Calcyon, a novel partner of clathrin light chain, stimulates clathrin-mediated endocytosis

In the central nervous system, clathrin-mediated endocytosis is crucial for efficient synaptic transmission. Clathrin-coated vesicle assembly and disassembly is regulated by some 30 adaptor and accessory proteins, most of which interact with clathrin heavy chain. Using the calcyon cytosolic domain as bait, we isolated clathrin light chain in a yeast two-hybrid screen. The interaction domain was mapped to the heavy chain binding domain and C-terminal regions of light chain. Further, the addition of the calcyon C terminus stimulated clathrin self-assembly in a dose-dependent fashion. Calcyon, which is a single transmembrane protein predominantly expressed in brain, localized to vesicular compartments within pre- and postsynaptic structures. There was a high degree of overlap in the distribution of LC and calcyon in neuronal dendrites, spines, and cell bodies. Co-immunoprecipitation studies further suggested an association of calcyon with the clathrin-mediated endocytic machinery. Compared with controls, HEK293 cells overexpressing calcyon exhibited significantly enhanced transferrin uptake but equivalent levels of recycling. Conversely, transferrin uptake was largely abolished in neocortical neurons obtained from mice homozygous for a calcyon null allele, whereas recycling proceeded at wild type levels. Collectively, these data indicate a role for calcyon in clathrin-mediated endocytosis in brain.     

SLC30A3 (ZnT3) Oligomerization by Dityrosine Bonds Regulates Its Subcellular Localization and Metal Transport Capacity

Non-covalent and covalent homo-oligomerization of membrane proteins regulates their subcellular localization and function. Here, we described a novel oligomerization mechanism affecting solute carrier family 30 member 3/zinc transporter 3 (SLC30A3/ZnT3). Oligomerization was mediated by intermolecular covalent dityrosine bonds. Using mutagenized ZnT3 expressed in PC12 cells, we identified two critical tyrosine residues necessary for dityrosine-mediated ZnT3 oligomerization. ZnT3 carrying the Y372F mutation prevented ZnT3 oligomerization, decreased ZnT3 targeting to synaptic-like microvesicles (SLMVs), and decreased resistance to zinc toxicity. Strikingly, ZnT3 harboring the Y357F mutation behaved as a “gain-of-function” mutant as it displayed increased ZnT3 oligomerization, targeting to SLMVs, and increased resistance to zinc toxicity. Single and double tyrosine ZnT3 mutants indicate that the predominant dimeric species is formed between tyrosine 357 and 372. ZnT3 tyrosine dimerization was detected under normal conditions and it was enhanced by oxidative stress. Covalent species were also detected in other SLC30A zinc transporters localized in different subcellular compartments. These results indicate that covalent tyrosine dimerization of a SLC30A family member modulates its subcellular localization and zinc transport capacity. We propose that dityrosine-dependent membrane protein oligomerization may regulate the function of diverse membrane protein in normal and disease states.     

Phosphatidylinositol 4‐kinase IIα function at endosomes is regulated by the ubiquitin ligase Itch

Phosphatidylinositol (PI) 4‐phosphate (PI(4)P) and its metabolizing enzymes serve important functions in cell signalling and membrane traffic. PI 4‐kinase type IIα (PI4KIIα) regulates Wnt signalling, endosomal sorting of signalling receptors, and promotes adaptor protein recruitment to endosomes and the trans‐Golgi network. Here we identify the E3 ubiquitin ligase Itch as binding partner and regulator of PI4KIIα function. Itch directly associates with and ubiquitinates PI4KIIα, and both proteins colocalize on endosomes containing Wnt‐activated frizzled 4 (Fz4) receptor. Depletion of PI4KIIα or Itch regulates Wnt signalling with corresponding changes in Fz4 internalization and degradative sorting. These findings unravel a new molecular link between phosphoinositide‐regulated endosomal membrane traffic, ubiquitin and the modulation of Wnt signalling.     

