacuolar protein sorting mutants is blocked in one branch of the exocytic pathway

Exocytic vesicles that accumulate in a temperature-sensitive sec6 mutant at a restrictive temperature can be separated into at least two populations with different buoyant densities and unique cargo molecules. Using a sec6 mutant background to isolate vesicles, we have found that vacuolar protein sorting mutants that block an endosome-mediated route to the vacuole, including vps1, pep12, vps4, and a temperature-sensitive clathrin mutant, missort cargo normally transported by dense exocytic vesicles, such as invertase, into light exocytic vesicles, whereas transport of cargo specific to the light exocytic vesicles appears unaffected. Immunoisolation experiments confirm that missorting, rather than a changed property of the normally dense vesicles, is responsible for the altered density gradient fractionation profile. The vps41Delta and apl6Delta mutants, which block transport of only the subset of vacuolar proteins that bypasses endosomes, sort exocytic cargo normally. Furthermore, a vps10Delta sec6 mutant, which lacks the sorting receptor for carboxypeptidase Y (CPY), accumulates both invertase and CPY in dense vesicles. These results suggest that at least one branch of the yeast exocytic pathway transits through endosomes before reaching the cell surface. Consistent with this possibility, we show that immunoisolated clathrin-coated vesicles contain invertase.

Biological basket weaving: formation and function of clathrin-coated vesicles

There has recently been considerable progress in understanding the regulation of clathrin-coated vesicle (CCV) formation and function. These advances are due to the determination of the structure of a number of CCV coat components at molecular resolution and the identification of novel regulatory proteins that control CCV formation in the cell. In addition, pathways of (a) phosphorylation, (b) receptor signaling, and (c) lipid modification that influence CCV formation, as well as the interaction between the cytoskeleton and CCV transport pathways are becoming better defined. It is evident that although clathrin coat assembly drives CCV formation, this fundamental reaction is modified by different regulatory proteins, depending on where CCVs are forming in the cell. This regulatory difference likely reflects the distinct biological roles of CCVs at the plasma membrane and trans-Golgi network, as well as the distinct properties of these membranes themselves. Tissue-specific functions of CCVs require even more-specialized regulation and defects in these pathways can now be correlated with human diseases.

Ordering of compartments in the yeast endocytic pathway

We have characterized the morphology of the yeast endocytic pathway leading from the plasma membrane to the vacuole by following the trafficking of positively charged nanogold in combination with compartment identification using immunolocalization of t-SNARE proteins. The first endocytic compartment, termed the early/recycling endosome, contains the t-SNARE, Tlg1p. The next compartment, the prevacuolar compartment, contains Pep12p. After transport to the prevacuolar compartment, where vacuolar enzymes are seen on their way to the vacuole, endocytic content is delivered to the late endosome and on to the vacuole, both of which are devoid of Pep12p immunolabel. Traffic to the prevacuolar compartment is reduced in strains mutant for the Rab5 homologs, Vps21p, Ypt52p, and Ypt53p and in vps27 mutant cells. On the other hand, traffic to the early recycling endosome is less dependent on Rab5 homologs and does not require Vps27p.

Clathrin Function in Yeast Endocytosis

The process of endocytosis is a complex series of events involving the coordinated activity of many proteins. In animal cells, clathrin plays a vital role in the invagination of the plasma membrane leading to formation of vesicles during endocytosis. The study of endocytosis in yeast cells has been hindered by a debate about the role of clathrin in early internalization steps. This review summarizes the evidence for and against clathrin's involvement in internalization from the yeast plasma membrane.

The human intersectin genes and their spliced variants are differentially expressed

Human intersectins (ITSN1 and ITSN2) are members of a conserved family of proteins involved in clathrin-mediated endocytosis. A short and a long isoform with different protein domain compositions have been described for both human intersectins. Here, we have resolved the exon/intron structure of the ITSN2 gene to explain the genomic origin of its alternatively spliced transcripts. Comparison of the two ITSN human genes shows a high level of conservation in their genomic organization, including the main alternative splicing events. An extensive tissue expression analysis of the two predominant transcripts as well as other minor variants shows that ITSN expression is under tissue and developmental controls. Their differential expression is made more evident when the expression of both intersectins is studied by in situ hybridization in mouse brain.

