Early-arriving Syp1p and Ede1p function in endocytic site placement and formation in budding yeast

Mechanistic divergences of endocytic clathrin-coated vesicle formation in mammals, yeasts and plants

Clathrin-coated vesicles (CCVs), generated by clathrin-mediated endocytosis (CME), are essential eukaryotic trafficking organelles that transport extracellular and plasma membrane-bound materials into the cell. In this Review, we explore mechanisms of CME in mammals, yeasts and plants, and highlight recent advances in the characterization of endocytosis in plants. Plants separated from mammals and yeast over 1.5 billion years ago, and plant cells have distinct biophysical parameters that can influence CME, such as extreme turgor pressure. Plants can therefore provide a wider perspective on fundamental processes in eukaryotic cells. We compare key mechanisms that drive CCV formation and explore what these mechanisms might reveal about the core principles of endocytosis across the tree of life. Fascinatingly, CME in plants appears to more closely resemble that in mammalian cells than that in yeasts, despite plants being evolutionarily further from mammals than yeast. Endocytic initiation appears to be highly conserved across these three systems, requiring similar protein domains and regulatory processes. Clathrin coat proteins and their honeycomb lattice structures are also highly conserved. However, major differences are found in membrane-bending mechanisms. Unlike in mammals or yeast, plant endocytosis occurs independently of actin, highlighting that mechanistic assumptions about CME across different systems should be made with caution.

SGIP1 binding to the α-helical H9 domain of cannabinoid receptor 1 promotes axonal surface expression

Endocannabinoid signalling mediated by cannabinoid receptor 1 (CB1R, also known as CNR1) is critical for homeostatic neuromodulation of both excitatory and inhibitory synapses. This requires highly polarised axonal surface expression of CB1R, but how this is achieved remains unclear. We previously reported that the α-helical H9 domain in the intracellular C terminus of CB1R contributes to axonal surface expression by an unknown mechanism. Here, we show in rat primary neuronal cultures that the H9 domain binds to the endocytic adaptor protein SGIP1 to promote CB1R expression in the axonal membrane. Overexpression of SGIP1 increases CB1R axonal surface localisation but has no effect on CB1R lacking the H9 domain (CB1RΔH9). Conversely, SGIP1 knockdown reduces axonal surface expression of CB1R but does not affect CB1RΔH9. Furthermore, SGIP1 knockdown diminishes CB1R-mediated inhibition of presynaptic Ca2+ influx in response to neuronal activity. Taken together, these data advance mechanistic understanding of endocannabinoid signalling by demonstrating that SGIP1 interaction with the H9 domain underpins axonal CB1R surface expression to regulate presynaptic responsiveness.

The EH domain-containing protein, EdeA, is involved in endocytosis, cell wall integrity, and pathogenicity in Aspergillus fumigatus

Endocytosis has been extensively studied in yeasts, where it plays crucial roles in growth, signaling regulation, and cell-surface receptor internalization. However, the biological functions of endocytosis in pathogenic filamentous fungi remain largely unexplored. In this study, we aimed to functionally characterize the roles of EdeA, an ortholog of the Saccharomyces cerevisiae endocytic protein Ede1, in Aspergillus fumigatus. EdeA was observed to be distributed as patches on the plasma membrane and concentrated in the subapical collar of hyphae, a localization characteristic of endocytic proteins. Loss of edeA caused defective hyphal polarity, reduced conidial production, and fewer sites of endocytosis initiations than that of the parental wild type. Notably, the edeA null mutant exhibited increased sensitivity to cell wall-disrupting agents, indicating a role for EdeA in maintaining cell wall integrity in A. fumigatus. This observation was further supported by the evidence showing that the thickness of the cell wall in the ΔedeA mutant increased, accompanied by abnormal activation of MpkA, a key component in the cell wall integrity pathway. Additionally, the ΔedeA mutant displayed increased pathogenicity in the Galleria mellonella wax moth infection model, possibly due to alterations in cell wall morphology. Site-directed mutagenesis identified the conserved residue E348 within the third EH (Eps15 homology) domain of EdeA as crucial for its subcellular localization and functions. In conclusion, our results highlight the involvement of EdeA in endocytosis, hyphal polarity, cell wall integrity, and pathogenicity in A. fumigatus.

IMPORTANCE

Aspergillus fumigatus is a significant human pathogenic fungus known to cause invasive aspergillosis, a disease with a high mortality rate. Understanding the basic principles of A. fumigatus pathogenicity is crucial for developing effective strategies against this pathogen. Previous research has underscored the importance of endocytosis in the infection capacity of pathogenic yeasts; however, its biological function in pathogenic mold remains largely unexplored. Our characterization of EdeA in A. fumigatus sheds light on the role of endocytosis in the development, stress response, and pathogenicity of pathogenic molds. These findings suggest that the components of the endocytosis process may serve as potential targets for antifungal therapy.

Roles of CgEde1 and CgMca in Development and Virulence of Colletotrichum gloeosporioides

Anthracnose, induced by Colletotrichum gloeosporioides, poses a substantial economic threat to rubber tree yields and various other tropical crops. Ede1, an endocytic scaffolding protein, plays a crucial role in endocytic site initiation and maturation in yeast. Metacaspases, sharing structural similarities with caspase family proteases, are essential for maintaining cell fitness. To enhance our understanding of the growth and virulence of C. gloeosporioides, we identified a homologue of Ede1 (CgEde1) in C. gloeosporioides. The knockout of CgEde1 led to impairments in vegetative growth, conidiation, and pathogenicity. Furthermore, we characterized a weakly interacted partner of CgEde1 and CgMca (orthologue of metacaspase). Notably, both the single mutant ΔCgMca and the double mutant ΔCgEde1/ΔCgMca exhibited severe defects in conidiation and germination. Polarity establishment and pathogenicity were also disrupted in these mutants. Moreover, a significantly insoluble protein accumulation was observed in ΔCgMca and ΔCgEde1/ΔCgMca strains. These findings elucidate the mechanism by which CgEde1 and CgMca regulates the growth and pathogenicity of C. gloeosporioides. Their regulation involves influencing conidiation, polarity establishment, and maintaining cell fitness, providing valuable insights into the intricate interplay between CgEde1 and CgMca in C. gloeosporioides.