Phosphoinositide 3-kinase regulates beta2-adrenergic receptor endocytosis by AP-2 recruitment to the receptor/beta-arrestin complex

Internalization of beta-adrenergic receptors (betaARs) occurs by the sequential binding of beta-arrestin, the clathrin adaptor AP-2, and clathrin. D-3 phosphoinositides, generated by the action of phosphoinositide 3-kinase (PI3K) may regulate the endocytic process; however, the precise molecular mechanism is unknown. Here we demonstrate that betaARKinase1 directly interacts with the PIK domain of PI3K to form a cytosolic complex. Overexpression of the PIK domain displaces endogenous PI3K from betaARK1 and prevents betaARK1-mediated translocation of PI3K to activated beta2ARs. Furthermore, disruption of the betaARK1/PI3K interaction inhibits agonist-stimulated AP-2 adaptor protein recruitment to the beta2AR and receptor endocytosis without affecting the internalization of other clathrin dependent processes such as internalization of the transferrin receptor. In contrast, AP-2 recruitment is enhanced in the presence of D-3 phospholipids, and receptor internalization is blocked in presence of the specific phosphatidylinositol-3,4,5-trisphosphate lipid phosphatase PTEN. These findings provide a molecular mechanism for the agonist-dependent recruitment of PI3K to betaARs, and support a role for the localized generation of D-3 phosphoinositides in regulating the recruitment of the receptor/cargo to clathrin-coated pits.     

The zinc transporter ZnT3 interacts with AP-3 and it is preferentially targeted to a distinct synaptic vesicle subpopulation

Synaptic vesicles (SV) are generated by two different mechanisms, one AP-2 dependent and one AP-3 dependent. It has been uncertain, however, whether these mechanisms generate SV that differ in molecular composition. We explored this hypothesis by analyzing the targeting of ZnT3 and synaptophysin both to PC12 synaptic-like microvesicles (SLMV) as well as SV isolated from wild-type and AP-3-deficient mocha brains. ZnT3 cytosolic tail interacted selectively with AP-3 in cell-free assays. Accordingly, pharmacological disruption of either AP-2- or AP-3-dependent SLMV biogenesis preferentially reduced synaptophysin or ZnT3 targeting, respectively; suggesting that these antigens were concentrated in different vesicles. As predicted, immuno-isolated SLMV revealed that ZnT3 and synaptophysin were enriched in different vesicle populations. Likewise, morphological and biochemical analyses in hippocampal neurons indicated that these two antigens were also present in distinct but overlapping domains. ZnT3 SV content was reduced in AP-3-deficient neurons, but synaptophysin was not altered in the AP-3 null background. Our evidence indicates that neuroendocrine cells assemble molecularly heterogeneous SV and suggests that this diversity could contribute to the functional variety of synapses.     

μ2‐Dependent endocytosis of N‐cadherin is regulated by β‐catenin to facilitate neurite outgrowth

Circuit formation in the brain requires neurite outgrowth throughout development to establish synaptic contacts with target cells. Active endocytosis of several adhesion molecules facilitates the dynamic exchange of these molecules at the surface and promotes neurite outgrowth in developing neurons. The endocytosis of N‐cadherin, a calcium‐dependent adhesion molecule, has been implicated in the regulation of neurite outgrowth, but the mechanism remains unclear. Here, we identified that a fraction of N‐cadherin internalizes through clathrin‐mediated endocytosis (CME). Two tyrosine‐based motifs in the cytoplasmic domain of N‐cadherin recognized by the μ2 subunit of the AP‐2 adaptor complex are responsible for CME of N‐cadherin. Moreover, β‐catenin, a core component of the N‐cadherin adhesion complex, inhibits N‐cadherin endocytosis by masking the 2 tyrosine‐based motifs. Removal of β‐catenin facilitates μ2 binding to N‐cadherin, thereby increasing clathrin‐mediated N‐cadherin endocytosis and neurite outgrowth without affecting the steady‐state level of surface N‐cadherin. These results identify and characterize the mechanism controlling N‐cadherin endocytosis through β‐catenin‐regulated μ2 binding to modulate neurite outgrowth.      

