All three PACSIN isoforms bind to endocytic proteins and inhibit endocytosis

PACSIN proteins bind tubulin and promote microtubule assembly

PACSINs are intracellular adapter proteins involved in vesicle transport, membrane dynamics and actin reorganisation. In this study, we report a novel role for PACSIN proteins as components of the centrosome involved in microtubule dynamics. Glutathione S-transferase (GST)-tagged PACSIN proteins interacted with protein complexes containing alpha- and gamma-tubulin in brain homogenate. Analysis of cell lysates showed that all three endogenous PACSINs co-immunoprecipitated dynamin, alpha-tubulin and gamma-tubulin. Furthermore, PACSINs bound only to unpolymerised tubulin, not to microtubules purified from brain. In agreement, the cellular localisation of endogenous PACSIN 2 was not affected by the microtubule depolymerising reagent nocodazole. By light microscopy, endogenous PACSIN 2 localised next to gamma-tubulin at purified centrosomes from NIH 3T3 cells. Finally, reduction of PACSIN 2 protein levels with small-interfering RNA (siRNA) resulted in impaired microtubule nucleation from centrosomes, whereas microtubule centrosome splitting was not affected, suggesting a role for PACSIN 2 in the regulation of tubulin polymerisation. These findings suggest a novel function for PACSIN proteins in dynamic microtubuli nucleation.

Stimulus-specific modulation of the cation channel TRPV4 by PACSIN 3

TRPV4, a member of the vanilloid subfamily of the transient receptor potential (TRP) channels, is activated by a variety of stimuli, including cell swelling, moderate heat, and chemical compounds such as synthetic 4alpha-phorbol esters. TRPV4 displays a widespread expression in various cells and tissues and has been implicated in diverse physiological processes, including osmotic homeostasis, thermo- and mechanosensation, vasorelaxation, tuning of neuronal excitability, and bladder voiding. The mechanisms that regulate TRPV4 in these different physiological settings are currently poorly understood. We have recently shown that the relative amount of TRPV4 in the plasma membrane is enhanced by interaction with the SH3 domain of PACSIN 3, a member of the PACSIN family of proteins involved in synaptic vesicular membrane trafficking and endocytosis. Here we demonstrate that PACSIN 3 strongly inhibits the basal activity of TRPV4 and its activation by cell swelling and heat, while leaving channel gating induced by the synthetic ligand 4alpha-phorbol 12,13-didecanoate unaffected. A single proline mutation in the SH3 domain of PACSIN 3 abolishes its inhibitory effect on TRPV4, indicating that PACSIN 3 must bind to the channel to modulate its function. In line herewith, mutations at specific proline residues in the N terminus of TRPV4 abolish binding of PACSIN 3 and render the channel insensitive to PACSIN 3-induced inhibition. Taken together, these data suggest that PACSIN 3 acts as an auxiliary protein of TRPV4 channel that not only affects the channel's subcellular localization but also modulates its function in a stimulus-specific manner.

Roles of F-BAR/PCH proteins in the regulation of membrane dynamics and actin reorganization

The Pombe Cdc15 Homology (PCH) proteins have emerged in many species as important coordinators of signaling pathways that regulate actomyosin assembly and membrane dynamics. The hallmark of the PCH proteins is the presence of a Fes/CIP4 homology-Bin/Amphiphysin/Rvsp (F-BAR) domain; therefore they are commonly referred to as F-BAR proteins. The prototype F-BAR protein, Cdc15p of Schizosaccharomyces pombe, has a role in the formation of the contractile actomyosin ring during cytokinesis. Vertebrate F-BAR proteins have an established role in binding phospholipids and they participate in membrane deformations, for instance, during the internalization of transmembrane receptors. This way the F-BAR proteins will function as linkers between the actin polymerization apparatus and the machinery regulating membrane dynamics. Interestingly, some members of the F-BAR proteins are implicated in inflammatory or neurodegenerative disorders and the observations can be expected to have clinical implications for the treatment of the diseases.