Differential regulation of the dopamine D2 and D3 receptors by G protein-coupled receptor kinases and beta-arrestins

The D(2) and D(3) receptors (D(2)R and D(3)R), which are potential targets for antipsychotic drugs, have a similar structural architecture and signaling pathway. Furthermore, in some brain regions they are expressed in the same cells, suggesting that differences between the two receptors might lie in other properties such as their regulation. In this study we investigated, using COS-7 and HEK-293 cells, the mechanism underlying the intracellular trafficking of the D(2)R and D(3)R. Activation of D(2)R caused G protein-coupled receptor kinase-dependent receptor phosphorylation, a robust translocation of beta-arrestin to the cell membrane, and profound receptor internalization. The internalization of the D(2)R was dynamin-dependent, suggesting that a clathrin-coated endocytic pathway is involved. In addition, the D(2)R, upon agonist-mediated internalization, localized to intracellular compartments distinct from those utilized by the beta(2)-adrenergic receptor. However, in the case of the D(3)R, only subtle agonist-mediated receptor phosphorylation, beta-arrestin translocation to the plasma membrane, and receptor internalization were observed. Interchange of the second and third intracellular loops of the D(2)R and D(3)R reversed their phenotypes, implicating these regions in the regulatory properties of the two receptors. Our studies thus indicate that functional distinctions between the D(2)R and D(3)R may be found in their desensitization and cellular trafficking properties. The differences in their regulatory properties suggest that they have distinct physiological roles in the brain.

Aux1p/Swa2p is required for cortical endoplasmic reticulum inheritance in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the endoplasmic reticulum (ER) is found at the periphery of the cell and around the nucleus. The segregation of ER through the mother-bud neck may occur by more than one mechanism because perinuclear, but not peripheral ER, requires microtubules for this event. To identify genes whose products are required for cortical ER inheritance, we have used a Tn3-based transposon library to mutagenize cells expressing a green fluorescent protein-tagged ER marker protein (Hmg1p). This approach has revealed that AUX1/SWA2 plays a role in ER inheritance. The COOH terminus of Aux1p/Swa2p contains a J-domain that is highly related to the J-domain of auxilin, which stimulates the uncoating of clathrin-coated vesicles. Deletion of the J-domain of Aux1p/Swa2p leads to vacuole fragmentation and membrane accumulation but does not affect the migration of peripheral ER into daughter cells. These findings suggest that Aux1p/Swa2p may be a bifunctional protein with roles in membrane traffic and cortical ER inheritance. In support of this hypothesis, we find that Aux1p/Swa2p localizes to ER membranes.

Parathyroid hormone receptor internalization is independent of protein kinase A and phospholipase C activation

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) binding to their common receptor stimulates second messenger accumulation, receptor phosphorylation, and internalization. LLC-PK(1) cells expressing a green fluorescent protein-tagged PTH/PTHrP receptor show time- and dose-dependent receptor internalization. The internalized receptors colocalize with clathrin-coated pits. Internalization is stimulated by PTH analogs that bind to and activate the PTH/PTHrP receptor. Cell lines expressing a mutant protein kinase A regulatory subunit that is resistant to cAMP and/or a mutant receptor (DSEL mutant) that does not activate phospholipase C internalize their receptors normally. In addition, internalization of the wild-type receptor and the DSEL mutant is stimulated by the PTH analog [Gly(1),Arg(19)]hPTH-(1-28), which does not stimulate phospholipase C. Forskolin, IBMX, and the active phorbol ester, phorbol-12-myristate-13-acetate, did not promote receptor internalization or increase PTH-induced internalization. These data indicate that ligand-induced internalization of the PTH/PTHrP receptor requires both ligand binding and receptor activation but does not involve stimulation of adenylate cyclase/protein kinase A or phospholipase C/protein kinase C.