Molecular and functional characterization of clathrin- and AP-2-binding determinants within a disordered domain of auxilin

Uncoating of clathrin-coated vesicles requires the J-domain protein auxilin for targeting hsc70 to the clathrin coats and for stimulating the hsc70 ATPase activity. This results in the release of hsc70-complexed clathrin triskelia and concomitant dissociation of the coat. To understand the complex role of auxilin in uncoating and clathrin assembly in more detail, we analyzed the molecular organization of its clathrin-binding domain (amino acids 547-813). CD spectroscopy of auxilin fragments revealed that the clathrin-binding domain is almost completely disordered in solution. By systematic mapping using synthetic peptides and by site-directed mutagenesis, we identified short peptide sequences involved in clathrin heavy chain and AP-2 binding and evaluated their significance for the function of auxilin. Some of the binding determinants, including those containing sequences 674DPF and 636WDW, showed dual specificity for both clathrin and AP-2. In contrast, the two DLL motifs within the clathrin-binding domain were exclusively involved in clathrin binding. Surprisingly, they interacted not only with the N-terminal domain of the heavy chain, but also with the distal domain. Moreover, both DLL peptides proved to be essential for clathrin assembly and uncoating. In addition, we found that the motif 726NWQ is required for efficient clathrin assembly activity. Auxilin shares a number of protein-protein interaction motifs with other endocytic proteins, including AP180. We demonstrate that AP180 and auxilin compete for binding to the alpha-ear domain of AP-2. Like AP180, auxilin also directly interacts with the ear domain of beta-adaptin. On the basis of our data, we propose a refined model for the uncoating mechanism of clathrin-coated vesicles.     

Cell entry of bovine ephemeral fever virus requires activation of Src‐JNK‐AP1 and PI3K‐Akt‐NF‐κB pathways as well as Cox‐2‐mediated PGE2/EP receptor signalling to enhance clathrin‐mediated virus endocytosis

Although we have previously demonstrated that cell entry of bovine ephemeral fever virus (BEFV) follows a clathrin‐mediated and dynamin 2‐dependent endocytosis pathway, the cellular mechanism mediating virus entry remains unknown. Here, we report that BEFV triggers simultaneously Src‐JNK‐AP1 and PI3K‐Akt‐NF‐κB signalling pathways in the stage of virus binding to induce clathrin and dynamin 2 expressions, while vesicular stomatitis virus only activates Src‐JNK signalling to enhance its entry. Activation of these pathways by ultraviolet‐inactivated BEFV suggests a role for virus binding but not viral internalization and gene expression. By blocking these signalling pathways with specific inhibitors, BEFV‐induced expressions of clathrin and dynamin 2 were significantly diminished. By labelling BEFV with 3,3′‐dilinoleyloxacarbocyanine perchlorate to track viral entry, we found that virus entry was hindered by both Src and Akt inhibitors, suggesting that these signalling pathways are crucial for efficient virus entry. In addition, BEFV also triggers Cox‐2‐catalysed prostaglandin E₂ (PGE₂) synthesis and induces expressions of G‐protein‐coupled E‐prostanoid (EP) receptors 2 and 4, leading to amplify signal cascades of Src‐JNK‐AP1 and PI3K‐Akt‐NF‐κB, which elevates both clathrin and dynamin 2 expressions. Furthermore, pretreatment of cells with adenylate cyclase (cAMP) inhibitor SQ22536 reduced BEFV‐induced Src phosphorylation as well as clathrin and dynamin 2 expressions. Our findings reveal for the first time that BEFV activates the Cox‐2‐mediated PGE₂/EP receptor signalling pathways, further enhancing Src‐JNK‐AP1 in a cAMP‐dependent manner and PI3K‐Akt‐NF‐κB in a cAMP‐independent manner. Accordingly, BEFV stimulates PGE₂/EP receptor signalling amplifying Src‐JNK‐AP1 and PI3K‐Akt‐NF‐κB pathways in an autocrine or paracrine fashion to enhance virus entry.     

