Distinct role of TGN-resident clathrin adaptors for Vps21p activation in the TGN-endosome trafficking pathway

Clathrin-mediated vesicle trafficking plays central roles in post-Golgi transport. In yeast (Saccharomyces cerevisiae), the AP-1 complex and GGA adaptors are predicted to generate distinct transport vesicles at the trans-Golgi network (TGN), and the epsin-related proteins Ent3p and Ent5p (collectively Ent3p/5p) act as accessories for these adaptors. Recently, we showed that vesicle transport from the TGN is crucial for yeast Rab5 (Vps21p)-mediated endosome formation, and that Ent3p/5p are crucial for this process, whereas AP-1 and GGA adaptors are dispensable. However, these observations were incompatible with previous studies showing that these adaptors are required for Ent3p/5p recruitment to the TGN, and thus the overall mechanism responsible for regulation of Vps21p activity remains ambiguous. Here, we investigated the functional relationships between clathrin adaptors in post-Golgi-mediated Vps21p activation. We show that AP-1 disruption in the ent3Δ5Δ mutant impaired transport of the Vps21p guanine nucleotide exchange factor Vps9p transport to the Vps21p compartment and severely reduced Vps21p activity. Additionally, GGA adaptors, the phosphatidylinositol-4-kinase Pik1p and Rab11 GTPases Ypt31p and Ypt32p were found to have partially overlapping functions for recruitment of AP-1 and Ent3p/5p to the TGN. These findings suggest a distinct role of clathrin adaptors for Vps21p activation in the TGN-endosome trafficking pathway.

The yeast endocytic early/sorting compartment exists as an independent sub-compartment within the trans-Golgi network

Although budding yeast has been extensively used as a model organism for studying organelle functions and intracellular vesicle trafficking, whether it possesses an independent endocytic early/sorting compartment that sorts endocytic cargos to the endo-lysosomal pathway or the recycling pathway has long been unclear. The structure and properties of the endocytic early/sorting compartment differ significantly between organisms; in plant cells, the trans-Golgi network (TGN) serves this role, whereas in mammalian cells a separate intracellular structure performs this function. The yeast syntaxin homolog Tlg2p, widely localizing to the TGN and endosomal compartments, is presumed to act as a Q-SNARE for endocytic vesicles, but which compartment is the direct target for endocytic vesicles remained unanswered. Here we demonstrate by high-speed and high-resolution 4D imaging of fluorescently labeled endocytic cargos that the Tlg2p-residing compartment within the TGN functions as the early/sorting compartment. After arriving here, endocytic cargos are recycled to the plasma membrane or transported to the yeast Rab5-residing endosomal compartment through the pathway requiring the clathrin adaptors GGAs. Interestingly, Gga2p predominantly localizes at the Tlg2p-residing compartment, and the deletion of GGAs has little effect on another TGN region where Sec7p is present but suppresses dynamics of the Tlg2-residing early/sorting compartment, indicating that the Tlg2p- and Sec7p-residing regions are discrete entities in the mutant. Thus, the Tlg2p-residing region seems to serve as an early/sorting compartment and function independently of the Sec7p-residing region within the TGN.

Role of phosphatidylserine in the localization of cell surface membrane proteins in yeast

Phosphatidylserine (PS) is a constituent of the cell membrane, being especially abundant in the cytoplasmic leaflet, and plays important roles in a number of cellular functions, including the formation of cell polarity and intracellular vesicle transport. Several studies in mammalian cells have suggested the role of PS in retrograde membrane traffic through endosomes, but in yeast, where PS is localized primarily at the plasma membrane (PM), the role in intracellular organelles remains unclear. Additionally, it is reported that polarized endocytic site formation is defective in PS-depleted yeast cells, but the role in the endocytic machinery has not been well understood. In this study, to clarify the role of PS in the endocytic pathway, we analyzed the effect of PS depletion on endocytic internalization and post-endocytic transport. We demonstrated that in cell lacking the PS synthase Cho1p (cho1Δ cell), binding and internalization of mating pheromone α-factor into the cell was severely impaired. Interestingly, the processes of endocytosis were mostly unaffected, but protein transport from the trans-Golgi network (TGN) to the PM was defective and localization of cell surface proteins was severely impaired in cho1Δ cells. We also showed that PS accumulated in intracellular compartments in cells lacking Rcy1p and Vps52p, both of which are implicated in endosome-to-PM transport via the TGN, and that the number of Snx4p-residing endosomes was increased in cho1Δ cells. These results suggest that PS plays a crucial role in the transport and localization of cell surface membrane proteins.Key words: phosphatidylserine, endocytosis, recycling, vesicle transport.