Proteasome assembly chaperone translation upon stress requires Ede1 phase separation at the plasma membrane

Proteome adaptation is key to cells surviving stresses. Increased translation of proteasome assembly chaperones (PACs) is critical for increasing proteasome assembly and cell degradative capacity. The endocytic protein Ede1 recruits PAC mRNA to cortical actin patches in Saccharomyces cerevisiae for translation upon stress. We show, through genetic and pharmacological studies, that this is mediated by the capacity of Ede1 to phase separate. PAC expression is maintained when we exchange the phase separating domains from Ede1 for those of unrelated proteins. Without these phase separating regions, PAC expression is not induced upon stress, preventing increased proteasome assembly, causing cell death. This work identifies a mechanism underpinning Ede1-mediated increased translation of specific mRNAs at a time when general translation is repressed.

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.

Light-dependent inhibition of clathrin-mediated endocytosis in yeast unveils conserved functions of the AP2 complex

Clathrin-mediated endocytosis (CME) is an essential cellular process, conserved among eukaryotes. Yeast constitutes a powerful genetic model to dissect the complex endocytic machinery, yet there is a lack of specific pharmacological agents to interfere with CME in these organisms. TL2 is a light-regulated peptide inhibitor targeting the AP2-β-adaptin/β-arrestin interaction and that can photocontrol CME with high spatiotemporal precision in mammalian cells. Here, we study endocytic protein dynamics by live-cell imaging of the fluorescently tagged coat-associated protein Sla1-GFP, demonstrating that TL2 retains its inhibitory activity in S. cerevisiae spheroplasts. This is despite the β-adaptin/β-arrestin interaction not being conserved in yeast. Our data indicate that the AP2 α-adaptin is the functional target of activated TL2. We identified as interacting partners for the α-appendage, the Eps15 and epsin homologues Ede1 and Ent1. This demonstrates that endocytic cargo loading and sensing can be executed by conserved molecular interfaces, regardless of the proteins involved.

Insights of Endocytosis Signaling in Health and Disease

Endocytosis in mammalian cells is a fundamental cellular machinery that regulates vital physiological processes, such as the absorption of metabolites, release of neurotransmitters, uptake of hormone cellular defense, and delivery of biomolecules across the plasma membrane. A remarkable characteristic of the endocytic machinery is the sequential assembly of the complex proteins at the plasma membrane, followed by internalization and fusion of various biomolecules to different cellular compartments. In all eukaryotic cells, functional characterization of endocytic pathways is based on dynamics of the protein complex and signal transduction modules. To coordinate the assembly and functions of the numerous parts of the endocytic machinery, the endocytic proteins interact significantly within and between the modules. Clathrin-dependent and -independent endocytosis, caveolar pathway, and receptor mediated endocytosis have been attributed to a greater variety of physiological and pathophysiological roles such as, autophagy, metabolism, cell division, apoptosis, cellular defense, and intestinal permeabilization. Notably, any defect or alteration in the endocytic machinery results in the development of pathological consequences associated with human diseases such as cancer, cardiovascular diseases, neurological diseases, and inflammatory diseases. In this review, an in-depth endeavor has been made to illustrate the process of endocytosis, and associated mechanisms describing pathological manifestation associated with dysregulated endocytosis machinery.

Interactions between Membraneless Condensates and Membranous Organelles at the Presynapse: A Phase Separation View of Synaptic Vesicle Cycle

Action potential-induced neurotransmitter release in presynaptic boutons involves coordinated actions of a large list of proteins that are associated directly or indirectly with membrane structures including synaptic vesicles and plasma membranes. These proteins are often highly abundant in different synaptic bouton sub-compartments, and they rarely act alone. Instead, these proteins interact with each other forming intricate and distinct molecular complexes. Many of these complexes form condensed clusters on membrane surfaces. This review summarizes findings in recent years showing that many of presynaptic protein complex assemblies are formed via phase separation. These protein condensates extensively interact with lipid membranes via distinct modes, forming various mesoscale structures by different mode of organizations between membraneless condensates and membranous organelles. We discuss that such mesoscale interactions could have deep implications on mobilization, exocytosis, and retrieval of synaptic vesicles.

The Syp1/FCHo2 protein induces septin filament bundling through its intrinsically disordered domain

The septin collar of budding yeast is an ordered array of septin filaments that serves a scaffolding function for the cytokinetic machinery at the bud neck and compartmentalizes the membrane between mother and daughter cell. How septin architecture is aided by septin-binding proteins is largely unknown. Syp1 is an endocytic protein that was implicated in the timely recruitment of septins to the newly forming collar through an unknown mechanism. Using advanced microscopy and in vitro reconstitution assays, we show that Syp1 is able to align laterally and tightly pack septin filaments, thereby forming flat bundles or sheets. This property is shared by the Syp1 mammalian counterpart FCHo2, thus emphasizing conserved protein functions. Interestingly, the septin-bundling activity of Syp1 resides mainly in its intrinsically disordered region. Our data uncover the mechanism through which Syp1 promotes septin collar assembly and offer another example of functional diversity of unstructured protein domains.

CRISPR/Cas9-mediated point mutations improve α-amylase secretion in Saccharomyces cerevisiae

The rapid expansion of the application of pharmaceutical proteins and industrial enzymes requires robust microbial workhorses for high protein production. The budding yeast Saccharomyces cerevisiae is an attractive cell factory due to its ability to perform eukaryotic post-translational modifications and to secrete proteins. Many strategies have been used to engineer yeast platform strains for higher protein secretion capacity. Herein, we investigated a line of strains that have previously been selected after UV random mutagenesis for improved α-amylase secretion. A total of 42 amino acid altering point mutations identified in this strain line were reintroduced into the parental strain AAC to study their individual effects on protein secretion. These point mutations included missense mutations (amino acid substitution), nonsense mutations (stop codon generation), and frameshift mutations. For comparison, single gene deletions for the corresponding target genes were also performed in this study. A total of 11 point mutations and seven gene deletions were found to effectively improve α-amylase secretion. These targets were involved in several bioprocesses, including cellular stresses, protein degradation, transportation, mRNA processing and export, DNA replication, and repair, which indicates that the improved protein secretion capacity in the evolved strains is the result of the interaction of multiple intracellular processes. Our findings will contribute to the construction of novel cell factories for recombinant protein secretion.

