The EH-domain-containing protein Pan1 is required for normal organization of the actin cytoskeleton in Saccharomyces cerevisiae

New biomarkers underlying acetic acid tolerance in the probiotic yeast Saccharomyces cerevisiae var. boulardii

Evolutionary engineering experiments, in combination with omics technologies, revealed genetic markers underpinning the molecular mechanisms behind acetic acid stress tolerance in the probiotic yeast Saccharomyces cerevisiae var. boulardii. Here, compared to the ancestral Ent strain, evolved yeast strains could quickly adapt to high acetic acid levels (7 g/L) and displayed a shorter lag phase of growth. Bioinformatic-aided whole-genome sequencing identified genetic changes associated with enhanced strain robustness to acetic acid: a duplicated sequence in the essential endocytotic PAN1 gene, mutations in a cell wall mannoprotein (dan4Thr192del), a lipid and fatty acid transcription factor (oaf1Ser57Pro) and a thiamine biosynthetic enzyme (thi13Thr332Ala). Induction of PAN1 and its associated endocytic complex SLA1 and END3 genes was observed following acetic acid treatment in the evolved-resistant strain when compared to the ancestral strain. Genome-wide transcriptomic analysis of the evolved Ent acid-resistant strain (Ent ev16) also revealed a dramatic rewiring of gene expression among genes associated with cellular transport, metabolism, oxidative stress response, biosynthesis/organization of the cell wall, and cell membrane. Some evolved strains also displayed better growth at high acetic acid concentrations and exhibited adaptive metabolic profiles with altered levels of secreted ethanol (4.0-6.4% decrease), glycerol (31.4-78.5% increase), and acetic acid (53.0-60.3% increase) when compared to the ancestral strain. Overall, duplication/mutations and transcriptional alterations are key mechanisms driving improved acetic acid tolerance in probiotic strains. We successfully used adaptive evolutionary engineering to rapidly and effectively elucidate the molecular mechanisms behind important industrial traits to obtain robust probiotic yeast strains for myriad biotechnological applications. KEY POINTS: •Acetic acid adaptation of evolutionary engineered robust probiotic yeast S. boulardii •Enterol ev16 with altered genetic and transcriptomic profiles survives in up to 7 g/L acetic acid •Improved acetic acid tolerance of S. boulardii ev16 with mutated PAN1, DAN4, OAF1, and THI13 genes.

Cytotoxicity of AuCu-Cu2S Nanocomposites: Implications for Biological Evaluation of the Nanocomposite Effect on Bombyx mori Silkworms and Cell Lines

AuCu-Cu2S nanocomposites are unique materials with exceptional properties that have recently received a lot of interest. However, little is known about their potential toxicity in terrestrial organisms and their subsequent effects on the environment. Therefore, it is essential to develop effective methodologies for evaluating AuCu-Cu2S nanocomposites in biological systems. This study reports the biological evaluation of the AuCu-Cu2S nanocomposite from animal and cell entity levels. The Bombyx mori silkworm was used as a model organism to study the effects of different concentrations of AuCu-Cu2S on silkworm development. Transcriptome analysis was also carried out to examine the genetic modulation exerted by the treatment. Moreover, biocompatibility and cytotoxicity of AuCu-Cu2S were evaluated in human bronchial epithelial cells 16HBE, human lung adenocarcinoma, and the insect Spodoptera frugiperda cell sf9 cell lines. The results showed that although AuCu-Cu2S at ≤400 ppm can prolong the eating habit of silkworms and promote the weight of the cocoon layer, there was an increase in silkworm mortality and a decrease in moth formation at a concentration of ≥800 ppm. The genetic regulation by AuCu-Cu2S treatment showed varying effects in the silkworm, primarily related to functions such as transport and catabolism, metabolism of cofactors and vitamins, xenobiotic biodegradation, amino acid, and carbohydrate. 16HBE, PC-9, and sf9 treated with 300 ppm of AuCu-Cu2S showed viability percentages of 60, 20, and 90%, respectively. Thus, AuCu-Cu2S at low concentrations serves as a safe and biocompatible material for the sf9 cell lines but is lethal to 16HBE and PC-9. This research could aid in understanding the biological effects and biocompatibility of AuCu-Cu2S nanocomposites, particularly in the field of biochemistry; however, the mechanisms involved need further exploration.

The Early Endocytosis Gene PAL1 Contributes to Stress Tolerance and Hyphal Formation in Candida albicans

The endocytic and secretory pathways of the fungal pathogen Candida albicans are fundamental to various key cellular processes such as cell growth, cell wall integrity, protein secretion, hyphal formation, and pathogenesis. Our previous studies focused on several candidate genes involved in early endocytosis, including ENT2 and END3, that play crucial roles in such processes. However, much remains to be discovered about other endocytosis-related genes and their contributions toward Candida albicans secretion and virulence. In this study, we examined the functions of the early endocytosis gene PAL1 using a reverse genetics approach based on CRISPR-Cas9-mediated gene deletion. Saccharomyces cerevisiae Pal1 is a protein in the early coat complex involved in clathrin-mediated endocytosis that is later internalized with the coat. The C. albicans pal1Δ/Δ null mutant demonstrated increased resistance to the antifungal agent caspofungin and the cell wall stressor Congo Red. In contrast, the null mutant was more sensitive to the antifungal drug fluconazole and low concentrations of SDS than the wild type (WT) and the re-integrant (KI). While pal1Δ/Δ can form hyphae and a biofilm, under some hyphal-inducing conditions, it was less able to demonstrate filamentous growth when compared to the WT and KI. The pal1Δ/Δ null mutant had no defect in clathrin-mediated endocytosis, and there were no changes in virulence-related processes compared to controls. Our results suggest that PAL1 has a role in susceptibility to antifungal agents, cell wall integrity, and membrane stability related to early endocytosis.