Endocytosis and downregulation of G protein-coupled receptors

The number of G protein-coupled receptors (GPCRs) displayed at the cell surface is a critical determinant of physiological responsiveness to native ligands and drugs. Downregulation of the number of adrenergic and dopaminergic receptors present on specific neurons can be induced by receptor agonists or by drugs that increase extracellular concentrations of catecholamines such as dopamine. Thus agonist-induced downregulation of GPCRs is of potentially great importance to the treatment of Parkinson's Disease. However, little is known about biochemical mechanisms that mediate GPCR downregulation. Recent studies of cloned GPCRs have provided exciting insights into specific mechanisms that control endocytosis of receptors from the plasma membrane and regulate proteolytic degradation of receptors. In this review we briefly survey representative studies establishing that multiple mechanisms of GPCR membrane trafficking can function in downregulation function both in neural and non-neural cell types. Then we focus on our present view of mechanisms mediating regulated proteolysis of GPCRs, highlighting recent progress in understanding membrane trafficking of GPCRs from the cell surface to lysosomes. Finally we discuss emerging evidence regarding a specific mechanism that modulates sorting of certain GPCRs between recycling and degradative pathways.

Yeast Gga coat proteins function with clathrin in Golgi to endosome transport

Gga proteins represent a newly recognized, evolutionarily conserved protein family with homology to the "ear" domain of the clathrin adaptor AP-1 gamma subunit. Yeast cells contain two Gga proteins, Gga1p and Gga2p, that have been proposed to act in transport between the trans-Golgi network and endosomes. Here we provide genetic and physical evidence that yeast Gga proteins function in trans-Golgi network clathrin coats. Deletion of Gga2p (gga2Delta), the major Gga protein, accentuates growth and alpha-factor maturation defects in cells carrying a temperature-sensitive allele of the clathrin heavy chain gene. Cells carrying either gga2Delta or a deletion of the AP-1 beta subunit gene (apl2Delta) alone are phenotypically normal, but cells carrying both gga2Delta and apl2Delta are defective in growth, alpha-factor maturation, and transport of carboxypeptidase S to the vacuole. Disruption of both GGA genes and APL2 results in cells so severely compromised in growth that they form only microcolonies. Gga proteins can bind clathrin in vitro and cofractionate with clathrin-coated vesicles. Our results indicate that yeast Gga proteins play an important role in cargo-selective clathrin-mediated protein traffic from the trans-Golgi network to endosomes.

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.

Protein and lipid requirements for endocytosis

Genetic and biochemical studies in yeast and animal cells have led to the identification of many components required for endocytosis. In this review, we summarize our understanding of the endocytic machinery with an emphasis on the proteins regulating the internalization step of endocytosis and endosome fusion. Even though the overall endocytic machinery appears to be conserved between yeast and animals, clear differences exist. We also discuss the roles of phosphoinositides, sterols, and sphingolipid precursors in endocytosis, because in addition to proteins, these lipids have emerged as important determinants in the spatial and most likely temporal specificity of endocytic membrane trafficking events.

Molecular mechanisms underlying endocytosis and sorting of ErbB receptor tyrosine kinases

The major process that regulates the amplitude and kinetics of signal transduction by tyrosine kinase receptors is endocytic removal of active ligand-receptor complexes from the cell surface, and their subsequent sorting to degradation or to recycling. Using the ErbB family of receptor tyrosine kinases we exemplify the diversity of the down regulation process, and concentrate on two sorting steps whose molecular details are emerging. These are the Eps15-mediated sorting to clathrin-coated regions of the plasma membrane and the c-Cbl-mediated targeting of receptors to lysosomal degradation. Like in yeast cells, sorting involves not only protein phosphorylation but also conjugation of ubiquitin molecules. The involvement of other molecules is reviewed and recent observations that challenge the negative regulatory role of endocytosis are described. Finally, we discuss the relevance of receptor down regulation to cancer therapy.