Condensation of Ede1 promotes the initiation of endocytosis

Clathrin-mediated endocytosis is initiated by a network of weakly interacting proteins through a poorly understood mechanism. Ede1, the yeast homolog of mammalian Eps15, is an early-arriving endocytic protein and a key initiation factor. In the absence of Ede1, most other early endocytic proteins lose their punctate localization and endocytic uptake is decreased. We show that in yeast cells, cytosolic concentration of Ede1 is buffered at a critical level. Excess amounts of Ede1 form large condensates which recruit other endocytic proteins and exhibit properties of phase-separated liquid droplets. We demonstrate that the central region of Ede1, containing a coiled-coil and a prion-like region, is essential for both the condensate formation and the function of Ede1 in endocytosis. The functionality of Ede1 mutants lacking the central region can be partially rescued by an insertion of heterologous prion-like domains. Conversely, fusion of a heterologous lipid-binding domain with the central region of Ede1 can promote clustering into stable plasma membrane domains. We propose that the ability of Ede1 to form condensed networks supports the clustering of early endocytic proteins and promotes the initiation of endocytosis.

Structural basis of an endocytic checkpoint that primes the AP2 clathrin adaptor for cargo internalization

Clathrin-mediated endocytosis (CME) is the main route of internalization from the plasma membrane. It is known that the heterotetrameric AP2 clathrin adaptor must open to simultaneously engage membrane and endocytic cargo, yet it is unclear how transmembrane cargos are captured to catalyze CME. Using cryogenic-electron microscopy, we discover a new way in which mouse AP2 can reorganize to expose membrane- and cargo-binding pockets, which is not observed in clathrin-coated structures. Instead, it is stimulated by endocytic pioneer proteins called muniscins, which do not enter vesicles. Muniscin-engaged AP2 is primed to rearrange into the vesicle-competent conformation on binding the tyrosine cargo internalization motif (YxxΦ). We propose adaptor priming as a checkpoint to ensure cargo internalization.

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.

Role of Clathrin and Dynamin in Clathrin Mediated Endocytosis/Synaptic Vesicle Recycling and Implications in Neurological Diseases

Endocytosis is a process essential to the health and well-being of cell. It is required for the internalisation and sorting of “cargo”—the macromolecules, proteins, receptors and lipids of cell signalling. Clathrin mediated endocytosis (CME) is one of the key processes required for cellular well-being and signalling pathway activation. CME is key role to the recycling of synaptic vesicles [synaptic vesicle recycling (SVR)] in the brain, it is pivotal to signalling across synapses enabling intracellular communication in the sensory and nervous systems. In this review we provide an overview of the general process of CME with a particular focus on two key proteins: clathrin and dynamin that have a central role to play in ensuing successful completion of CME. We examine these two proteins as they are the two endocytotic proteins for which small molecule inhibitors, often of known mechanism of action, have been identified. Inhibition of CME offers the potential to develop therapeutic interventions into conditions involving defects in CME. This review will discuss the roles and the current scope of inhibitors of clathrin and dynamin, providing an insight into how further developments could affect neurological disease treatments.

SGIP1α, but Not SGIP1, is an Ortholog of FCHo Proteins and Functions as an Endocytic Regulator

Src homology 3-domain growth factor receptor-bound 2-like interacting protein 1 (SGIP1), originally known as a regulator of energy homeostasis, was later found to be an ortholog of Fer/Cip4 homology domain-only (FCHo) proteins and to function during endocytosis. SGIP1α is a longer splicing variant in mouse brains that contains additional regions in the membrane phospholipid-binding domain (MP) and C-terminal region, but functional consequences with or without additional regions between SGIP1 and SGIP1α remain elusive. Moreover, many previous studies have either inadvertently used SGIP1 instead of SGIP1α or used the different isoforms with or without additional regions indiscriminately, resulting in further confusion. Here, we report that the additional region in the MP is essential for SGIP1α to deform membrane into tubules and for homo-oligomerization, and SGIP1, which lacks this region, fails to perform these functions. Moreover, only SGIP1α rescued endocytic defects caused by FCHo knock-down. Thus, our results indicate that SGIP1α, but not SGIP1, is the functional ortholog of FCHos, and SGIP1 and SGIP1α are not functionally redundant. These findings suggest that caution should be taken in interpreting the role of SGIP1 in endocytosis.

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.

Clathrin: the molecular shape shifter

Clathrin is best known for its contribution to clathrin-mediated endocytosis yet it also participates to a diverse range of cellular functions. Key to this is clathrin's ability to assemble into polyhedral lattices that include curved football or basket shapes, flat lattices or even tubular structures. In this review, we discuss clathrin structure and coated vesicle formation, how clathrin is utilised within different cellular processes including synaptic vesicle recycling, hormone desensitisation, spermiogenesis, cell migration and mitosis, and how clathrin's remarkable 'shapeshifting' ability to form diverse lattice structures might contribute to its multiple cellular functions.

Spatial regulation of clathrin-mediated endocytosis through position-dependent site maturation

During clathrin-mediated endocytosis (CME), over 50 different proteins assemble on the plasma membrane to reshape it into a cargo-laden vesicle. It has long been assumed that cargo triggers local CME site assembly in Saccharomyces cerevisiae based on the discovery that cortical actin patches, which cluster near exocytic sites, are CME sites. Quantitative imaging data reported here lead to a radically different view of which CME steps are regulated and which steps are deterministic. We quantitatively and spatially describe progression through the CME pathway and pinpoint a cargo-sensitive regulatory transition point that governs progression from the initiation phase of CME to the internalization phase. Thus, site maturation, rather than site initiation, accounts for the previously observed polarized distribution of actin patches in this organism. While previous studies suggested that cargo ensures its own internalization by regulating either CME initiation rates or frequency of abortive events, our data instead identify maturation through a checkpoint in the pathway as the cargo-sensitive step.