Candida albicans END3 Mediates Endocytosis and Has Subsequent Roles in Cell Wall Integrity, Morphological Switching, and Tissue Invasion

The role of endocytosis in Candida albicans secretion, filamentation, and virulence remains poorly understood, despite its importance as a fundamental component of intracellular trafficking. Given that secretory mutants display defects in endocytosis, we have focused our attention on endocytic mutants to understand the interconnection between endocytosis and other secretory pathways. Using a reverse-genetic approach based upon CRISPR-Cas9 mediated gene deletion, we studied the functions of the gene END3, which plays a key role in clathrin-based endocytosis. In the end3Δ/Δ null mutant, clathrin-mediated endocytosis was substantially reduced. While in vitro growth, cell morphology, and vacuoles appeared normal, the mutant was impaired in actin patch formation, filamentous growth, biofilm formation, cell wall integrity, and extracellular protease secretion. In addition, susceptibility to various antifungal agents was altered. Consistent with the inability to form hyphae, in an in vitro keratinocyte infection model, the null mutant displayed reduced damage of mammalian adhesion zippers and host cell death. Thus, C. albicans END3 has a role in efficient endocytosis that is required for cell wall integrity, protein secretion, hyphal formation, and virulence-related processes. These findings suggest that impaired endocytosis subsequently affects other secretory pathways, providing evidence of the interconnection between these processes. IMPORTANCE Candida albicans is a fungal commensal organism that can cause serious opportunistic infections in immunocompromised patients leading to substantial complications and mortality. A better understanding of the microbe's biology to develop more effective therapeutic and diagnostic tools is required as invasive candidiasis is a problem of continued clinical importance. This study focuses on endocytosis, an important but incompletely understood cellular mechanism needed to uptake nutrients and communicate with a cell's environment. In this study, we have assessed the role of endocytosis in cell wall integrity, biofilm formation, and tissue invasion in C. albicans. These findings will improve our understanding of cellular mechanisms underlying endocytosis and will inform us of the interconnection with other intracellular transport processes.

Candida albicans ENT2 Contributes to Efficient Endocytosis, Cell Wall Integrity, Filamentation, and Virulence

Epsins play a pivotal role in the formation of endocytic vesicles and potentially provide a linkage between endocytic and other trafficking pathways. We identified a Candida albicans epsin, ENT2, that bears homology to the Saccharomyces cerevisiae early endocytosis genes ENT1 and ENT2 and studied its functions by a reverse genetic approach utilizing CRISPR-Cas9-mediated gene deletion. The C. albicans ent2Δ/Δ null mutant displayed cell wall defects and altered antifungal drug sensitivity. To define the role of C. albicans ENT2 in endocytosis, we performed assays with the lipophilic dye FM4-64 that revealed greatly reduced uptake in the ent2Δ/Δ mutant. Next, we showed that the C. albicans ent2Δ/Δ mutant was unable to form hyphae and biofilms. Assays for virulence properties in an in vitro keratinocyte infection model demonstrated reduced damage of mammalian adhesion zippers and host cell death from the ent2Δ/Δ mutant. We conclude that C. albicans ENT2 has a role in efficient endocytosis, a process that is required for maintaining cell wall integrity, hyphal formation, and virulence-defining traits. IMPORTANCE The opportunistic fungal pathogen Candida albicans is an important cause of invasive infections in hospitalized patients and a source of considerable morbidity and mortality. Despite its clinical importance, we still need to improve our ability to diagnose and treat this common pathogen. In order to support these advancements, a greater understanding of the biology of C. albicans is needed. In these studies, we are focused on the fundamental biological process of endocytosis, of which little is directly known in C. albicans. In addition to studying the function of a key gene in this process, we are examining the role of endocytosis in the virulence-related processes of filamentation, biofilm formation, and tissue invasion. These studies will provide greater insight into the role of endocytosis in causing invasive fungal infections.

Proteomic Identification and Quantification of Snake Venom Biomarkers in Venom and Plasma Extracellular Vesicles

The global exploration of snakebites requires the use of quantitative omics approaches to characterize snake venom as it enters into the systemic circulation. These omics approaches give insights into the venom proteome, but a further exploration is warranted to analyze the venom-reactome for the identification of snake venom biomarkers. The recent discovery of extracellular vesicles (EVs), and their critical cellular functions, has presented them as intriguing sources for biomarker discovery and disease diagnosis. Herein, we purified EV’s from the snake venom (svEVs) of Crotalus atrox and C. oreganus helleri, and from plasma of BALB/c mice injected with venom from each snake using EVtrap in conjunction with quantitative mass spectrometry for the proteomic identification and quantification of svEVs and plasma biomarkers. Snake venom EVs from C. atrox and C. o. helleri were highly enriched in 5′ nucleosidase, L-amino acid oxidase, and metalloproteinases. In mouse plasma EVs, a bioinformatic analysis for revealed upregulated responses involved with cytochrome P450, lipid metabolism, acute phase inflammation immune, and heat shock responses, while downregulated proteins were associated with mitochondrial electron transport, NADH, TCA, cortical cytoskeleton, reticulum stress, and oxidative reduction. Altogether, this analysis will provide direct evidence for svEVs composition and observation of the physiological changes of an envenomated organism.

The Role of Secretory Pathways in Candida albicans Pathogenesis

Candida albicans is a fungus that is a commensal organism and a member of the normal human microbiota. It has the ability to transition into an opportunistic invasive pathogen. Attributes that support pathogenesis include secretion of virulence-associated proteins, hyphal formation, and biofilm formation. These processes are supported by secretion, as defined in the broad context of membrane trafficking. In this review, we examine the role of secretory pathways in Candida virulence, with a focus on the model opportunistic fungal pathogen, Candida albicans.

Live-cell imaging of early coat protein dynamics during clathrin-mediated endocytosis

Clathrin-mediated endocytosis is an essential process that is mediated by the stepwise appearance or disappearance of many different proteins at the plasma membrane. In the budding yeast, these proteins are categorized into at least five modules, according to their spatiotemporal dynamics. Among them, the dynamics of proteins in the late coat module are well characterized, but those in the early coat module still remain unclear because of the lack of a suitable fluorescent marker with sufficient brightness to allow analysis. To examine the dynamics of early coat proteins, in this study we tagged four representative early coat proteins with 3GFP, and expressed them in a single cell. This cell exhibited a significant increase in the fluorescence intensity of early coat proteins relative to that of each 3GFP-tagged protein. Using this strain, we performed a detailed analysis of early coat proteins, including their precise lifetime, changes in fluorescence intensity, and motility on the plasma membrane. We found that early coat proteins move on the plasma membrane before internalization. Additionally, we expressed these 3GFP-tagged proteins in mutants with deletion of genes related to endocytosis, and found four mutants - end3Δ, las17Δ, sla2Δ, and clc1Δ- in which the lifetime of early coat proteins was markedly increased. Interestingly, deletion of the CLC1 gene dramatically reduced the internalization of early coat proteins whereas internalization of actin patches was largely unchanged, suggesting that the clc1Δ mutant might have a defect in the link between the early coat and actin modules.