Eps15/Pan1p is a master regulator of the late stages of the endocytic pathway

Endocytosis is a multistep process involving the sequential recruitment and action of numerous proteins. This process can be divided into two phases: an early phase, in which sites of endocytosis are formed, and a late phase in which clathrin-coated vesicles are formed and internalized into the cytosol, but how these phases link to each other remains unclear. In this study, we demonstrate that anchoring the yeast Eps15-like protein Pan1p to the peroxisome triggers most of the events occurring during the late phase at the peroxisome. At this ectopic location, Pan1p recruits most proteins that function in the late phases-including actin nucleation promoting factors-and then initiates actin polymerization. Pan1p also recruited Prk1 kinase and actin depolymerizing factors, thereby triggering disassembly immediately after actin assembly and inducing dissociation of endocytic proteins from the peroxisome. These observations suggest that Pan1p is a key regulator for initiating, processing, and completing the late phase of endocytosis.

Cooperative regulation of endocytic vesicle transport by yeast Eps15-like protein Pan1p and epsins

Dynamic actin filaments are required for the formation and internalization of endocytic vesicles. Yeast actin cables serve as a track for the translocation of endocytic vesicles to early endosomes, but the molecular mechanisms regulating the interaction between vesicles and the actin cables remain ambiguous. Previous studies have demonstrated that the yeast Eps15-like protein Pan1p plays an important role in this interaction, and that interaction is not completely lost even after deletion of the Pan1p actin-binding domain, suggesting that additional proteins mediate association of the vesicle with the actin cable. Other candidates for mediating the interaction are endocytic coat proteins Sla2p (yeast Hip1R) and Ent1p/2p (yeast epsins), as these proteins can bind to both the plasma membrane and the actin filament. Here, we investigated the degree of redundancy in the actin-binding activities of Pan1p, Sla2p, and Ent1p/2p involved in the internalization and transport of endocytic vesicles. Expression of the nonphosphorylatable form of Pan1p, Pan1-18TA, caused abnormal accumulation of both actin cables and endocytic vesicles, and this accumulation was additively suppressed by deletion of the actin-binding domains of both Pan1p and Ent1p. Interestingly, deletion of the actin-binding domains of Pan1p and Ent1p in cells lacking the ENT2 gene resulted in severely defective internalization of endocytic vesicles and recruitment of actin cables to the site of endocytosis. These results suggest that Pan1p and Ent1p/2p cooperatively regulate the interaction between the endocytic vesicle and the actin cable.

Rab5-mediated endosome formation is regulated at the trans-Golgi network

Early endosomes, also called sorting endosomes, are known to mature into late endosomes via the Rab5-mediated endolysosomal trafficking pathway. Thus, early endosome existence is thought to be maintained by the continual fusion of transport vesicles from the plasma membrane and the trans-Golgi network (TGN). Here we show instead that endocytosis is dispensable and post-Golgi vesicle transport is crucial for the formation of endosomes and the subsequent endolysosomal traffic regulated by yeast Rab5 Vps21p. Fittingly, all three proteins required for endosomal nucleotide exchange on Vps21p are first recruited to the TGN before transport to the endosome, namely the GEF Vps9p and the epsin-related adaptors Ent3/5p. The TGN recruitment of these components is distinctly controlled, with Vps9p appearing to require the Arf1p GTPase, and the Rab11s, Ypt31p/32p. These results provide a different view of endosome formation and identify the TGN as a critical location for regulating progress through the endolysosomal trafficking pathway.