Septin architecture and function in budding yeast

The septins constitute a conserved family of guanosine phosphate-binding and filament-forming proteins widespread across eukaryotic species. Septins appear to have two principal functions. One is to form a cortical diffusion barrier, like the septin collar at the bud neck of Saccharomyces cerevisiae, which prevents movement of membrane-associated proteins between the mother and daughter cells. The second is to serve as a polymeric scaffold for recruiting the proteins required for critical cellular processes to particular subcellular areas. In the last decade, structural information about the different levels of septin organization has appeared, but crucial structural determinants and factors responsible for septin assembly remain largely unknown. This review highlights recent findings on the architecture and function of septins and their remodeling with an emphasis on mitotically dividing budding yeasts.

Fungal plasma membrane domains

The plasma membrane (PM) performs a plethora of physiological processes, the coordination of which requires spatial and temporal organization into specialized domains of different sizes, stability, protein/lipid composition and overall architecture. Compartmentalization of the PM has been particularly well studied in the yeast Saccharomyces cerevisiae, where five non-overlapping domains have been described: The Membrane Compartments containing the arginine permease Can1 (MCC), the H+-ATPase Pma1 (MCP), the TORC2 kinase (MCT), the sterol transporters Ltc3/4 (MCL), and the cell wall stress mechanosensor Wsc1 (MCW). Additional cortical foci at the fungal PM are the sites where clathrin-dependent endocytosis occurs, the sites where the external pH sensing complex PAL/Rim localizes, and sterol-rich domains found in apically grown regions of fungal membranes. In this review, we summarize knowledge from several fungal species regarding the organization of the lateral PM segregation. We discuss the mechanisms of formation of these domains, and the mechanisms of partitioning of proteins there. Finally, we discuss the physiological roles of the best-known membrane compartments, including the regulation of membrane and cell wall homeostasis, apical growth of fungal cells and the newly emerging role of MCCs as starvation-protective membrane domains.

Endocytic FgEde1 regulates virulence and autophagy in Fusarium graminearum

Endocytosis plays critical roles in cellular processes, including nutrient uptake and signal transduction. Ede1 is an endocytic scaffolding protein that contributes to endocytic site initiation and maturation in yeast. However, the functions of Ede1 in phytopathogenic fungi are not known. Here, we identified functions of FgEde1 (FGSG_05182) in Fusarium graminearum. Deletion of FgEde1 resulted in defects in hyphal growth, conidiation and ascospore development. The FgEde1 deletion mutant showed reduced deoxynivalenol (DON) production and virulence in wheat. Furthermore, the FgEde1 deletion mutant also exhibited increased resistance to osmotic and oxidative stress as well as cell-wall perturbing agents. Importantly, deletion of FgEde1 increased the severity of autophagy in hyphae. Taken together, these results reveal that FgEde1 is involved in hyphal growth, asexual and sexual reproduction, virulence, stress responses, and autophagy in F. graminearum.

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.

Identification of Suppressor of Clathrin Deficiency-1 (SCD1) and Its Connection to Clathrin-Mediated Endocytosis in Saccharomyces cerevisiae

Clathrin is a major coat protein involved in vesicle formation during endocytosis and transport in the endosomal/trans Golgi system. Clathrin is required for normal growth of yeast (Saccharomyces cerevisiae) and in some genetic backgrounds deletion of the clathrin heavy chain gene (CHC1) is lethal. Our lab defined a locus referred to as “suppressor of clathrin deficiency” (SCD1). In the presence of the scd1-v allele (“v” – viable), yeast cells lacking clathrin heavy chain survive but grow slowly, are morphologically abnormal and have many membrane trafficking defects. In the presence of scd1-i (“i”- inviable), chc1∆ causes lethality. As a strategy to identify SCD1, we used pooled linkage analysis and whole genome sequencing. Here, we report that PAL2 (YHR097C) is the SCD1 locus. pal2∆ is synthetic lethal with chc1∆; whereas a deletion of its paralog, PAL1, is not synthetic lethal with clathrin deficiency. Like Pal1, Pal2 has two NPF motifs that are potential binding sites for EH domain proteins such as the early endocytic factor Ede1, and Pal2 associates with Ede1. Also, GFP-tagged Pal2p localizes to cortical patches containing other immobile phase endocytic coat factors. Overall, our data show that PAL2 is the SCD1 locus and the Pal2 protein has characteristics of an early factor involved in clathrin-mediated endocytosis.

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.

Systematic Nanoscale Analysis of Endocytosis Links Efficient Vesicle Formation to Patterned Actin Nucleation

Clathrin-mediated endocytosis is an essential cellular function in all eukaryotes that is driven by a self-assembled macromolecular machine of over 50 different proteins in tens to hundreds of copies. How these proteins are organized to produce endocytic vesicles with high precision and efficiency is not understood. Here, we developed high-throughput superresolution microscopy to reconstruct the nanoscale structural organization of 23 endocytic proteins from over 100,000 endocytic sites in yeast. We found that proteins assemble by radially ordered recruitment according to function. WASP family proteins form a circular nanoscale template on the membrane to spatially control actin nucleation during vesicle formation. Mathematical modeling of actin polymerization showed that this WASP nano-template optimizes force generation for membrane invagination and substantially increases the efficiency of endocytosis. Such nanoscale pre-patterning of actin nucleation may represent a general design principle for directional force generation in membrane remodeling processes such as during cell migration and division.

Phospho-regulation of intrinsically disordered proteins for actin assembly and endocytosis

Actin filament assembly contributes to the endocytic pathway pleiotropically, with active roles in clathrin-dependent and clathrin-independent endocytosis as well as subsequent endosomal trafficking. Endocytosis comprises a series of dynamic events, including the initiation of membrane curvature, bud invagination, vesicle abscission and subsequent vesicular transport. The ultimate success of endocytosis requires the coordinated activities of proteins that trigger actin polymerization, recruit actin-binding proteins (ABPs) and organize endocytic proteins (EPs) that promote membrane curvature through molecular crowding or scaffolding mechanisms. A particularly interesting phenomenon is that multiple EPs and ABPs contain a substantial percentage of intrinsically disordered regions (IDRs), which can contribute to protein coacervation and phase separation. In addition, intrinsically disordered proteins (IDPs) frequently contain sites for post-translational modifications (PTMs) such as phosphorylation, and these modifications exhibit a high preference for IDR residues [Groban ES et al. (2006) PLoS Comput Biol 2, e32]. PTMs are implicated in regulating protein function by modulating the protein conformation, protein-protein interactions and the transition between order and disorder states of IDPs. The molecular mechanisms by which IDRs of ABPs and EPs fine-tune actin assembly and endocytosis remain mostly unexplored and elusive. In this review, we analyze protein sequences of budding yeast EPs and ABPs, and discuss the potential underlying mechanisms for regulating endocytosis and actin assembly through the emerging concept of IDR-mediated protein multivalency, coacervation, and phase transition, with an emphasis on the phospho-regulation of IDRs. Finally, we summarize the current understanding of how these mechanisms coordinate actin cytoskeleton assembly and membrane curvature formation during endocytosis in budding yeast.