The Effects of Destruxin A on Relish and Rel Gene Regulation to the Suspected Immune-Related Genes of Silkworm

Destruxin A (DA), a cyclodepsipeptidic mycotoxin of entomopathogenic fungus, Metarhizium anisopliae, has anti-immunity activity against insects, but the mechanism of immune regulation is not clear yet. In our previous experiment, the significant expression changes of Bm_nscaf2838_045, Bm_nscaf2674_066, and Bm_nscaf2767_133 genes in a silkworm’s hemocytes were found, which suggested that these genes might be involved in insect’s innate immunity. In the current experiment, the silkworm cell line Bm12 was used to survey the expression levels of these genes after the cells were treated with DA and the transcription factors BmRel, BmRelish1 and BmRelish2 were silenced by specific siRNA. The results indicated that, after the cells were treated by DA, the gene expression level of BmRelish2 was significantly downregulated, but BmRel and BmRelish1 were not changed. The results also showed that the gene expression levels of Bm_nscaf2838_045 and Bm_nscaf2674_066 had similar phenomena, i.e., downregulation with individual BmRelish1 gene silence or DA treatment, upregulation with combination of BmRelish1 gene silence and DA treatment, upregulation with individual BmRelish2 gene silence, and downregulation with combination of BmRelish2 gene silence plus DA treatment, but no changes in the BmRel gene silence combined with DA treatment. For the Bm_nscaf2767_133 gene, the downregulated expressions were found in individual BmRelish2 gene silence or DA treatment, upregulation in the combination treatment of BmRelish2 gene silence plus DA, and the individual treatment of BmRel or BmRelish1 silence. It is suggested that expressions of the Bm_nscaf2838_045 and Bm_nscaf2674_066 genes are closely related to the Imd signal pathway, but Bm_nscaf2767_133 genes might involve in both Toll and Imd pathways. Furthermore, the BmRelish1 gene acts as an activator and the BmRelish2 gene acts as a repressor for both Bm_nscaf2838_045 and Bm_nscaf2674_066 gene expressions. It also implies that DA may participate in the splicing process of BmRelish where BmRelish2 was promoted. Our research will provide new insights on the understanding of the activity mechanisms of destruxins.

Pan1 regulates transitions between stages of clathrin-mediated endocytosis

The Saccharomyces cerevisiae endocytic protein Pan1 is critical for coat interactions during three transitions of the endocytic pathway. Pan1 depletion arrests endocytosis and causes actin misregulation, leading to actin flares that are connected to the coat but not the membrane. The Pan1 central region is critical for endocytic and essential functions.

Endocytosis is a well-conserved process by which cells invaginate small portions of the plasma membrane to create vesicles containing extracellular and transmembrane cargo proteins. Dozens of proteins and hundreds of specific binding interactions are needed to coordinate and regulate these events. Saccharomyces cerevisiae is a powerful model system with which to study clathrin-mediated endocytosis (CME). Pan1 is believed to be a scaffolding protein due to its interactions with numerous proteins that act throughout the endocytic process. Previous research characterized many Pan1 binding interactions, but due to Pan1's essential nature, the exact mechanisms of Pan1's function in endocytosis have been difficult to define. We created a novel Pan1-degron allele, Pan1-AID, in which Pan1 can be specifically and efficiently degraded in <1 h upon addition of the plant hormone auxin. The loss of Pan1 caused a delay in endocytic progression and weakened connections between the coat/actin machinery and the membrane, leading to arrest in CME. In addition, we determined a critical role for the central region of Pan1 in endocytosis and viability. The regions important for endocytosis and viability can be separated, suggesting that Pan1 may have a distinct role in the cell that is essential for viability.

Molecular analysis of Arp2/3 complex activation in cells

Many forms of cellular motility are driven by the growth of branched networks of actin filaments, which push against a membrane. In the dendritic nucleation model, Arp2/3 complex is critical, binding to the side of an existing mother filament, nucleating a new daughter filament, and thus creating a branch. Spatial and temporal regulation of Arp2/3 activity is critical for efficient generation of force and movement. A diverse collection of Arp2/3 regulatory proteins has been identified. They bind to and/or activate Arp2/3 complex via an acidic motif with a conserved tryptophan residue. We tested this model for Arp2/3 regulator function in vivo, by examining the roles of multiple Arp2/3 regulators in endocytosis in living yeast cells. We measured the molecular composition of the actin network in cells with mutations that removed the acidic motifs of the four Arp2/3 regulators previously shown to influence the proper function of the actin network. Unexpectedly, we did not find a simple or direct correlation between defects in patch assembly and movement and changes in the composition and dynamics of dendritic nucleation proteins. Taken together our data does not support the simple hypothesis that the primary role for Arp2/3 regulators is to recruit and activate Arp2/3. Rather our data suggests that these regulators may be playing more subtle roles in establishing functional networks in vivo.

Secretory carrier membrane proteins

Secretory carrier membrane proteins (SCAMPs) are a family of integral membrane proteins that play roles in mediating exocytosis in animal cells. However, relatively little is known about the subcellular localization, trafficking, and function of SCAMPs in plants. Several recent studies in plant cells indicate that plant SCAMPs share many similarities with their mammalian homologs although there are differences. In this review, we will first summarize and compare animal and plant SCAMPs in terms of their subcellular localization, trafficking, and possible functions. We will then present a phylogenetic analysis of plant and animal SCAMPs. Finally, we will present expression analysis on selective Arabidopsis SCAMPs in the hope of pointing to directions for functional characterization of plant SCAMPs in the future.

Regulation of clathrin coat assembly by Eps15 homology domain-mediated interactions during endocytosis

ETOC: The EH domain is a highly conserved protein–protein interaction domain involved in endocytosis. The EH domains of yeast endocytic proteins, Pan1p, End3p, and Ede1p, have a redundant function and are required for efficient recruitment of several endocytic proteins to sites of endocytosis in order to facilitate clathrin coat assembly.

Clathrin-mediated endocytosis involves a coordinated series of molecular events regulated by interactions among a variety of proteins and lipids through specific domains. One such domain is the Eps15 homology (EH) domain, a highly conserved protein–protein interaction domain present in a number of proteins distributed from yeast to mammals. Several lines of evidence suggest that the yeast EH domain–containing proteins Pan1p, End3p, and Ede1p play important roles during endocytosis. Although genetic and cell-biological studies of these proteins suggested a role for the EH domains in clathrin-mediated endocytosis, it was unclear how they regulate clathrin coat assembly. To explore the role of the EH domain in yeast endocytosis, we mutated those of Pan1p, End3p, or Ede1p, respectively, and examined the effects of single, double, or triple mutation on clathrin coat assembly. We found that mutations of the EH domain caused a defect of cargo internalization and a delay of clathrin coat assembly but had no effect on assembly of the actin patch. We also demonstrated functional redundancy among the EH domains of Pan1p, End3p, and Ede1p for endocytosis. Of interest, the dynamics of several endocytic proteins were differentially affected by various EH domain mutations, suggesting functional diversity of each EH domain.