Functional complementation reveals that 9 of the 13 human V-ATPase subunits can functionally substitute for their yeast orthologs

Vacuolar-type H⁺-ATPase (V-ATPase) is a highly conserved proton pump responsible for acidification of intracellular organelles and potential drug target. It is a multisubunit complex comprising a cytoplasmic V₁ domain responsible for ATP hydrolysis and a membrane-embedded V_o domain that contributes to proton translocation across the membrane. Saccharomyces cerevisiae V-ATPase is composed of 14 subunits, deletion of any one of which results in well-defined growth defects. As the structure of V-ATPase and the function of each subunit have been well-characterized in yeast, this organism has been recognized as a preferred model for studies of V-ATPases. In this study, to assess the functional relatedness of the yeast and human V-ATPase subunits, we investigated whether human V-ATPase subunits can complement calcium- or pH-sensitive growth, acidification of the vacuolar lumen, assembly of the V-ATPase complex, and protein sorting in yeast mutants lacking the equivalent yeast genes. These assessments revealed that 9 of the 13 human V-ATPase subunits can partially or fully complement the function of the corresponding yeast subunits. Importantly, sequence similarity was not necessarily correlated with functional complementation. We also found that besides all V_o domain subunits, the V₁ F subunit is required for proper assembly of the V_o domain at the endoplasmic reticulum. Furthermore, the human H subunit fully restored the level of vacuolar acidification, but only partially rescued calcium-sensitive growth, suggesting a specific role of the H subunit in V-ATPase activity. These findings provide important insights into functional homologies between yeast and human V-ATPases.

Distinct roles for plasma membrane PtdIns(4)P and PtdIns(4,5)P2 during receptor-mediated endocytosis in yeast

Clathrin-mediated endocytosis requires the coordinated assembly of various endocytic proteins and lipids at the plasma membrane. Accumulating evidence demonstrates a crucial role for phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P₂] in endocytosis but specific roles for phosphatidylinositol-4-phosphate [PtdIns(4)P], other than as the biosynthetic precursor of PtdIns(4,5)P₂, have not been clarified. In this study we investigated the roles of PtdIns(4)P and PtdIns(4,5)P₂ in receptor-mediated endocytosis through the construction of temperature-sensitive (ts) mutants for the phosphatidylinositol 4-kinases (PI4-kinases) Stt4p and Pik1p and the 1-phosphatidylinositol-4-phosphate 5-kinase [PtdIns(4) 5-kinase] Mss4p. Quantitative analyses of endocytosis revealed that both the stt4^tspik1^ts and mss4^ts mutants have a severe defect in endocytic internalization. Live-cell imaging of endocytic protein dynamics in stt4^tspik1^ts and mss4^ts mutants revealed that PtdIns(4)P is required for the recruitment of the α-factor receptor Ste2p to clathrin-coated pits, whereas PtdIns(4,5)P₂ is required for membrane internalization. We also found that the localization to endocytic sites of the ENTH/ANTH domain-bearing clathrin adaptors, Ent1p, Ent2p, Yap1801p and Yap1802p, is significantly impaired in the stt4^tspik1^ts mutant but not in the mss4^ts mutant. These results suggest distinct roles in successive steps for PtdIns(4)P and PtdIns(4,5)P₂ during receptor-mediated endocytosis.

[Rab GTPases networks in membrane traffic in Saccharomyces cerevisiae]