ARHGAP4 is a novel RhoGAP that mediates inhibition of cell motility and axon outgrowth

This report examines the structure and function of ARHGAP4, a novel RhoGAP whose structural features make it ideally suited to regulate the cytoskeletal dynamics that control cell motility and axon outgrowth. Our studies show that ARHGAP4 inhibits the migration of NIH/3T3 cells and the outgrowth of hippocampal axons. ARHGAP4 contains an N-terminal FCH domain, a central GTPase activating (GAP) domain and a C-terminal SH3 domain. Our structure/function analyses show that the FCH domain appears to be important for spatially localizing ARHGAP4 to the leading edges of migrating NIH/3T3 cells and to axon growth cones. Our analyses also show that the GAP domain and C-terminus are necessary for ARHGAP4-mediated inhibition of cell and axon motility. These observations suggest that ARHGAP4 can act as a potent inhibitor of cell and axon motility when it is localized to the leading edge of motile cells and axons.

Protective up-regulation of CK2 by mutant huntingtin in cells co-expressing NMDA receptors

Huntington's disease is caused by a polyglutamine expansion in the huntingtin (htt) protein, and previous data indicate that over-activation of NMDA receptors (NMDARs) may be involved in the selective degeneration of cells expressing NR1/NR2B NMDARs. We used Kinetworkstrade mark multi-immunoblotting screens to examine expression of 76 protein kinases, 18 protein phosphatases, 25 heat shock/stress proteins, and 27 apoptosis proteins in human embryonic kidney 293 cells transfected with NR1/NR2B and htt containing 15 (htt-15Q; wild-type) or 138 (htt-138Q; mutant) glutamine repeats. Follow-up experiments revealed several proteins involved in the heat-shock response pathway to be up-regulated in the soluble fraction from cells expressing htt-138Q, including protein phosphatase 5 and cyclin-dependent kinase 5. Increased expression in the soluble fraction of htt-138Q-expressing cells was also noted for the stress- and calcium-activated protein-serine/threonine kinase casein kinase 2, a change which was confirmed in striatal tissue of yeast artificial chromosome transgenic mice expressing full-length mutant htt. Inhibition of casein kinase 2 activity in cultured striatal neurons from these mice significantly exacerbated NMDAR-mediated toxicity, as assessed by labeling of apoptotic nuclei. Our findings are consistent with up-regulation of components of the stress response pathway in the presence of polyglutamine-expanded htt and NR1/NR2B which may reflect an attempt at the cellular level to ameliorate the detrimental effects of mutant htt expression.

The K15 protein of Kaposi's sarcoma-associated herpesvirus recruits the endocytic regulator intersectin 2 through a selective SH3 domain interaction

Kaposi's sarcoma-associated herpesvirus, also known as human herpesvirus 8, is closely associated with several cancers including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The rightmost end of the KSHV genome encodes a protein, K15, with multiple membrane-spanning segments and an intracellular carboxy-terminal tail that contains several conserved motifs with the potential to recruit interaction domains (i.e., SH2, SH3, TRAF) of host cell proteins. K15 has been implicated in downregulating B cell receptor (BCR) signaling through its conserved motifs and may thereby play a role in maintaining viral latency and/or preventing apoptosis of the infected B cells. However, K15's mode of action is largely unknown. We have used mass spectrometry, domain and peptide arrays, and surface plasmon resonance to identify binding partners for a conserved proline-rich sequence (PPLP) in the K15 cytoplasmic tail. We show that the PPLP motif selectively binds the SH3-C domain of an endocytic adaptor protein, Intersectin 2 (ITSN2). This interaction can be observed both in vitro and in cells, where K15 and ITSN2 colocalize to discrete compartments within the B cell. The ability of K15 to associate with ITSN2 suggests a new role for the K15 viral protein in intracellular protein trafficking.