Symmetry breaking and the establishment of cell polarity in budding yeast

Cell polarity is typically oriented by external cues such as cell-cell contacts, chemoattractants, or morphogen gradients. In the absence of such cues, however, many cells can spontaneously polarize in a random direction, suggesting the existence of an internal polarity-generating mechanism whose direction can be spatially biased by external cues. Spontaneous 'symmetry-breaking' polarization is likely to involve an autocatalytic process set off by small random fluctuations. Here we review recent work on the nature of the autocatalytic process in budding yeast and on the question of why polarized cells only develop a single 'front'.

Investigating topological and functional features of multimodular proteins

To generate functional modules as functionally and structurally cohesive formations in protein interaction networks (PINs) constitutes an important step towards understanding how modules communicate on a higher level of the PIN organisation that underlies cell functionality. However, we need to understand how individual modules communicate and are organized into the higher-order structure(s) of the PIN organization that underlies cell functionality. In an attempt to contribute to this understanding, we make an assumption that the proteins reappearing in several modules, termed here as multimodular proteins (MMPs), may be useful in building higher-order structure(s) as they may constitute communication points between different modules. In this paper, we investigate common properties shared by these proteins and compare them with the properties of so-called single-modular proteins (SMPs) by analyzing three aspects: functional aspect, that is, annotation of the proteins, topological aspect that is betweenness centrality of the proteins, and lethality. Furthermore, we investigate the interconnectivity role of some proteins that are identified as functionally and topologically important.

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

The epsins are a family of adaptors involved in recruiting other endocytic proteins, binding of ubiquitylated cargo and induction of membrane curvature. These molecules bear a characteristic epsin N-terminal homology (ENTH) domain and multiple peptide motifs that mediate protein-protein interactions. We have previously demonstrated that the ENTH domain of epsin is involved in Cdc42 signaling regulation. Here, we present evidence that yeast epsin 2 (Ent2) plays a signaling role during cell division. We observed that overexpression of the ENTH domain of Ent2 (ENTH2), but not Ent1, promoted the formation of chains of cells and aberrant septa. This dominant-negative effect resulted from ENTH2-mediated interference with septin assembly pathways. We mapped the ENTH2 determinants responsible for induction of the phenotype and found them to be important for efficient binding to the septin regulatory protein, Bem3. Supporting a physiological role for epsin 2 in cell division, the protein localized to sites of polarized growth and cytokinesis and rescued a defect in cell division induced by Bem3 misregulation. Collectively, our findings provide a potential molecular mechanism linking endocytosis (via epsin 2) with signaling pathways regulating cell division.

Import of nuclear-encoded mitochondrial proteins: a cotranslational perspective

A growing amount of evidence suggests that the cytosolic translation of nuclear-encoded mitochondrial proteins and their subsequent import into mitochondria are tightly coupled in a process termed cotranslational import. In addition to the original posttranslational view of mitochondrial protein import, early literature also provides both in vitro and in vivo experimental evidence supporting the simultaneous existence of a cotranslational protein-import mechanism in mitochondria. Recent investigations have started to reveal the cotranslational import mechanism which is initiated by transporting either a translation complex or a translationally competent mRNA encoding a mitochondrial protein to the mitochondrial surface. The intracellular localization of mRNA to the mitochondrial surface has emerged as the latest addition to our understanding of mitochondrial biogenesis. It is mediated by targeting elements within the mRNA molecule in association with potential mRNA-binding proteins.

Negative regulation of the actin-regulating kinase Prk1p by patch localization-induced autophosphorylation

The Prk1 family of protein kinases are important regulators of endocytosis and actin cytoskeleton in some eukaryotic cells. In budding yeast, Prk1p phosphorylates numerous endocytic proteins including Pan1p and Sla1p. Prk1p has been observed to undergo autophosphorylation in vivo. In this study, we determined the sites and underlying role of the autophosphorylation. Two sites located in the noncatalytic region were identified to be the autophosphorylation sites. When the sites were mutated, the non-autophosphorylatable Prk1p phosphorylated Pan1p and Sla1p more efficiently than the wild-type kinase, suggesting a negative effect of the autophosphorylation. In addition, the dynamic properties of actin and the coat complex were also altered in the autophosphorylation mutant cells. Interestingly, the autophosphorylation of Prk1p was dependent on cortical localization of the kinase and could be induced by phosphorylated Sla1p. These results suggest that the autophosphorylation of Prk1p may represent a feedback mechanism possibly involved in fine-tuning the pace of progression during actin-coupled endocytosis.

Characterization of nuclear localization and nuclear export signals of yeast actin-binding protein Pan1

Pan1 is an actin patch-associated protein involved in endocytosis. Our studies revealed that in oleate-grown cells Pan1 is located in the nucleus as well as in patches. One of three putative nuclear localization signals (NLS) of Pan1, NLS2, directed beta-galactosidase (beta-gal) to the nucleus. However, GFP-Pan1(886-1219), containing NLS2, was found in the cytoplasm indicating that it may contain a nuclear export signal (NES). A putative Pan1 NES, overlapping with NLS3, re-addressed NLS(H2B)-NES/NLS3-beta-gal from the nucleus to the cytoplasm. Inactivation of the NES allowed NLS3 to be effective. Thus, Pan1 contains functional NLSs and a NES and appears to shuttle in certain circumstances.

Scd5p mediates phosphoregulation of actin and endocytosis by the type 1 phosphatase Glc7p in yeast

Pan1p plays essential roles in both actin and endocytosis in yeast. It interacts with, and regulates the function of, multiple endocytic proteins and actin assembly machinery. Phosphorylation of Pan1p by the kinase Prk1p down-regulates its activity, resulting in disassembly of the endocytic vesicle coat complex and termination of vesicle-associated actin polymerization. In this study, we focus on the mechanism that acts to release Pan1p from phosphorylation inhibition. We show that Pan1p is dephosphorylated by the phosphatase Glc7p, and the dephosphorylation is dependent on the Glc7p-targeting protein Scd5p, which itself is a phosphorylation target of Prk1p. Scd5p links Glc7p to Pan1p in two ways: directly by interacting with Pan1p and indirectly by interacting with the Pan1p-binding protein End3p. Depletion of Glc7p from the cells causes defects in cell growth, actin organization, and endocytosis, all of which can be partially suppressed by deletion of the PRK1 gene. These results suggest that Glc7p antagonizes the activity of the Prk1p kinase in regulating the functions of Pan1p and possibly other actin- and endocytosis-related proteins.