Intracellular membrane trafficking between membranous compartments is essential for organelle biogenesis, structure, and identity. Rab/Ypt GTPases are well-characterized regulators of intracellular membrane trafficking, functioning as molecular switches that alternate between GTP- and GDP-bound forms. In Saccharomyces cerevisiae, 11 Rab/Ypt GTPases have been identified and their functions are known to be conserved in their mammalian counterparts. In yeast, the secretory pathway is regulated by sequential activation and inactivation (the so-called Rab cascade) of three types of yeast Rab protein -Ypt1p, Ypt31p/32p and Sec4p -via specific guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). In addition to these Rabs, we and others have recently demonstrated that Ypt6p is predominantly localized to the early Golgi compartment, and functions as another regulator of anterograde transport for intra-Golgi trafficking in the secretory pathway. On the other hand, the endocytic pathway is known to be regulated by three yeast Rab5s (Vps21p, Ypt52p and Ypt53p) and one Rab7 (Ypt7p). Rab5 and Rab7 are key determinants of endosome identity, and the Rab5-Rab7 cascade is important for the progression from early to late endosome. Our recent study demonstrates that the endocytic pathway branches into two vacuolar targeting pathways, the Rab5-dependent vacuole protein sorting (VPS) pathway and the Rab5-independent pathway. In this review, we focus on recent advances in our understanding of molecular mechanisms that regulate the localization and activity of yeast Rab GTPases in intracellular membrane trafficking.

Yeast Eps15-like endocytic protein Pan1p regulates the interaction between endocytic vesicles, endosomes and the actin cytoskeleton

The actin cytoskeleton plays important roles in the formation and internalization of endocytic vesicles. In yeast, endocytic vesicles move towards early endosomes along actin cables, however, the molecular machinery regulating interaction between endocytic vesicles and actin cables is poorly understood. The Eps15-like protein Pan1p plays a key role in actin-mediated endocytosis and is negatively regulated by Ark1 and Prk1 kinases. Here we show that pan1 mutated to prevent phosphorylation at all 18 threonines, pan1-18TA, displayed almost the same endocytic defect as ark1Δ prk1Δ cells, and contained abnormal actin concentrations including several endocytic compartments. Early endosomes were highly localized in the actin concentrations and displayed movement along actin cables. The dephosphorylated form of Pan1p also caused stable associations between endocytic vesicles and actin cables, and between endocytic vesicles and endosomes. Thus Pan1 phosphorylation is part of a novel mechanism that regulates endocytic compartment interactions with each other and with actin cables.

DOI:http://dx.doi.org/10.7554/eLife.10276.001

The cells of all eukaryotes – including plants, animals and fungi – absorb many substances that they need from their surroundings by forming pockets around them, and then pinching off these pockets to create structures called vesicles. Clathrin is a protein that acts as a scaffold for these vesicles.

Inside a eukaryotic cell, clathrin-coated vesicles first go to a structure known as an endosome, possibly by following a track made from filaments of a protein called actin. Researchers have shown previously that a yeast protein called Pan1 binds to actin filaments and helps the cells to create clathrin-coated vesicles. However it was unclear if the Pan1 protein is also important for transporting clathrin-coated vesicles to endosomes.

Previous studies have also shown that adding phosphate groups on to the Pan1 protein prevents it from binding to clathrin-coated vesicles or actin filaments. Now, Toshima et al. show that a mutant version of the Pan1 protein, which cannot be modified in this way, can bind stably to both clathrin-coated vesicles and the actin filaments and connect them together. The experiments also showed that, in yeast cells that only produce the mutant version of Pan1, clathrin-coated vesicles bind stably to endosomes without the need for actin. Thus, these findings show that the addition of phosphate groups onto Pan1 is part of a mechanism that regulates the interactions between clathrin-coated vesicles, endosomes and actin filaments.

Following on from this work, one future challenge is to find which proteins directly connect clathrin-coated vesicles with endosomes. It will also be important to investigate if similar mechanisms are used in the cells of mammals.

DOI:http://dx.doi.org/10.7554/eLife.10276.002

Srv2/CAP is required for polarized actin cable assembly and patch internalization during clathrin-mediated endocytosis

The dynamic assembly and disassembly of actin filaments is essential for the formation and transport of vesicles during endocytosis. In yeast, two types of actin structures, namely cortical patches and cytoplasmic cables, play a direct role in endocytosis, but how their interaction is regulated remains unclear. Here we show that Srv2/CAP, an evolutionally conserved actin regulator, is required for efficient endocytosis due to its role in the formation of the actin patches that aid in initial vesicle invagination and the actin cables that these move along. Deletion of the SRV2 gene results in the appearance of aberrant fragmented actin cables that frequently moved past actin patches, the sites of endocytosis. We find that the C-terminal CARP domain of Srv2p is vitally important for the proper assembly of actin patches and cables; we also demonstrate that Srv2's N terminal HFD domain is required for its localization to actin patches, specifically to the ADP-actin rich region through an interaction with cofilin. These results demonstrate the in vivo roles of Srv2p in the regulation of the actin cytoskeleton during clathrin-mediated endocytosis.