A Flow Cytometry-Based Phenotypic Screen To Identify Novel Endocytic Factors in Saccharomyces cerevisiae

Endocytosis is a fundamental process for internalizing material from the plasma membrane, including many transmembrane proteins that are selectively internalized depending on environmental conditions. In most cells, the main route of entry is clathrin-mediated endocytosis (CME), a process that involves the coordinated activity of over 60 proteins; however, there are likely as-yet unidentified proteins involved in cargo selection and/or regulation of endocytosis. We performed a mutagenic screen to identify novel endocytic genes in Saccharomyces cerevisiae expressing the methionine permease Mup1 tagged with pHluorin (pHl), a pH-sensitive GFP variant whose fluorescence is quenched upon delivery to the acidic vacuole lumen. We used fluorescence-activated cell sorting to isolate mutagenized cells with elevated fluorescence, resulting from failure to traffic Mup1-pHl cargo to the vacuole, and further assessed subcellular localization of Mup1-pHl to characterize the endocytic defects in 256 mutants. A subset of mutant strains was classified as having general endocytic defects based on mislocalization of additional cargo proteins. Within this group, we identified mutations in four genes encoding proteins with known roles in endocytosis: the endocytic coat components SLA2, SLA1, and EDE1, and the ARP3 gene, whose product is involved in nucleating actin filaments to form branched networks. All four mutants demonstrated aberrant dynamics of the endocytic machinery at sites of CME; moreover, the arp3^R346H mutation showed reduced actin nucleation activity in vitro. Finally, whole genome sequencing of two general endocytic mutants identified mutations in conserved genes not previously implicated in endocytosis, KRE33 and IQG1, demonstrating that our screening approach can be used to identify new components involved in endocytosis.

Local actin polymerization during endocytic carrier formation

Extracellular macromolecules, pathogens and cell surface proteins rely on endocytosis to enter cells. Key steps of endocytic carrier formation are cargo molecule selection, plasma membrane folding and detachment from the cell surface. While dedicated proteins mediate each step, the actin cytoskeleton contributes to all. However, its role can be indirect to the actual molecular events driving endocytosis. Here, we review our understanding of the molecular steps mediating local actin polymerization during the formation of endocytic carriers. Clathrin-mediated endocytosis is the least reliant on local actin polymerization, as it is only engaged to counter forces induced by membrane tension or cytoplasmic pressure. Two opposite situations are coated pit formation in yeast and at the basolateral surface of polarized mammalian cells which are, respectively, dependent and independent on actin polymerization. Conversely, clathrin-independent endocytosis forming both nanometer [CLIC (clathrin-independent carriers)/GEEC (glycosylphosphatidylinositol (GPI)-anchored protein enriched endocytic compartments), caveolae, FEME (fast endophilin-mediated endocytosis) and IL-2β (interleukin-2β) uptake] and micrometer carriers (macropinocytosis) are dependent on actin polymerization to power local membrane deformation and carrier budding. A variety of endocytic adaptors can recruit and activate the Cdc42/N-WASP or Rac1/WAVE complexes, which, in turn, engage the Arp2/3 complex, thereby mediating local actin polymerization at the membrane. However, the molecular steps for RhoA and formin-mediated actin bundling during endocytic pit formation remain unclear.

The Sla1 adaptor-clathrin interaction regulates coat formation and progression of endocytosis

Clathrin-mediated endocytosis is a fundamental transport pathway that depends on numerous protein-protein interactions. Testing the importance of the adaptor protein-clathrin interaction for coat formation and progression of endocytosis in vivo has been difficult due to experimental constrains. Here, we addressed this question using the yeast clathrin adaptor Sla1, which is unique in showing a cargo endocytosis defect upon substitution of 3 amino acids in its clathrin-binding motif (sla1^AAA ) that disrupt clathrin binding. Live-cell imaging showed an impaired Sla1-clathrin interaction causes reduced clathrin levels but increased Sla1 levels at endocytic sites. Moreover, the rate of Sla1 recruitment was reduced indicating proper dynamics of both clathrin and Sla1 depend on their interaction. sla1^AAA cells showed a delay in progression through the various stages of endocytosis. The Arp2/3-dependent actin polymerization machinery was present for significantly longer time before actin polymerization ensued, revealing a link between coat formation and activation of actin polymerization. Ultimately, in sla1^AAA cells a larger than normal actin network was formed, dramatically higher levels of various machinery proteins other than clathrin were recruited, and the membrane profile of endocytic invaginations was longer. Thus, the Sla1-clathrin interaction is important for coat formation, regulation of endocytic progression and membrane bending.

Two subunits of the exocyst, Sec3p and Exo70p, can function exclusively on the plasma membrane

The exocyst is an octameric complex that tethers secretory vesicles to the plasma membrane in preparation for fusion. We anchored each subunit with a transmembrane (TM) domain at its N- or C-terminus. Only N-terminally anchored TM-Sec3p and C-terminally anchored Exo70p-TM proved functional. These findings orient the complex with respect to the membrane and establish that Sec3p and Exo70p can function exclusively on the membrane. The functions of TM-Sec3p and Exo70p-TM were largely unaffected by blocks in endocytic recycling, suggesting that they act on the plasma membrane rather than on secretory vesicles. Cytosolic pools of the other exocyst subunits were unaffected in TM-sec3 cells, while they were partially depleted in exo70-TM cells. Blocking actin-dependent delivery of secretory vesicles in act1-3 cells results in loss of Sec3p from the purified complex. Our results are consistent with a model in which Sec3p and Exo70p can function exclusively on the plasma membrane while the other subunits are brought to them on secretory vesicles.