Eps15 and Dap160 control synaptic vesicle membrane retrieval and synapse development

Epidermal growth factor receptor pathway substrate clone 15 (Eps15) is a protein implicated in endocytosis, endosomal protein sorting, and cytoskeletal organization. Its role is, however, still unclear, because of reasons including limitations of dominant-negative experiments and apparent redundancy with other endocytic proteins. We generated Drosophila eps15-null mutants and show that Eps15 is required for proper synaptic bouton development and normal levels of synaptic vesicle (SV) endocytosis. Consistent with a role in SV endocytosis, Eps15 moves from the center of synaptic boutons to the periphery in response to synaptic activity. The endocytic protein, Dap160/intersectin, is a major binding partner of Eps15, and eps15 mutants phenotypically resemble dap160 mutants. Analyses of eps15 dap160 double mutants suggest that Eps15 functions in concert with Dap160 during SV endocytosis. Based on these data, we hypothesize that Eps15 and Dap160 promote the efficiency of endocytosis from the plasma membrane by maintaining high concentrations of multiple endocytic proteins, including dynamin, at synapses.

A novel pathway that coordinates mitotic exit with spindle position

In budding yeast, the spindle position checkpoint (SPC) delays mitotic exit until the mitotic spindle moves into the neck between the mother and bud. This checkpoint works by inhibiting the mitotic exit network (MEN), a signaling cascade initiated and controlled by Tem1, a small GTPase. Tem1 is regulated by a putative guanine exchange factor, Lte1, but the function and regulation of Lte1 remains poorly understood. Here, we identify novel components of the checkpoint that operate upstream of Lte1. We present genetic evidence in agreement with existing biochemical evidence for the molecular mechanism of a pathway that links microtubule-cortex interactions with Lte1 and mitotic exit. Each component of this pathway is required for the spindle position checkpoint to delay mitotic exit until the spindle is positioned correctly.

Localization of Gts1p in cortical actin patches of yeast and its possible role in endocytosis

Herein we report that Gts1p fused with green-fluorescent protein (GFP) is localized in the cortical actin patch besides nuclei in yeast and the cortical Gts1p changed its position together with the patch depending on the cell-cycle phase, while nuclear Gts1p accumulated predominantly in the budding phase. Whereas Gts1p does not directly bind to actin, it associated mainly with the actin-associated protein Pan1p. In the GTS1-deleted transformant gts1Delta, the number of cells containing either a fragmented vacuole or an enlarged single central vacuole increased and the uptake of the hydrophilic dye Lucifer yellow (LY) in the vacuole decreased. Further, gts1Delta transformed with a mutant Gts1p having two cysteine-to-alanine substitutions in a zinc finger resembling that of GTPase-activating proteins of ADP-ribosylation factors (ARF-GAP) neither recovered the LY uptake unlike gts1Delta transformed with the wild-type GTS1, nor reduced the average size of central vacuoles as much as the latter did. These results suggested that Gts1p in the actin patch is involved in the fluid-phase endocytosis and membrane trafficking for vacuole formation and that the putative ARF-GAP domain in Gts1p plays an important role in these functions.

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.

Functional analysis ofCandida albicans genes whoseSaccharomyces cerevisiae homologues are involved in endocytosis

PCR-based techniques for directed gene alterations have become standard tools in Candida albicans. To help to increase the speed of functional analysis of Candida albicans genes, we previously constructed and updated a modular set of pFA-plasmid vectors for PCR-based gene targeting in C. albicans. Here we report the functional analyses of C. albicans ORFs whose homologues in S. cerevisiae are involved in endocytosis, to explore their potential involvement in polarized cell growth. Three C. albicans genes, ABP1, BZZ1 and EDE1, were found to be non-essential. Yeast and hyphal morphogenesis were not affected by the individual deletions and the mutant strains appeared wild-type-like under the different growth conditions tested. On the other hand, deletion of both alleles of the C. albicans PAN1 homologue was not feasible. Promoter shut-down experiments using a MET3p-PAN1/pan1 strain indicated severe growth defects and abolished endocytosis, indicating that PAN1 is an essential gene. Subcellular distribution of CaAbp1 and CaPan1 was analysed via GFP-tagged proteins. Both proteins were found to localize at the cortex and at hyphal tips in a patch-like manner, supporting their role in endocytosis. Localization patterns of Abp1 and Pan1, however, were distinct from that of the FM4-64 stained Spitzenkörper.

Negative regulation of yeast Eps15-like Arp2/3 complex activator, Pan1p, by the Hip1R-related protein, Sla2p, during endocytosis

Control of actin assembly nucleated by the Arp2/3 complex plays a crucial role during budding yeast endocytosis. The yeast Eps15-related Arp2/3 complex activator, Pan1p, is essential for endocytic internalization and proper actin organization. Pan1p activity is negatively regulated by Prk1 kinase phosphorylation after endocytic internalization. Phosphorylated Pan1p is probably then dephosphorylated in the cytosol. Pan1p is recruited to endocytic sites approximately 25 s before initiation of actin polymerization, suggesting that its Arp2/3 complex activation activity is kept inactive during early stages of endocytosis by a yet-to-be-identified mechanism. However, how Pan1p is maintained in an inactive state is not clear. Using tandem affinity purification-tagged Pan1p, we identified End3p as a stoichiometric component of the Pan1p complex, and Sla2p, a yeast Hip1R-related protein, as a novel binding partner of Pan1p. Interestingly, Sla2p specifically inhibited Pan1p Arp2/3 complex activation activity in vitro. The coiled-coil region of Sla2p was important for Pan1p inhibition, and a pan1 partial loss-of-function mutant suppressed the temperature sensitivity, endocytic phenotypes, and actin phenotypes observed in sla2DeltaCC mutant cells that lack the coiled-coil region. Overall, our results establish that Sla2p's regulation of Pan1p plays an important role in controlling Pan1p-stimulated actin polymerization during endocytosis.

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.

Pan1p, an actin cytoskeleton-associated protein, is required for growth of yeast on oleate medium

Pan1p is a yeast actin cytoskeleton-associated protein localized in actin patches. It activates the Arp2/3 complex, which is necessary for actin polymerization and endocytosis. We isolated the pan1-11 yeast mutant unable to grow on oleate as a sole carbon source and, therefore, exhibiting the Oleate- phenotype. In addition, mutant cells are temperature-sensitive and grow more slowly on glycerol or succinate-containing medium but similarly to the wild type on ethanol, pyruvate or acetate-containing media; this indicates proper functioning of the mitochondrial respiratory chain. However, growth on ethanol medium is compromised when oleic acid is present. Cells show growth arrest in the apical growth phase, and accumulation of cells with abnormally elongated buds is observed. The growth defects of pan1-11 are suppressed by overexpression of the END3 gene encoding a protein that binds Pan1p. The morphology of peroxisomes and induction of peroxisomal enzymes are normal in pan1-11, indicating that the defect in growth on oleate medium does not result from impairment in peroxisome function. The pan1-11 allele has a deletion of a fragment encoding amino acids 1109-1126 that are part of (QPTQPV)7 repeats. Surprisingly, the independently isolated pan1-9 mutant, which expresses a truncated form of Pan1p comprising aa 1-859, is able to grow on all media tested. Our results indicate that Pan1p, and possibly other components of the actin cytoskeleton, are necessary to properly regulate growth of dividing cells in response to the presence of some alternative carbon sources in the medium.