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

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

Analysis of subcellular localization and function of the yeast Rab6 homologue, Ypt6p, using a novel amino-terminal tagging strategy

Ypt6p, the yeast homologue of mammalian Rab6, is involved in the multiple processes regulated by membrane trafficking such as vacuole maturation and membrane protein recycling. Although several lines of evidence suggest that Ypt6p is possibly localized to multiple membrane compartments, the precise localization of endogenous Ypt6p remains to be elucidated. In this study, we developed a novel method for N-terminal tagging of endogenous protein based on homologous recombination and investigated the subcellular localization and function of Ypt6p. Ypt6p and its GTP-bound form were predominantly localized to the cis- to medial-Golgi compartments whereas the GDP-bound form of Ypt6p was localized to the cytosol. Ric1p, a component of the specific GEF complex for Ypt6p, largely colocalized with Ypt6p in the early Golgi, and localization of Ypt6p changed to the cytosol in ric1Δ cells. On the other hand, Gyp6p, a putative GAP for Ypt6p, was localized to the trans-Golgi compartment and deletion of GYP6 increased the localization of Ypt6p at the trans-Golgi, suggesting that Gyp6p promotes the dissociation of Ypt6p from the Golgi when arriving at the trans-Golgi compartment. Additionally, we demonstrated that overexpression of the GDP-bound form of Ypt6p caused defective vacuole formation and recycling of Snc1p to the plasma membrane. These results suggest that the GTP-binding activity of Ypt6p is necessary for intra-Golgi trafficking and protein recycling in the early Golgi compartment.

Bifurcation of the endocytic pathway into Rab5-dependent and -independent transport to the vacuole

The yeast Rab5 homologue, Vps21p, is known to be involved both in the vacuolar protein sorting (VPS) pathway from the trans-Golgi network to the vacuole, and in the endocytic pathway from the plasma membrane to the vacuole. However, the intracellular location at which these two pathways converge remains unclear. In addition, the endocytic pathway is not completely blocked in yeast cells lacking all Rab5 genes, suggesting the existence of an unidentified route that bypasses the Rab5-dependent endocytic pathway. Here we show that convergence of the endocytic and VPS pathways occurs upstream of the requirement for Vps21p in these pathways. We also identify a previously unidentified endocytic pathway mediated by the AP-3 complex. Importantly, the AP-3-mediated pathway appears mostly intact in Rab5-disrupted cells, and thus works as an alternative route to the vacuole/lysosome. We propose that the endocytic traffic branches into two routes to reach the vacuole: a Rab5-dependent VPS pathway and a Rab5-independent AP-3-mediated pathway.

Localization and functional requirement of yeast Na+/H+ exchanger, Nhx1p, in the endocytic and protein recycling pathway

Acidification of the lumen of intracellular organelles is important for post-transcriptional processing, endosomal maturation, receptor recycling, and vesicle trafficking, being regulated by an intricate balance between H+ influx through vacuolar-type H+-ATPase and efflux through ion channels and transporters, such as the Na+/H+ exchanger (NHE). The eukaryotic NHE family comprises two major subgroups, one residing in the plasma membrane and the other in intracellular organelles. While mammalian intracellular NHE isoforms are localized to various organelles, including the mid-trans-Golgi compartments, early and late endosomes, and recycling endosomes, Nhx1p, the sole NHE in yeast, has been reported to be localized predominantly to the late endosomal/prevacuolar compartment. Here, using live cell imaging, we demonstrated that Nhx1p is localized to the trans-Golgi network compartments, late endosomes, and recycling endosomes, similar to mammalian intracellular NHE isoforms. Loss of Nhx1p led to accumulation of components of the retromer and endosomal sorting complex required for transport complexes, but not trans-Golgi compartments, in aberrant prevacuolar compartments. Importantly, Nhx1p was also required for recycling of the plasma membrane vesicle SNAP receptor Snc1p. These observations suggest that Nhx1p plays an important role in regulation of the luminal pH of various intracellular organelles, and that this regulation is critical for the protein recycling pathway as well as the endocytic pathway.