ORP-Mediated ER Contact with Endocytic Sites Facilitates Actin Polymerization

Oxysterol binding protein-related proteins (ORPs) are conserved lipid binding polypeptides, enriched at ER contacts sites. ORPs promote non-vesicular lipid transport and work as lipid sensors in the context of many cellular tasks, but the determinants of their distinct localization and function are not understood. Here, we demonstrate that the yeast endocytic invaginations associate with the ER and that this association specifically requires the ORPs Osh2 and Osh3, which bridge the endocytic myosin-I Myo5 to the ER integral-membrane VAMP-associated protein (VAP) Scs2. Disruption of the ER contact with endocytic sites using ORP, VAP, myosin-I, or reticulon mutants delays and weakens actin polymerization and interferes with vesicle scission. Finally, we provide evidence suggesting that ORP-dependent sterol transfer facilitates actin polymerization at endocytic sites.

The TORC2-Dependent Signaling Network in the Yeast Saccharomyces cerevisiae

To grow, eukaryotic cells must expand by inserting glycerolipids, sphingolipids, sterols, and proteins into their plasma membrane, and maintain the proper levels and bilayer distribution. A fungal cell must coordinate growth with enlargement of its cell wall. In Saccharomyces cerevisiae, a plasma membrane-localized protein kinase complex, Target of Rapamicin (TOR) complex-2 (TORC2) (mammalian ortholog is mTORC2), serves as a sensor and master regulator of these plasma membrane- and cell wall-associated events by directly phosphorylating and thereby stimulating the activity of two types of effector protein kinases: Ypk1 (mammalian ortholog is SGK1), along with a paralog (Ypk2); and, Pkc1 (mammalian ortholog is PKN2/PRK2). Ypk1 is a central regulator of pathways and processes required for plasma membrane lipid and protein homeostasis, and requires phosphorylation on its T-loop by eisosome-associated protein kinase Pkh1 (mammalian ortholog is PDK1) and a paralog (Pkh2). For cell survival under various stresses, Ypk1 function requires TORC2-mediated phosphorylation at multiple sites near its C terminus. Pkc1 controls diverse processes, especially cell wall synthesis and integrity. Pkc1 is also regulated by Pkh1- and TORC2-dependent phosphorylation, but, in addition, by interaction with Rho1-GTP and lipids phosphatidylserine (PtdSer) and diacylglycerol (DAG). We also describe here what is currently known about the downstream substrates modulated by Ypk1-mediated and Pkc1-mediated phosphorylation.

Syp1 regulates the clathrin-mediated and clathrin-independent endocytosis of multiple cargo proteins through a novel sorting motif

Internalization of proteins from the plasma membrane (PM) allows for cell-surface composition regulation, signaling of network modulation, and nutrient uptake. Clathrin-mediated endocytosis (CME) is a major internalization route for PM proteins. During CME, endocytic adaptor proteins bind cargoes at the cell surface and link them to the PM and clathrin coat. Muniscins are a conserved family of endocytic adaptors, including Syp1 in budding yeast and its mammalian orthologue, FCHo1. These adaptors bind cargo via a C-terminal μ-homology domain (μHD); however, few cargoes exhibiting muniscin-dependent endocytosis have been identified, and the sorting sequence recognized by the µHD is unknown. To reveal Syp1 cargo-sorting motifs, we performed a phage display screen and used biochemical methods to demonstrate that the Syp1 µHD binds DxY motifs in the previously identified Syp1 cargo Mid2 and the v-SNARE Snc1. We also executed an unbiased visual screen, which identified the peptide transporter Ptr2 and the ammonium permease Mep3 as Syp1 cargoes containing DxY motifs. Finally, we determined that, in addition to regulating cargo entry through CME, Syp1 can promote internalization of Ptr2 through a recently identified clathrin-independent endocytic pathway that requires the Rho1 GTPase. These findings elucidate the mechanism of Syp1 cargo recognition and its role in trafficking.

Selection and stabilization of endocytic sites by Ede1, a yeast functional homologue of human Eps15

During clathrin-mediated endocytosis (CME), endocytic-site maturation can be divided into two stages corresponding to the arrival of the early and late proteins at the plasma membrane. The early proteins are required to capture cargo and position the late machinery, which includes proteins involved in actin assembly and membrane scission. However, the mechanism by which early-arriving proteins select and stabilize endocytic sites is not known. Ede1, one of the earliest proteins recruited to endocytic sites, facilitates site initiation and stabilization. Deletion of EDE1 results in fewer CME initiations and defects in the timing of vesicle maturation. Here we made truncation mutants of Ede1 to better understand how different domains contribute to its recruitment to CME sites, site selection, and site maturation. We found that the minimal domains required for efficient Ede1 localization at CME sites are the third EH domain, the proline-rich region, and the coiled-coil region. We also found that many strains expressing ede1 truncations could support a normal rate of site initiation but still had defects in site-maturation timing, indicating separation of Ede1 functions. When expressed in yeast, human Eps15 localized to the plasma membrane, where it recruited late-phase CME proteins and supported productive endocytosis, identifying it as an Ede1 functional homologue.

Septin-Associated Protein Kinases in the Yeast Saccharomyces cerevisiae

Septins are a family of eukaryotic GTP-binding proteins that associate into linear rods, which, in turn, polymerize end-on-end into filaments, and further assemble into other, more elaborate super-structures at discrete subcellular locations. Hence, septin-based ensembles are considered elements of the cytoskeleton. One function of these structures that has been well-documented in studies conducted in budding yeast Saccharomyces cerevisiae is to serve as a scaffold that recruits regulatory proteins, which dictate the spatial and temporal control of certain aspects of the cell division cycle. In particular, septin-associated protein kinases couple cell cycle progression with cellular morphogenesis. Thus, septin-containing structures serve as signaling platforms that integrate a multitude of signals and coordinate key downstream networks required for cell cycle passage. This review summarizes what we currently understand about how the action of septin-associated protein kinases and their substrates control information flow to drive the cell cycle into and out of mitosis, to regulate bud growth, and especially to direct timely and efficient execution of cytokinesis and cell abscission. Thus, septin structures represent a regulatory node at the intersection of many signaling pathways. In addition, and importantly, the activities of certain septin-associated protein kinases also regulate the state of organization of the septins themselves, creating a complex feedback loop.