The function of the endocytic scaffold protein Pan1p depends on multiple domains

Pan1p is an essential protein of the yeast Saccharomyces cerevisiae that is required for the internalization step of endocytosis and organization of the actin cytoskeleton. Pan1p, which binds several other endocytic proteins, is composed of multiple protein-protein interaction domains including two Eps15 Homology (EH) domains, a coiled-coil domain, an acidic Arp2/3-activating region, and a proline-rich domain. In this study, we have induced high-level expression of various domains of Pan1p in wild-type cells to assess the dominant consequences on viability, endocytosis, and actin organization. We found that the most severe phenotypes, with blocked endocytosis and aggregated actin, required expression of nearly full length Pan1p, and also required the endocytic regulatory protein kinase Prk1p. The central coiled-coil domain was the smallest fragment whose overexpression caused any dominant effects; these effects were more pronounced by inclusion of the second EH domain. Co-overexpressing nonoverlapping amino- and carboxy-terminal fragments did not mimic the effects of the intact protein, whereas fragments that overlapped within the coiled-coil region could. Yeast two-hybrid and in vivo coimmunoprecipitation analyses suggest that Pan1 may form dimers or higher order oligomers. Collectively, our data support a view of Pan1p as a dimeric/oligomeric scaffold whose functions require both the amino- and carboxy-termini, linked by the central region.

Negative Regulation of Phosphatidylinositol 4,5-Bisphosphate Levels by the INP51-associated Proteins TAX4 and IRS4

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) is an important second messenger in signaling pathways in organisms ranging from yeast to mammals, but the regulation of PI(4,5)P(2) levels remains unclear. Here we present evidence that PI(4,5)P(2) levels in Saccharomyces cerevisiae are down-regulated by the homologous and functionally redundant proteins TAX4 and IRS4. The EPS15 homology domain-containing proteins TAX4 and IRS4 bind and activate the PI(4,5)P 5-phosphatase INP51 via an Asn-Pro-Phe motif in INP51. Furthermore, the INP51-TAX4/IRS4 complex negatively regulates the cell integrity pathway. Thus, TAX4 and IRS4 are novel regulators of PI(4,5)P(2) and PI(4,5)P(2)-dependent signaling. The interaction between TAX4/IRS4 and INP51 is analogous to the association of EPS15 with the 5-phosphatase synaptojanin 1 in mammalian cells, suggesting that EPS15 is an activator of synaptojanin 1.

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.

Dap160/Intersectin Acts as a Stabilizing Scaffold Required for Synaptic Development and Vesicle Endocytosis

We describe the isolation of mutations in dynamin-associated protein 160 kDa (dap160), the Drosophila homolog of intersectin, a putative adaptor for proteins involved in endocytosis, cytoskeletal regulation, and signaling. We show that partial loss-of-function mutants display temperature-sensitive (ts) paralysis, whereas null mutants show ts defects in endocytosis. Loss-of-function mutants exhibit bouton overgrowth at larval neuromuscular junctions (NMJs), but evoked neurotransmission is normal. Mutant NMJs show a mild endocytic defect at 22 degrees C, which is strongly enhanced at 34 degrees C. The levels of dynamin, synaptojanin and endophilin are severely reduced in dap160 mutant NMJs, suggesting that Dap160 serves to stabilize an endocytic macromolecular complex. Electron microscopy reveals fewer vesicles, aberrant large vesicles, and an accumulation of endocytic intermediates at active and periactive zones in mutant terminals. Our data suggest that Dap160, like dynamin, is involved in synaptic vesicle retrieval at active and periactive zones.

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.

Identification of novel recognition motifs and regulatory targets for the yeast actin-regulating kinase Prk1p

Prk1p is a serine/threonine kinase involved in the regulation of the actin cytoskeleton organization in the yeast Saccharomyces cerevisiae. Previously, we have identified LxxQxTG as the phosphorylation site of Prk1p. In this report, the recognition sequence for Prk1p is investigated more thoroughly. It is found that the presence of a hydrophobic residue at the position of P-5 is necessary for Prk1p phosphorylation and L, I, V, and M are all able to confer the phosphorylation at various efficiencies. The residue flexibility at P-2 has also been identified to include Q, N, T, and S. A homology-based three-dimensional model of the kinase domain of Prk1p provided some structural interpretations for these substrate specificities. The characterization of the [L/I/V/M]xx[Q/N/T/S]xTG motif led to the identification of a spectrum of potential targets for Prk1p from yeast genome. One of them, Scd5p, which contains three LxxTxTG motifs and is previously known to be important for endocytosis and actin organization, has been chosen to demonstrate its relationship with Prk1p. Phosphorylation of Scd5p by Prk1p at the three LxxTxTG motifs could be detected in vitro and in vivo, and deletion of PRK1 suppressed the defects in actin cytoskeleton and endocytosis in one of the scd5 mutants. These results allowed us to conclude that Scd5p is likely another regulatory target of Prk1p.

Regulation of the yeast amphiphysin homologue Rvs167p by phosphorylation

The yeast amphiphysin homologue Rvs167p plays a role in regulation of the actin cytoskeleton, endocytosis, and sporulation. Rvs167p is a phosphoprotein in vegetatively growing cells and shows increased phosphorylation upon treatment with mating pheromone. Previous work has shown that Rvs167p can be phosphorylated in vitro by the cyclin-dependent kinase Pho85p complexed with its cyclin Pcl2p. Using chymotryptic phosphopeptide mapping, we have identified the sites on which Rvs167p is phosphorylated in vitro by Pcl2p-Pho85p. We have shown that these same sites are phosphorylated in vivo during vegetative growth and that phosphorylation at two of these sites is Pcl-Pho85p dependent. In cells treated with mating pheromone, the MAP kinase Fus3p is needed for full phosphorylation of Rvs167p. Functional genomics and genetics experiments revealed that mutation of other actin cytoskeleton genes compromises growth of a strain in which phosphorylation of Rvs167p is blocked by mutation. Phosphorylation of Rvs167p inhibits its interaction in vitro with Las17p, an activator of the Arp2/3 complex, as well as with a novel protein, Ymr192p. Our results suggest that phosphorylation of Rvs167p by a cyclin-dependent kinase and by a MAP kinase is an important mechanism for regulating protein complexes involved in actin cytoskeleton function.