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.

Requirements for recruitment of a G protein-coupled receptor to clathrin-coated pits in budding yeast

Endocytic internalization of G protein-coupled receptors (GPCRs) plays a critical role in down-regulation of GPCR signaling. The yeast mating pheromone receptor Ste2p has been used as a model to investigate mechanisms of signal transduction, modification, and endocytic internalization of GPCRs. We previously used a fluorescently labeled mating pheromone derivative to reveal unappreciated molecular and spatiotemporal features of GPCR endocytosis in budding yeast. Here, we identify recruitment of Ste2p to preexisting clathrin-coated pits (CCPs) as a key step regulated by receptor phosphorylation and subsequent ubiquitination upon ligand binding. The yeast casein kinase I homologue Yck2p directly phosphorylates six serine residues located in the C-terminal tail of Ste2p, and mutation of these serine residues to alanine significantly decreased recruitment of Ste2p to CCPs. We also found that the clathrin adaptors Ent1p, Ent2p, and Ede1p work cooperatively to recruit ubiquitinated Ste2p to CCPs. In addition, ubiquitination has a role in ligand-independent constitutive recruitment of Ste2p to CCPs, although this process is much slower than ligand-induced recruitment. These results suggest that ubiquitination of Ste2p is indispensable for recruiting Ste2p to CCPs in both ligand-dependent and ligand-independent endocytosis.

PtdIns(4,5)P2 turnover is required for multiple stages during clathrin- and actin-dependent endocytic internalization

The lipid phosphatidylinositol-4,5-bisphosphate (PtdIns[4,5]P(2)) appears to play an important role in endocytosis. However, the timing of its formation and turnover, and its specific functions at different stages during endocytic internalization, have not been established. In this study, Sla2 ANTH-GFP and Sjl2-3GFP were expressed as functional fusion proteins at endogenous levels to quantitatively explore PtdIns(4,5)P(2) dynamics during endocytosis in yeast. Our results indicate that PtdIns(4,5)P(2) levels increase and decline in conjunction with coat and actin assembly and disassembly, respectively. Live-cell image analysis of endocytic protein dynamics in an sjl1Delta sjl2Delta mutant, which has elevated PtdIns(4,5)P(2) levels, revealed that the endocytic machinery is still able to assemble and disassemble dynamically, albeit nonproductively. The defects in the dynamic behavior of the various endocytic proteins in this double mutant suggest that PtdIns(4,5)P(2) turnover is required for multiple stages during endocytic vesicle formation. Furthermore, our results indicate that PtdIns(4,5)P(2) turnover may act in coordination with the Ark1/Prk1 protein kinases in stimulating disassembly of the endocytic machinery.

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.

Spatial dynamics of receptor-mediated endocytic trafficking in budding yeast revealed by using fluorescent alpha-factor derivatives

Much progress defining the order and timing of endocytic internalization events has come as a result of real-time, live-cell fluorescence microscopy. Although the availability of numerous endocytic mutants makes yeast an especially valuable organism for functional analysis of endocytic dynamics, a serious limitation has been the lack of a fluorescent cargo for receptor-mediated endocytosis. We have now synthesized biologically active fluorescent mating-pheromone derivatives and demonstrated that receptor-mediated endocytosis in budding yeast occurs via the clathrin- and actin-mediated endocytosis pathway. We found that endocytic proteins first assemble into patches on the plasma membrane, and then alpha-factor associates with the patches. Internalization occurs next, concomitant with actin assembly at patches. Additionally, endocytic vesicles move toward early endosomes on actin cables. Early endosomes also associate with actin cables, and they actively move toward endocytic sites to capture vesicles being released from the plasma membrane. Thus, endocytic vesicle formation and capture of the newly released vesicles by early endosomes occur in a highly concerted manner, mediated by the actin cytoskeleton.