Phosphatidylinositol-4-kinase type II alpha is a component of adaptor protein-3-derived vesicles

A membrane fraction enriched in vesicles containing the adaptor protein (AP) -3 cargo zinc transporter 3 was generated from PC12 cells and was used to identify new components of these organelles by mass spectrometry. Proteins prominently represented in the fraction included AP-3 subunits, synaptic vesicle proteins, and lysosomal proteins known to be sorted in an AP-3-dependent way or to interact genetically with AP-3. A protein enriched in this fraction was phosphatidylinositol-4-kinase type IIalpha (PI4KIIalpha). Biochemical, pharmacological, and morphological analyses supported the presence of PI4KIIalpha in AP-3-positive organelles. Furthermore, the subcellular localization of PI4KIIalpha was altered in cells from AP-3-deficient mocha mutant mice. The PI4KIIalpha normally present both in perinuclear and peripheral organelles was substantially decreased in the peripheral membranes of AP-3-deficient mocha fibroblasts. In addition, as is the case for other proteins sorted in an AP-3-dependent way, PI4KIIalpha content was strongly reduced in nerve terminals of mocha hippocampal mossy fibers. The functional relationship between AP-3 and PI4KIIalpha was further explored by PI4KIIalpha knockdown experiments. Reduction of the cellular content of PI4KIIalpha strongly decreased the punctate distribution of AP-3 observed in PC12 cells. These results indicate that PI4KIIalpha is present on AP-3 organelles where it regulates AP-3 function.     

The schizophrenia susceptibility factor dysbindin and its associated complex sort cargoes from cell bodies to the synapse

Dysbindin assembles into the biogenesis of lysosome-related organelles complex 1 (BLOC-1), which interacts with the adaptor protein complex 3 (AP-3), mediating a common endosome-trafficking route. Deficiencies in AP-3 and BLOC-1 affect synaptic vesicle composition. However, whether AP-3-BLOC-1-dependent sorting events that control synapse membrane protein content take place in cell bodies upstream of nerve terminals remains unknown. We tested this hypothesis by analyzing the targeting of phosphatidylinositol-4-kinase type II α (PI4KIIα), a membrane protein present in presynaptic and postsynaptic compartments. PI4KIIα copurified with BLOC-1 and AP-3 in neuronal cells. These interactions translated into a decreased PI4KIIα content in the dentate gyrus of dysbindin-null BLOC-1 deficiency and AP-3-null mice. Reduction of PI4KIIα in the dentate reflects a failure to traffic from the cell body. PI4KIIα was targeted to processes in wild-type primary cultured cortical neurons and PC12 cells but failed to reach neurites in cells lacking either AP-3 or BLOC-1. Similarly, disruption of an AP-3-sorting motif in PI4KIIα impaired its sorting into processes of PC12 and primary cultured cortical neuronal cells. Our findings indicate a novel vesicle transport mechanism requiring BLOC-1 and AP-3 complexes for cargo sorting from neuronal cell bodies to neurites and nerve terminals.     

The monomeric clathrin assembly protein, AP180, regulates contractile vacuole size in Dictyostelium discoideum

AP180, one of many assembly proteins and adaptors for clathrin, stimulates the assembly of clathrin lattices on membranes, but its unique contribution to clathrin function remains elusive. In this study we identified the Dictyostelium discoideum ortholog of the adaptor protein AP180 and characterized a mutant strain carrying a deletion in this gene. Imaging GFP-labeled AP180 showed that it localized to punctae at the plasma membrane, the contractile vacuole, and the cytoplasm and associated with clathrin. AP180 null cells did not display defects characteristic of clathrin mutants and continued to localize clathrin punctae on their plasma membrane and within the cytoplasm. However, like clathrin mutants, AP180 mutants, were osmosensitive. When immersed in water, AP180 null cells formed abnormally large contractile vacuoles. Furthermore, the cycle of expansion and contraction for contractile vacuoles in AP80 null cells was twice as long as that of wild-type cells. Taken together, our results suggest that AP180 plays a unique role as a regulator of contractile vacuole morphology and activity in Dictyostelium.     