The Ark1/Prk1 family of protein kinases. Regulators of endocytosis and the actin skeleton

The Ark/Prk serine/threonine kinases initiate phosphorylation cycles that control the endocytic machinery in mammalian cells and in yeast, and the actin cytoskeleton in yeast. The members of this protein family are unified by homologies in their kinase domain, but are generally diverse in their other domains. The evolution of Ark/Prk family members in different organisms may have allowed the conserved role of the kinase domain, which is required for the phosphorylation of both endocytic and cytoskeletal components, to be coupled to other functional domains.

Rsp5p, a new link between the actin cytoskeleton and endocytosis in the yeast Saccharomyces cerevisiae

Rsp5p is an ubiquitin-protein ligase of Saccharomyces cerevisiae that has been implicated in numerous processes including transcription, mitochondrial inheritance, and endocytosis. Rsp5p functions at multiple steps of endocytosis, including ubiquitination of substrates and other undefined steps. We propose that one of the roles of Rsp5p in endocytosis involves maintenance and remodeling of the actin cytoskeleton. We report the following. (i) There are genetic interactions between rsp5 and several mutant genes encoding actin cytoskeletal proteins. rsp5 arp2, rsp5 end3, and rsp5 sla2 double mutants all show synthetic growth defects. Overexpressed wild-type RSP5 or mutant rsp5 genes with lesions of some WW domains suppress growth defects of arp2 and end3 cells. The defects in endocytosis, actin cytoskeleton, and morphology of arp2 are also suppressed. (ii) Rsp5p and Sla2p colocalize in abnormal F-actin-containing clumps in arp2 and pan1 mutants. Immunoprecipitation experiments confirmed that Rsp5p and Act1p colocalize in pan1 mutants. (iii) Rsp5p and Sla2p coimmunoprecipitate and partially colocalize to punctate structures in wild-type cells. These studies provide the first evidence for an interaction of an actin cytoskeleton protein with Rsp5p. (iv) rsp5-w1 mutants are resistant to latrunculin A, a drug that sequesters actin monomers and depolymerizes actin filaments, consistent with the fact that Rsp5p is involved in actin cytoskeleton dynamics.

Mot3 is a transcriptional repressor of ergosterol biosynthetic genes and is required for normal vacuolar function in Saccharomyces cerevisiae

The Saccharomyces cerevisiae MOT3 gene encodes a nuclear protein implicated in both repression and activation of transcription. However, a mot3 Delta mutation causes only mild phenotypes under normal growth conditions. To learn more about Mot3 function, we have performed a synthetic lethal screen. This screen identified PAN1, a gene required for normal endocytosis, and VPS41, a gene required for vacuolar fusion and protein targeting, suggesting a role for Mot3 in the regulation of membrane-related genes. Transcriptional analyses show that Mot3 represses transcription of ERG2, ERG6 and ERG9, genes required for ergosterol biosynthesis, during both aerobic and hypoxic growth. Chromatin immunoprecipitation experiments suggest that this repression is direct. Ergosterol has been shown to be required for endocytosis and homotypic vacuole fusion, providing a link between Mot3 and these processes. Consistent with these results, mot3 Delta mutants have a number of related defects, including impaired homotypic vacuole fusion and increased sterol levels. Taken together, our data suggest that proper transcriptional regulation of ergosterol biosynthetic genes by Mot3 is important for normal vacuolar function and probably for the endocytic membrane transport system.

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 couples the yeast endocytic machinery to proteins regulating actin dynamics

Sla1p is a protein required for cortical actin patch structure and organisation in budding yeast. Here we use a combination of immunofluorescence microscopy and biochemical approaches to demonstrate interactions of Sla1p both with proteins regulating actin dynamics and with proteins required for endocytosis. Using Sla1p-binding studies we reveal association of Sla1p with two proteins known to be important for activation of the Arp2/3 complex in yeast, Abp1p and the yeast WASP homologue Las17p/Bee1p. A recent report of Sla1p association with Pan1p puts Sla1p in the currently unique position of being the only yeast protein known to interact with all three known Arp2/3-activating proteins in yeast. Localisation of Sla1p at the cell cortex is, however, dependent on the EH-domain-containing protein End3p, which is part of the yeast endocytic machinery. Using spectral variants of GFP on Sla1p(YFP) and on Abp1p (CFP) we show for the first time that these proteins can exist in discrete complexes at the cell cortex. However, the detection of a significant FRET signal means that these proteins also come close together in a single complex, and it is in this larger complex that we propose that Sla1p binding to Abp1p and Las17p/Bee1p is able to link actin dynamics to the endocytic machinery. Finally, we demonstrate marked defects in both fluid-phase and receptor-mediated endocytosis in cells that do not express SLA1, indicating that Sla1p is central to the requirement in yeast to couple endocytosis with the actin cytoskeleton.

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.

Regulation of yeast actin cytoskeleton-regulatory complex Pan1p/Sla1p/End3p by serine/threonine kinase Prk1p

The serine/threonine kinase Prk1p is known to be involved in the regulation of the actin cytoskeleton organization in budding yeast. One possible function of Prk1p is the negative regulation of Pan1p, an actin patch regulatory protein that forms a complex in vivo with at least two other proteins, Sla1p and End3p. In this report, we identified Sla1p as another substrate for Prk1p. The phosphorylation of Sla1p by Prk1p was established in vitro with the use of immunoprecipitated Prk1p and in vivo with the use of PRK1 overexpression, and was further supported by the finding that immunoprecipitated Sla1p contained PRK1- and ARK1-dependent kinase activities. Stable complex formation between Prk1p and Sla1p/Pan1p in vivo could be observed once the phosphorylation reaction was blocked by mutation in the catalytic site of Prk1p. Elevation of Prk1p activities in wild-type cells resulted in a number of deficiencies, including those in colocalization of Pan1p and Sla1p, endocytosis, and cell wall morphogenesis, likely attributable to a disintegration of the Pan1p/Sla1p/End3p complex. These results lend a strong support to the model that the phosphorylation of the Pan1p/Sla1p/End3p complex by Prk1p is one of the important mechanisms by which the organization and functions of the actin cytoskeleton are regulated.