Phosphoregulation of Arp2/3-dependent actin assembly during receptor-mediated endocytosis

In both yeast and mammals, endocytic internalization is accompanied by a transient burst of actin polymerization. The yeast protein kinases Prk1p and Ark1p, which are related to the mammalian proteins GAK and AAK1, are key regulators of this process. However, the molecular mechanism(s) by which they regulate actin assembly at endocytic sites have not yet been determined. The Eps15-like yeast protein Pan1p is a Prk1p substrate that is essential for endocytic internalization and for proper actin organization. Pan1p is an Arp2/3 activator and here we show that this activity is dependent on F-actin binding. Mutation of all 15 Prk1p-targeted threonines in Pan1p to alanines mimicked the ark1Delta prk1Delta phenotype, demonstrating that Pan1p is a key Prk1p target in vivo. Moreover, phosphorylation by Prk1p inhibited the ability of Pan1p to bind to F-actin and to activate the Arp2/3 complex, thereby identifying the endocytic phosphoregulation mechanism of Prk1p. We conclude that Prk1p phosphorylation of Pan1p shuts off Arp2/3-mediated actin polymerization on endocytic vesicles, allowing them to fuse with endosomes.

Binding of 14-3-3beta regulates the kinase activity and subcellular localization of testicular protein kinase 1

Testicular protein kinase 1 (TESK1) is a serine/threonine kinase that phosphorylates cofilin and induces actin cytoskeletal reorganization. The kinase activity of TESK1 is stimulated by integrin-mediated signaling pathways, but the mechanism of regulation has remained unknown. By using the yeast two-hybrid system, we identified 14-3-3beta to be the binding protein of TESK1. Specific interaction between TESK1 and 14-3-3beta became evident in in vitro and in vivo co-precipitation assays. 14-3-3beta interacts with TESK1 through the C-terminal region of TESK1 and in a manner dependent on the phosphorylation of Ser-439 within an RXXSXP motif. Binding of 14-3-3beta inhibited the kinase activity of TESK1. During cell spreading on fibronectin, the TESK1/14-3-3beta interaction significantly decreased, in a time course that inversely correlated with increase in TESK1 kinase activity. Thus, the dissociation of 14-3-3beta from a TESK1/14-3-3beta complex is likely to be involved in the integrin-mediated TESK1 activation. In HeLa cells, TESK1, together with 14-3-3beta, accumulated at the cell periphery when cells were plated on fibronectin, whereas they were diffusely distributed in the cytoplasm in the case of non-stimulated cells. We propose that 14-3-3beta plays important roles in regulating the kinase activity of TESK1 and localizing TESK1 to cell adhesion sites following integrin stimulation.

Cell-type-specific expression of a TESK1 promoter-linked lacZ gene in transgenic mice

Testicular protein kinase 1 (TESK1) is a serine/threonine kinase highly expressed in testicular germ cells and has the potential to phosphorylate cofilin and induce actin cytoskeletal reorganization. We examined the expression of a lacZ reporter gene linked to a 9.0-kb 5'-flanking region of TESK1 gene in transgenic mice. A high level of lacZ expression was observed in testicular germ cells only at stages after pachytene spermatocytes, the expression patterns being similar to those of TESK1 mRNA in rat testis, determined by in situ hybridization. Expression of lacZ was also detected in renal proximal tubules, cardiac myocytes, and specific neurons in the central nervous system in adult transgenic mice. Whole-mount staining revealed the expression of lacZ in neural tissues in embryonic mice. These results suggest the cell-type- and stage-specific expression of TESK1 gene and the diverse and specific physiological functions of TESK1, including those in spermatogenesis and neural development.