Generation of an isogenic collection of yeast actin mutants and identification of three interrelated phenotypes

A large collection of yeast actin mutations has been previously isolated and used in numerous studies of actin cytoskeletal function. However, the various mutations have been in congenic, rather than isogenic, backgrounds, making it difficult to compare the subtle phenotypes that are characteristic of these mutants. We have therefore placed 27 mutations in an isogenic background. We used a subset of these mutants to compare the degree to which different actin alleles are defective in sporulation, endocytosis, and growth on NaCl-containing media. We found that the three phenotypes are highly correlated. The correlations are specific and not merely a reflection of general growth defects, because the phenotypes are not correlated with growth rates under normal conditions. Significantly, those actin mutants exhibiting the most severe phenotypes in all three processes have altered residues that cluster to a small region of the actin crystal structure previously defined as the fimbrin (Sac6p)-binding site. We examined the relationship between endocytosis and growth on salt and found that shifting wild-type or actin mutant cells to high salt reduces the rate of alpha-factor internalization. These results suggest that actin mutants may be unable to grow on salt because of additive endocytic defects (due to mutation and salt).

WW domains of Rsp5p define different functions: determination of roles in fluid phase and uracil permease endocytosis in Saccharomyces cerevisiae

Rsp5p, ubiquitin-protein ligase, an enzyme of the ubiquitination pathway, contains three WW domains that mediate protein-protein interactions. To determine if these domains adapt Rsp5p to a subset of substrates involved in numerous cellular processes, we generated mutations in individual or combinations of the WW domains. The rsp5-w1, rsp5-w2, and rsp5-w3 mutant alleles complement RSP5 deletions at 30 degrees. Thus, individual WW domains are not essential. Each rsp5-w mutation caused temperature-sensitive growth. Among variants with mutations in multiple WW domains, only rsp5-w1w2 complemented the deletion. Thus, the WW3 domain is sufficient for Rsp5p essential functions. To determine whether rsp5-w mutations affect endocytosis, fluid phase and uracil permease (Fur4p) endocytosis was examined. The WW3 domain is important for both processes. WW2 appears not to be important for fluid phase endocytosis whereas it is important for Fur4p endocytosis. In contrast, the WW1 domain affects fluid phase endocytosis, but it does not appear to function in Fur4p endocytosis. Thus, various WW domains play different roles in the endocytosis of these two substrates. Rsp5p is located in the cytoplasm in a punctate pattern that does not change during the cell cycle. Altering WW domains does not change the location of Rsp5p.

Functional morphology of the secretory pathway organelles in yeast

The glycoprotein secretory pathway of yeast serves mainly for cell surface growth and cell division. It involves a centrifugal transport of transit macromolecules among organelles, whose membranes contain resident proteins needed for driving the transport. These resident membrane proteins return by retrograde vesicular transport. Apart from this, the pathway involves endocytosis. The model yeast Saccharomyces cerevisiae and vertebrate cells were found to contain very similar gene products regulating the molecular mechanism of glycoprotein transport, and the cellular mechanism of their secretion pathways was therefore also presumed to be identical. Biochemists have postulated that, in S. cerevisiae, the translocation of peptides through the endoplasmic reticulum membranes into the lumen of ER cisternae and the core glycosylation is followed by a vector-mediated transport into the functional cascade of the Golgi system cisternae and between them. This is the site of maturation and sorting of glycoproteins, before the ultimate transport by other vectors involving either secretion from the cells (exocytosis across the plasmalemma into the cell wall) or transport into the lysosome-like vacuole via a prevacuolar compartment, which serves at the same time as a primary endosome. The established cellular model of secretion deals with budding yeast; interphase yeast cells, in which the secretion is limited and which predominate in exponential cultures, have not been taken into consideration. The quality of organelle imaging in S. cerevisiae ultra-thin sections depends on the fixation technique used and on specimen contrasting by metals. The results achieved by combinations of different techniques differ mostly in the imaging of bilayers of membrane interfaces and the transparence of the matrix phase. Fixation procedures are decisive for the results of topochemical localisations of cellular antigenic components or enzyme activities, which form the basis of the following survey of functional morphology of organelles involved in the yeast secretory pathway. The existing results of these studies do not confirm all aspects of the vertebrate model of the Golgi apparatus proposed by molecular geneticists to hold for S. cerevisiae, and alternative models of the cellular mechanism of secretion in this yeast are, therefore, also discussed.

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.