In vivo role for actin-regulating kinases in endocytosis and yeast epsin phosphorylation

The yeast actin-regulating kinases Ark1p and Prk1p are signaling proteins localized to cortical actin patches, which may be sites of endocytosis. Interactions between the endocytic proteins Pan1p and End3p may be regulated by Prk1p-dependent threonine phosphorylation of Pan1p within the consensus sequence [L/I]xxQxTG. We identified two Prk1p phosphorylation sites within the Pan1p-binding protein Ent1p, a yeast epsin homologue, and demonstrate Prk1p-dependent phosphorylation of both threonines. Converting both threonines to either glutamate or alanine mimics constitutively phosphorylated or dephosphorylated Ent1p, respectively. Synthetic growth defects were observed in a pan1-20 ENT1(EE) double mutant, suggesting that Ent1p phosphorylation negatively regulates the formation/activity of a Pan1p-Ent1p complex. Interestingly, pan1-20 ent2 Delta but not pan1-20 ent1 Delta double mutants had improved growth and endocytosis over the pan1-20 mutant. We found that actin-regulating Ser/Thr kinase (ARK) mutants exhibit endocytic defects and that overexpressing either wild-type or alanine-substituted Ent1p partially suppressed phenotypes associated with loss of ARK kinases, including growth, endocytosis, and actin localization defects. Consistent with synthetic growth defects of pan1-20 ENT1(EE) cells, overexpressing glutamate-substituted Ent1p was deleterious to ARK mutants. Surprisingly, overexpressing the related Ent2p protein could not suppress ARK kinase mutant phenotypes. These results suggest that Ent1p and Ent2p are not completely redundant and may perform opposing functions in endocytosis. These data support the model that, as for clathrin-dependent recycling of synaptic vesicles, yeast endocytic protein phosphorylation inhibits endocytic functions.

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.

A novel EH domain protein of Saccharomyces cerevisiae, Ede1p, involved in endocytosis

Sequencing of the entire genome of S. cerevisiae has revealed the existence of five proteins containing EH domains. These are protein-protein interaction modules first described in mammalian Eps15, a protein that is involved in clathrin-dependent endocytosis. Two of the yeast proteins, End3p and Pan1p, are required for the internalization step of endocytosis. We report characterization of the nonessential ORF YBL047c which, like Eps15, encodes a protein with three N-terminal EH domains. Deletion of YBL047c leads to a defective fluid-phase endocytosis and to defective internalization of the pheromone (alpha)-factor and uracil permease. We therefore named YBL047c EDE1, for EH Domains and Endocytosis. Ede1p expressed as a chromosomally encoded fusion to the green fluorescent protein is localized in punctate cortical spots that only partially colocalize with actin patches. This localization is maintained when actin is depolymerized. Deletion of EDE1 impairs the diploid budding pattern, but has only a small impact on actin cytoskeleton organization, in contrast to the effects observed in pan1 cells and many end mutants impaired in proteins colocalizing with cortical actin patches. Genetic interaction was observed between EDE1 and RSP5, which encodes the ubiquitin ligase Rsp5p essential for ubiquitin-dependent endocytosis of many plasma membrane proteins, thus further emphasizing the functional link between Rsp5p and the EH domain proteins. We also observed genetic interaction between EDE1, and END3 or PAN1, suggesting that Ede1p might be part of a yeast EH network implicated in endocytosis.

Pan1p, End3p, and S1a1p, three yeast proteins required for normal cortical actin cytoskeleton organization, associate with each other and play essential roles in cell wall morphogenesis

The EH domain proteins Pan1p and End3p of budding yeast have been known to form a complex in vivo and play important roles in organization of the actin cytoskeleton and endocytosis. In this report, we describe new findings concerning the function of the Pan1p-End3p complex. First, we found that the Pan1p-End3p complex associates with Sla1p, another protein known to be required for the assembly of cortical actin structures. Sla1p interacts with the first long repeat region of Pan1p and the N-terminal EH domain of End3p, thus leaving the Pan1p-End3p interaction, which requires the second long repeat of Pan1p and the C-terminal repeat region of End3p, undisturbed. Second, Pan1p, End3p, and Sla1p are also required for normal cell wall morphogenesis. Each of the Pan1-4, sla1Delta, and end3Delta mutants displays the abnormal cell wall morphology previously reported for the act1-1 mutant. These cell wall defects are also exhibited by wild-type cells overproducing the C-terminal region of Sla1p that is responsible for interactions with Pan1p and End3p. These results indicate that the functions of Pan1p, End3p, and Sla1p in cell wall morphogenesis may depend on the formation of a heterotrimeric complex. Interestingly, the cell wall abnormalities exhibited by these cells are independent of the actin cytoskeleton organization on the cell cortex, as they manifest despite the presence of apparently normal cortical actin cytoskeleton. Examination of several act1 mutants also supports this conclusion. These observations suggest that the Pan1p-End3p-Sla1p complex is required not only for normal actin cytoskeleton organization but also for normal cell wall morphogenesis in yeast.

Role of actin in regulated exocytosis and compensatory membrane retrieval: insights from an old acquaintance

This review summarizes new insights into the role of the actin cytoskeleton in exocytosis and compensatory membrane retrieval from mammalian regulated secretory cells. Data from our lab and others now indicate that the actin cytoskeleton is involved in exocytosis both as a negative regulator of membrane fusion under resting conditions and as a facilitator of movement of secretory granules to their site of fusion with the apical plasmalemma. Coating of docked secretory granules with actin filaments correlates with the dissociation of secretory-granule-associated rab3D, pointing out a novel role for rab proteins in modulating the actin cytoskeleton during regulated exocytosis. Compensatory membrane retrieval following regulated exocytosis is also critically dependent on the actin cytoskeleton both in initiating the formation of clathrin-coated retrieval vesicles and subsequent trafficking back into the cell. We propose that insertion of secretory granule membrane into the plasmalemma initiates a trigger for membrane retrieval, possibly by exposing sites where proteins involved in compensatory membrane retrieval are assembled. The results summarized in this review were derived primarily from investigations on the pancreatic acinar cell, an old friend who is providing modern wisdom not attainable in other simpler systems.

The EH network

The EH domain is an evolutionary conserved protein-protein interaction domain present in a growing number of proteins from yeast to mammals. Even though the domain was discovered just 5 years ago, a great deal has been learned regarding its three-dimensional structure and binding specificities. Moreover, a number of cellular ligands of the domain have been identified and demonstrated to define a complex network of protein-protein interactions in the eukaryotic cell. Interestingly, many of the EH-containing and EH-binding proteins display characteristics of endocytic "accessory" proteins, suggesting that the principal function of the EH network is to regulate various steps in endocytosis. In addition, recent evidence suggests that the EH network might work as an "integrator" of signals controlling cellular pathways as diverse as endocytosis, nucleocytosolic export, and ultimately cell proliferation.