Cofilin phosphorylation and actin reorganization activities of testicular protein kinase 2 and its predominant expression in testicular Sertoli cells

We previously identified testicular protein kinase 1 (TESK1), which phosphorylates cofilin and induces actin cytoskeletal reorganization. We now report identification and characterization of another member of a TESK family, testicular protein kinase 2 (TESK2), with 48% amino acid identity with TESK1. Like TESK1, TESK2 phosphorylated cofilin specifically at Ser-3 and induced formation of actin stress fibers and focal adhesions. Both TESK1 and TESK2 are highly expressed in the testis, but in contrast to TESK1, which is predominantly expressed in testicular germ cells, TESK2 is expressed predominantly in nongerminal Sertoli cells. Thus, TESK1 and TESK2 seem to play distinct roles in spermatogenesis. In HeLa cells, TESK1 was localized mainly in the cytoplasm, whereas TESK2 was localized mainly in the nucleus, which means that TESK1 and TESK2 likely have distinct cellular functions. Because the kinase-inactive mutant of TESK2 was localized in the cytoplasm, nuclear/cytoplasmic localization of TESK2 depends on its kinase activity. A TESK2 mutant lacking the C-terminal noncatalytic region had about a 10-fold higher kinase activity in vitro and, when expressed in HeLa cells, induced punctate actin aggregates in the cytoplasm and unusual condensation and fragmentation of nuclei, followed by apoptosis. Thus, we propose that the C-terminal region plays important roles in regulating the kinase activity and cellular functions of TESK2.

Cofilin phosphorylation by protein kinase testicular protein kinase 1 and its role in integrin-mediated actin reorganization and focal adhesion formation

Testicular protein kinase 1 (TESK1) is a serine/threonine kinase with a structure composed of a kinase domain related to those of LIM-kinases and a unique C-terminal proline-rich domain. Like LIM-kinases, TESK1 phosphorylated cofilin specifically at Ser-3, both in vitro and in vivo. When expressed in HeLa cells, TESK1 stimulated the formation of actin stress fibers and focal adhesions. In contrast to LIM-kinases, the kinase activity of TESK1 was not enhanced by Rho-associated kinase (ROCK) or p21-activated kinase, indicating that TESK1 is not their downstream effector. Both the kinase activity of TESK1 and the level of cofilin phosphorylation increased by plating cells on fibronectin. Y-27632, a specific inhibitor of ROCK, inhibited LIM-kinase-induced cofilin phosphorylation but did not affect fibronectin-induced or TESK1-induced cofilin phosphorylation in HeLa cells. Expression of a kinase-negative TESK1 suppressed cofilin phosphorylation and formation of stress fibers and focal adhesions induced in cells plated on fibronectin. These results suggest that TESK1 functions downstream of integrins and plays a key role in integrin-mediated actin reorganization, presumably through phosphorylating and inactivating cofilin. We propose that TESK1 and LIM-kinases commonly phosphorylate cofilin but are regulated in different ways and play distinct roles in actin reorganization in living cells.

Differential effects of progesterone and 17beta-estradiol on the Ca(2+) entry induced by thapsigargin and endothelin-1 in in situ endothelial cells

The effects of progesterone and 17beta-estradiol on Ca(2+) signaling in in situ endothelial cells were investigated using front-surface fluorometry of fura-2-loaded strips of porcine aortic valve. Progesterone inhibited the thapsigargin-induced sustained [Ca(2+)](i) elevation (IC(50)=33.9 microM, n=4), while 17beta-estradiol added a transient [Ca(2+)](i) elevation. Progesterone and 17beta-estradiol had no significant effect on the thapsigargin-induced [Ca(2+)](i) elevations in the absence of extracellular Ca(2+). A Mn(2+)-induced decline of fluorescent intensity at 360 nm excitation was accelerated by thapsigargin. This acceleration was completely reversed by progesterone, but not by 17beta-estradiol. Progesterone inhibited, and 17beta-estradiol enhanced the endothelin-1 (ET-1)-induced [Ca(2+)](i) elevation, while both had no effect on the ET-1-induced Ca(2+) release observed in the absence of extracellular Ca(2+) or in the pertussis toxin-treated strips. Progesterone and 17beta-estradiol thus had different effects on Ca(2+) signaling, especially on Ca(2+) influx, in endothelial cells.