The final cut: cell polarity meets cytokinesis at the bud neck in S. cerevisiae

Cell division is a fundamental but complex process that gives rise to two daughter cells. It includes an ordered set of events, altogether called "the cell cycle", that culminate with cytokinesis, the final stage of mitosis leading to the physical separation of the two daughter cells. Symmetric cell division equally partitions cellular components between the two daughter cells, which are therefore identical to one another and often share the same fate. In many cases, however, cell division is asymmetrical and generates two daughter cells that differ in specific protein inheritance, cell size, or developmental potential. The budding yeast Saccharomyces cerevisiae has proven to be an excellent system to investigate the molecular mechanisms governing asymmetric cell division and cytokinesis. Budding yeast is highly polarized during the cell cycle and divides asymmetrically, producing two cells with distinct sizes and fates. Many components of the machinery establishing cell polarization during budding are relocalized to the division site (i.e., the bud neck) for cytokinesis. In this review we recapitulate how budding yeast cells undergo polarized processes at the bud neck for cell division.

Polarized Exocytosis Induces Compensatory Endocytosis by Sec4p-Regulated Cortical Actin Polymerization

Polarized growth is maintained by both polarized exocytosis, which transports membrane components to specific locations on the cell cortex, and endocytosis, which retrieves these components before they can diffuse away. Despite functional links between these two transport pathways, they are generally considered to be separate events. Using live cell imaging, in vivo and in vitro protein binding assays, and in vitro pyrene-actin polymerization assays, we show that the yeast Rab GTPase Sec4p couples polarized exocytosis with cortical actin polymerization, which induces endocytosis. After polarized exocytosis to the plasma membrane, Sec4p binds Las17/Bee1p (yeast Wiskott-Aldrich Syndrome protein [WASp]) in a complex with Sla1p and Sla2p during actin patch assembly. Mutations that inactivate Sec4p, or its guanine nucleotide exchange factor (GEF) Sec2p, inhibit actin patch formation, whereas the activating sec4-Q79L mutation accelerates patch assembly. In vitro assays of Arp2/3-dependent actin polymerization established that GTPγS-Sec4p overrides Sla1p inhibition of Las17p-dependent actin nucleation. These results support a model in which Sec4p relocates along the plasma membrane from polarized sites of exocytic vesicle fusion to nascent sites of endocytosis. Activated Sec4p then promotes actin polymerization and triggers compensatory endocytosis, which controls surface expansion and kinetically refines cell polarization.

Eps15 membrane-binding and -bending activity acts redundantly with Fcho1 during clathrin-mediated endocytosis

Clathrin-mediated endocytosis involves a network of proteins that direct cargo capture while simultaneously facilitating membrane remodeling. Eps15 is a critical factor that binds and bends membranes and acts redundantly with Fcho1 to ensure clathrin lattice stability during the initial stages of plasma membrane invagination.

Clathrin coat assembly on membranes requires cytosolic adaptors and accessory proteins, which bridge triskeleons with the lipid bilayer and stabilize lattice architecture throughout the process of vesicle formation. In Caenorhabditis elegans, the prototypical AP-2 adaptor complex, which is activated by the accessory factor Fcho1 at the plasma membrane, is dispensable during embryogenesis, enabling us to define alternative mechanisms that facilitate clathrin-mediated endocytosis. Here we uncover a synthetic genetic interaction between C. elegans Fcho1 (FCHO-1) and Eps15 (EHS-1), suggesting that they function in a parallel and potentially redundant manner. Consistent with this idea, we find that the FCHO-1 EFC/F-BAR domain and the EHS-1 EH domains exhibit highly similar membrane-binding and -bending characteristics in vitro. Furthermore, we demonstrate a critical role for EHS-1 when FCHO-1 membrane-binding and -bending activity is specifically eliminated in vivo. Taken together, our data highlight Eps15 as an important membrane-remodeling factor, which acts in a partially redundant manner with Fcho proteins during the earliest stages of clathrin-mediated endocytosis.

Linking up at the BAR: Oligomerization and F-BAR protein function

As cells grow, move, and divide, they must reorganize and rearrange their membranes and cytoskeleton. The F-BAR protein family links cellular membranes with actin cytoskeletal rearrangements in processes including endocytosis, cytokinesis, and cell motility. Here we review emerging information on mechanisms of F-BAR domain oligomerization and membrane binding, and how these activities are coordinated with additional domains to accomplish scaffolding and signaling functions.

Identification of Cell Cycle Dependent Interaction Partners of the Septins by Quantitative Mass Spectrometry

The septins are a conserved family of GTP-binding proteins that, in the baker's yeast, assemble into a highly ordered array of filaments at the mother bud neck. These filaments undergo significant structural rearrangements during the cell cycle. We aimed at identifying key components that are involved in or regulate the transitions of the septins. By combining cell synchronization and quantitative affinity-purification mass-spectrometry, we performed a screen for specific interaction partners of the septins at three distinct stages of the cell cycle. A total of 83 interaction partners of the septins were assigned. Surprisingly, we detected DNA-interacting/nuclear proteins and proteins involved in ribosome biogenesis and protein synthesis predominantly present in alpha-factor arrested that do not display an assembled septin structure. Furthermore, two distinct sets of regulatory proteins that are specific for cells at S-phase with a stable septin collar or at mitosis with split septin rings were identified.

Complementary methods like SPLIFF and immunoprecipitation allowed us to more exactly define the spatial and temporal characteristics of selected hits of the AP-MS screen.