Differential recognition of a dileucine-based sorting signal by AP-1 and AP-3 reveals a requirement for both BLOC-1 and AP-3 in delivery of OCA2 to melanosomes

Cell types that generate unique lysosome-related organelles (LROs), such as melanosomes in melanocytes, populate nascent LROs with cargoes that are diverted from endosomes. Cargo sorting toward melanosomes correlates with binding via cytoplasmically exposed sorting signals to either heterotetrameric adaptor AP-1 or AP-3. Some cargoes bind both adaptors, but the relative contribution of each adaptor to cargo recognition and their functional interactions with other effectors during transport to melanosomes are not clear. Here we exploit targeted mutagenesis of the acidic dileucine-based sorting signal in the pigment cell-specific protein OCA2 to dissect the relative roles of AP-1 and AP-3 in transport to melanosomes. We show that binding to AP-1 or AP-3 depends on the primary sequence of the signal and not its position within the cytoplasmic domain. Mutants that preferentially bound either AP-1 or AP-3 each trafficked toward melanosomes and functionally complemented OCA2 deficiency, but AP-3 binding was necessary for steady-state melanosome localization. Unlike tyrosinase, which also engages AP-3 for optimal melanosomal delivery, both AP-1- and AP-3-favoring OCA2 variants required BLOC-1 for melanosomal transport. These data provide evidence for distinct roles of AP-1 and AP-3 in OCA2 transport to melanosomes and indicate that BLOC-1 can cooperate with either adaptor during cargo sorting to LROs.     

Pathophysiological concentrations of amyloid beta proteins directly inhibit rat brain and recombinant human type II phosphatidylinositol 4-kinase activity

We previously found that pathophysiological concentrations (< or = 10 nm) of an amyloid beta protein (Abeta25-35) reduced the plasma membrane phosphatidylinositol monophosphate level in cultured rat hippocampal neurons with a decrease in phosphatidylinositol 4-monophosphate-dependent Cl- -ATPase activity. As this suggested an inhibitory effect of Abeta25-35 on plasma membrane phosphatidylinositol 4-kinase (PI4K) activity, in vitro effects of Abetas on PI4K activity was examined using rat brain subcellular fractions and recombinant human type II PI4K (PI4KII). Abeta25-35 (10 nm) inhibited PI4KII activity, but neither PI 3-kinase (PI3K) nor type III PI4K (PI4KIII) activity, in microsomal fractions, while 100 nm Abeta25-35 inhibited PI3K activity in mitochondrial fractions. In plasma membrane-rich fractions, Abetas (> 0.5 nm) dose-dependently inhibited PI4KII activity, the maximal inhibition to 77-87% of control being reached around 10 nm of Abetas without significant changes in apparent Km values for ATP and PI, suggesting non-competitive inhibition by Abetas. The inhibition by 10 nm Abeta25-35 was reversible. In recombinant human PI4KIIalpha, inhibition profiles of Abetas were similar to those in rat brain plasma membranes. Therefore, pathophysiological concentrations of Abetas directly and reversibly inhibited plasma membrane PI4KII activity, suggesting that plasma membrane PI4KII is a target of Abetas in the pathogenesis of Alzheimer's disease.     