New Regulators of Clathrin-Mediated Endocytosis Identified in Saccharomyces cerevisiae by Systematic Quantitative Fluorescence Microscopy

Despite the importance of clathrin-mediated endocytosis (CME) for cell biology, it is unclear if all components of the machinery have been discovered and many regulatory aspects remain poorly understood. Here, using Saccharomyces cerevisiae and a fluorescence microscopy screening approach we identify previously unknown regulatory factors of the endocytic machinery. We further studied the top scoring protein identified in the screen, Ubx3, a member of the conserved ubiquitin regulatory X (UBX) protein family. In vivo and in vitro approaches demonstrate that Ubx3 is a new coat component. Ubx3-GFP has typical endocytic coat protein dynamics with a patch lifetime of 45 ± 3 sec. Ubx3 contains a W-box that mediates physical interaction with clathrin and Ubx3-GFP patch lifetime depends on clathrin. Deletion of the UBX3 gene caused defects in the uptake of Lucifer Yellow and the methionine transporter Mup1 demonstrating that Ubx3 is needed for efficient endocytosis. Further, the UBX domain is required both for localization and function of Ubx3 at endocytic sites. Mechanistically, Ubx3 regulates dynamics and patch lifetime of the early arriving protein Ede1 but not later arriving coat proteins or actin assembly. Conversely, Ede1 regulates the patch lifetime of Ubx3. Ubx3 likely regulates CME via the AAA-ATPase Cdc48, a ubiquitin-editing complex. Our results uncovered new components of the CME machinery that regulate this fundamental process.

Rho1- and Pkc1-dependent phosphorylation of the F-BAR protein Syp1 contributes to septin ring assembly

Septins often form filaments and rings at the neck of cellular appendages. Assembly of these structures must be coordinated with membrane remodeling. In budding yeast, the Rho1 GTPase and its effector, Pkc1, play a role in septin ring stabilization during budding at least partly through phosphorylation of the bud neck–associated F-BAR protein Syp1.

In many cell types, septins assemble into filaments and rings at the neck of cellular appendages and/or at the cleavage furrow to help compartmentalize the plasma membrane and support cytokinesis. How septin ring assembly is coordinated with membrane remodeling and controlled by mechanical stress at these sites is unclear. Through a genetic screen, we uncovered an unanticipated link between the conserved Rho1 GTPase and its effector protein kinase C (Pkc1) with septin ring stability in yeast. Both Rho1 and Pkc1 stabilize the septin ring, at least partly through phosphorylation of the membrane-associated F-BAR protein Syp1, which colocalizes asymmetrically with the septin ring at the bud neck. Syp1 is displaced from the bud neck upon Pkc1-dependent phosphorylation at two serines, thereby affecting the rigidity of the new-forming septin ring. We propose that Rho1 and Pkc1 coordinate septin ring assembly with membrane and cell wall remodeling partly by controlling Syp1 residence at the bud neck.

ENTH and ANTH domain proteins participate in AP2-independent clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is a major route of entry into eukaryotic cells. A core of evolutionarily ancient genes encodes many components of this system but much of our mechanistic understanding of CME is derived from a phylogenetically narrow sampling of a few model organisms. In the parasite Trypanosoma brucei, which is distantly related to the better characterised animals and fungi, exceptionally fast endocytic turnover aids its evasion of the host immune system. Although clathrin is absolutely essential for this process, the adaptor protein complex 2 (AP2) has been secondarily lost, suggesting mechanistic divergence. Here, we characterise two phosphoinositide-binding monomeric clathrin adaptors, T. brucei (Tb)EpsinR and TbCALM, which in trypanosomes are represented by single genes, unlike the expansions present in animals and fungi. Depletion of these gene products reveals essential, but partially redundant, activities in CME. Ultrastructural analysis of TbCALM and TbEpsinR double-knockdown cells demonstrated severe defects to clathrin-coated pit formation and morphology associated with a dramatic inhibition of endocytosis. Depletion of TbCALM alone, however, produced a distinct lysosomal segregation phenotype, indicating an additional non-redundant role for this protein. Therefore, TbEpsinR and TbCALM represent ancient phosphoinositide-binding proteins with distinct and vital roles in AP2-independent endocytosis.

Casein kinase 1 promotes initiation of clathrin-mediated endocytosis

In budding yeast, over 60 proteins functioning in at least five modules are recruited to endocytic sites with predictable order and timing. However, how sites of clathrin-mediated endocytosis are initiated and stabilized is not well understood. Here, the casein kinase 1 (CK1) Hrr25 is shown to be an endocytic protein and to be among the earliest proteins to appear at endocytic sites. Hrr25 absence or overexpression decreases or increases the rate of endocytic site initiation, respectively. Ede1, an early endocytic Eps15-like protein important for endocytic initiation, is an Hrr25 target and is required for Hrr25 recruitment to endocytic sites. Hrr25 phosphorylation of Ede1 is required for Hrr25-Ede1 interaction and promotes efficient initiation of endocytic sites. These observations indicate that Hrr25 kinase and Ede1 cooperate to initiate and stabilize endocytic sites. Analysis of the mammalian homologs CK1δ/ε suggests a conserved role for these protein kinases in endocytic site initiation and stabilization.

A second Las17 monomeric actin-binding motif functions in Arp2/3-dependent actin polymerization during endocytosis

During clathrin-mediated endocytosis (CME), actin assembly provides force to drive vesicle internalization. Members of the Wiskott-Aldrich syndrome protein (WASP) family play a fundamental role stimulating actin assembly. WASP family proteins contain a WH2 motif that binds globular actin (G-actin) and a central-acidic motif that binds the Arp2/3 complex, thus promoting the formation of branched actin filaments. Yeast WASP (Las17) is the strongest of five factors promoting Arp2/3-dependent actin polymerization during CME. It was suggested that this strong activity may be caused by a putative second G-actin-binding motif in Las17. Here, we describe the in vitro and in vivo characterization of such Las17 G-actin-binding motif (LGM) and its dependence on a group of conserved arginine residues. Using the yeast two-hybrid system, GST-pulldown, fluorescence polarization and pyrene-actin polymerization assays, we show that LGM binds G-actin and is necessary for normal Arp2/3-mediated actin polymerization in vitro. Live-cell fluorescence microscopy experiments demonstrate that LGM is required for normal dynamics of actin polymerization during CME. Further, LGM is necessary for normal dynamics of endocytic machinery components that are recruited at early, intermediate and late stages of endocytosis, as well as for optimal endocytosis of native CME cargo. Both in vitro and in vivo experiments show that LGM has relatively lower potency compared to the previously known Las17 G-actin-binding motif, WH2. These results establish a second G-actin-binding motif in Las17 and advance our knowledge on the mechanism of actin assembly during CME.

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.