Genetic analysis of the neuronal and ubiquitous AP-3 adaptor complexes reveals divergent functions in brain

Neurons express adaptor (AP)-3 complexes assembled with either ubiquitous (beta3A) or neuronal-specific (beta3B) beta3 isoforms. However, it is unknown whether these complexes indeed perform distinct functions in neuronal tissue. Here, we explore this hypothesis by using genetically engineered mouse models lacking either beta3A- or beta3B-containing AP-3 complexes. Somatic and neurological phenotypes were specifically associated with the ubiquitous and neuronal adaptor deficiencies, respectively. At the cellular level, AP-3 isoforms were localized to distinct neuronal domains. beta3B-containing AP-3 complexes were preferentially targeted to neuronal processes. Consistently, beta3B deficiency compromised synaptic zinc stores assessed by Timm's staining and the synaptic vesicle targeting of membrane proteins involved in zinc uptake (ZnT3 and ClC-3). Surprisingly, despite the lack of neurological symptoms, beta3A-deficient mouse brain possessed significantly increased synaptic zinc stores and synaptic vesicle content of ZnT3 and ClC-3. These observations indicate that the functions of beta3A- and beta3B-containing complexes are distinct and divergent. Our results suggest that concerted nonredundant functions of neuronal and ubiquitous AP-3 provide a mechanism to control the levels of selected membrane proteins in synaptic vesicles.     

Adaptor Protein2 (AP2) Orchestrates CXCR2‐Mediated Cell Migration

The chemokine receptor CXCR2 is vital for inflammation, wound healing, angiogenesis, cancer progression and metastasis. Adaptor protein 2 (AP2), a clathrin binding heterotetrameric protein comprised of α, β2, μ2 and σ2 subunits, facilitates clathrin‐mediated endocytosis. Mutation of the LLKIL motif in the CXCR2 carboxyl‐terminal domain (CTD) results in loss of AP2 binding to the receptor and loss of ligand‐mediated receptor internalization and chemotaxis. AP2 knockdown also results in diminished ligand‐mediated CXCR2 internalization, polarization and chemotaxis. Using knockdown/rescue approaches with AP2‐μ2 mutants, the binding domains were characterized in reference to CXCR2 internalization and chemotaxis. When in an open conformation, μ2 Patch 1 and Patch 2 domains bind tightly to membrane PIP2 phospholipids. When AP2‐μ2, is replaced with μ2 mutated in Patch 1 and/or Patch 2 domains, ligand‐mediated receptor binding and internalization are not lost. However, chemotaxis requires AP2‐μ2 Patch 1, but not Patch 2. AP2‐σ2 has been demonstrated to bind dileucine motifs to facilitate internalization. Expression of AP2‐σ2 V88D and V98S dominant negative mutants resulted in loss of CXCR2 mediated chemotaxis. Thus, AP2 binding to both membrane phosphatidylinositol phospholipids and dileucine motifs is crucial for directional migration or chemotaxis. Moreover, AP2‐mediated receptor internalization can be dissociated from AP2‐mediated chemotaxis.      

Structural analysis of the interaction between Dishevelled2 and clathrin AP-2 adaptor, a critical step in noncanonical Wnt signaling

Wnt association with its receptor, Frizzled (Fz), and recruitment by the latter of an adaptor, Dishevelled (Dvl), initiates signaling through at least two distinct pathways ("canonical" and "noncanonical"). Endocytosis and compartmentalization help determine the signaling outcome. Our previous work has shown that Dvl2 links at least one Frizzled family member (Fz4) to clathrin-mediated endocytosis by interacting with the μ2 subunit of the AP-2 clathrin adaptor, through both a classical endocytic tyrosine motif and a so-called "DEP domain." We report here the crystal structure of a chimeric protein that mimics the Dvl2-μ2 complex. The DEP domain binds at one end of the elongated, C-terminal domain of μ2. This domain:domain interface shows that parts of the μ2 surface distinct from the tyrosine-motif site can help recruit specific receptors or adaptors into a clathrin coated pit. Mutation of residues at the DEP-μ2 contact or in the tyrosine motif reduce affinity of Dvl2 for μ2 and block efficient internalization of Fz4 in response to ligation by Wnt5a. The crystal structure has thus allowed us to identify the specific interaction that leads to Frizzled uptake and to downstream, noncanonical signaling events.     

Multiple C-terminal motifs of the 46-kDa mannose 6-phosphate receptor tail contribute to efficient binding of medium chains of AP-2 and AP-3

The interaction of adaptor protein (AP) complexes with signal structures in the cytoplasmic domains of membrane proteins is required for intracellular sorting. Tyrosine- or dileucine-based motifs have been reported to bind to medium chain subunits (mu) of AP-1, AP-2, or AP-3. In the present study, we have examined the interaction of the entire 67-amino acid cytoplasmic domain of the 46-kDa mannose 6-phosphate receptor (MPR46-CT) containing tyrosine- as well as dileucine-based motifs with mu2 and mu3A chains using the yeast two-hybrid system. Both mu2 and mu3A bind specifically to the MPR46-CT. In contrast, mu3A fails to bind to the cytoplasmic domain of the 300-kDa mannose 6-phosphate receptor. Mutational analysis of the MPR46-CT revealed that the tyrosine-based motif and distal sequences rich in acidic amino acid residues are sufficient for effective binding to mu2. However, the dileucine motif was found to be one part of a consecutive complex C-terminal structure comprising tyrosine and dileucine motifs as well as clusters of acidic residues necessary for efficient binding of mu3A. Alanine substitution of 2 or 4 acidic amino acid residues of this cluster reduces the binding to mu3A much more than to mu2. The data suggest that the MPR46 is capable of interacting with different AP complexes using multiple partially overlapping sorting signals, which might depend on posttranslational modifications or subcellular localization of the receptor.     

Regulation of cargo‐selective endocytosis by dynamin 2 GTPase‐activating protein girdin

In clathrin‐mediated endocytosis (CME), specificity and selectivity for cargoes are thought to be tightly regulated by cargo‐specific adaptors for distinct cellular functions. Here, we show that the actin‐binding protein girdin is a regulator of cargo‐selective CME. Girdin interacts with dynamin 2, a GTPase that excises endocytic vesicles from the plasma membrane, and functions as its GTPase‐activating protein. Interestingly, girdin depletion leads to the defect in clathrin‐coated pit formation in the center of cells. Also, we find that girdin differentially interacts with some cargoes, which competitively prevents girdin from interacting with dynamin 2 and confers the cargo selectivity for CME. Therefore, girdin regulates transferrin and E‐cadherin endocytosis in the center of cells and their subsequent polarized intracellular localization, but has no effect on integrin and epidermal growth factor receptor endocytosis that occurs at the cell periphery. Our results reveal that girdin regulates selective CME via a mechanism involving dynamin 2, but not by operating as a cargo‐specific adaptor.     

Activation of phosphoinositide 3-kinase by interaction with Ras and by point mutation.

We have reported previously that Ras interacts with the catalytic subunit of phosphoinositide 3-kinase (PI 3-kinase) in a GTP-dependent manner. The affinity of the interaction of Ras-GTP with p85alpha/p110alpha is shown here to be approximately 150 nM. The site of interaction on the p110alpha and beta isoforms of PI 3-kinase lies between amino acid residues 133 and 314. A point mutation in this region, K227E, blocks the GTP-dependent interaction of PI 3-kinase p110alpha with Ras in vitro and the ability of Ras to activate PI 3-kinase in intact cells. In addition, this mutation elevates the basal activity of PI 3-kinase in intact cells, suggesting a direct influence of the Ras binding site on the catalytic activity of PI 3-kinase. Using an in vitro reconstitution assay, it is shown that the interaction of Ras-GTP, but not Ras-GDP, with PI 3-kinase leads to an increase in its enzymatic activity. This stimulation is synergistic with the effect of tyrosine phosphopeptide binding to p85, particularly at suboptimal peptide concentrations. These data show that PI 3-kinase is regulated by a number of mechanisms, and that Ras contributes to the activation of this lipid kinase synergistically with tyrosine kinases.