PACSIN 1 forms tetramers via its N-terminal F-BAR domain

The ability of protein kinase C and casein kinase 2 substrate in neurons (PACSIN)/syndapin proteins to self-polymerize is crucial for the simultaneous interactions with more than one Src homology 3 domain-binding partner or with lipid membranes. The assembly of this network has profound effects on the neural Wiskott-Aldrich syndrome protein-mediated attachment of the actin polymerization machinery to vesicle membranes as well as on the movement of the corresponding vesicles. Also, the sensing of vesicle membranes and/or the induction of membrane curvature are more easily facilitated in the presence of larger PACSIN complexes. The N-terminal Fes-CIP homology and Bin-Amphiphysin-Rvs (F-BAR) domains of several PACSIN-related proteins have been shown to mediate self-interactions, whereas studies using deletion mutants derived from closely related proteins led to the view that oligomerization depends on the formation of a trimeric complex via a coiled-coil region present in these molecules. To address whether the model of trimeric complex formation is applicable to PACSIN 1, the protein was recombinantly expressed and tested in four different assays for homologous interactions. The results showed that PACSIN 1 forms tetramers of about 240 kDa, with the self-interaction having a K(D) of 6.4 x 10(-8) M. Ultrastructural analysis of these oligomers after negative staining showed that laterally arranged PACSIN molecules bind to each other via a large globular domain and form a barrel-like structure. Together, these results demonstrate that the N-terminal F-BAR domain of PACSIN 1 forms the contact site for a tetrameric structure, which is able to simultaneously interact with multiple Src homology 3 binding partners.

Differential Requirements for ts-O45-G and Procollagen Biosynthetic Transport

Emerging experimental evidence favours the existence of cargo sorting occurring upon the endoplasmic reticulum (ER) exit. Recent studies revealed that, in contrast to the conventional secretory marker ts-O45-G, procollagen (PC I) exits the ER at sites not coated with coat protein II and is transported to the Golgi complex in carriers devoid of coat protein I. Here, we investigated whether PC I trafficking requires a different molecular machinery in comparison with the ts-O45-G. By combining colocalization of the cargoes with endogenous markers, downregulation of transport machinery by RNA interference and knock-ins by complementary DNA over-expression, we provide strong evidence that PC I and ts-O45-G have common but also different molecular requirements during pre- and post-Golgi trafficking events.

Intracellular trafficking of TRP channels

Thirteen years ago, it was suggested that exocytotic insertion of store-operated channels into the plasma membrane lead to increased Ca(2+) entry in non-excitable cells upon G protein-coupled or tyrosine kinase receptor stimulation. Since the discovery of the TRP channel superfamily and their involvement in receptor-induced Ca(2+) entry, many studies have shown that different members of the TRP superfamily translocate into the plasma membrane upon stimulation. While the exact molecular mechanism by which TRP channels insert into the plasma membrane is unknown, TRP-binding proteins have been shown to directly regulate this trafficking. This review summarizes recent advances related to the mechanism of TRP channel trafficking, focusing on the role of TRP-binding proteins.

Pombe Cdc15 homology (PCH) proteins: coordinators of membrane-cytoskeletal interactions

Cellular adhesion, motility, endocytosis, exocytosis and cytokinesis involve the coordinated reorganization of the cytoskeleton and of the plasma membrane. The 'Pombe Cdc15 homology' (PCH) family of adaptor proteins has recently been shown to coordinate the membrane and cytoskeletal dynamics involved in these processes by curving membranes, recruiting dynamin and controlling the architecture of the actin cytoskeleton. Mutations in PCH family members or proteins that interact with them are associated with autoinflammatory, neurological or neoplastic diseases. Here, we review the nature, actions and disease associations of the vertebrate PCH family members, highlighting their fundamental roles in the regulation of processes involving membrane-cytoskeletal interactions.

PACSIN3 overexpression increases adipocyte glucose transport through GLUT1

PACSIN family members regulate intracellular vesicle trafficking via their ability to regulate cytoskeletal rearrangement. These processes are known to be involved in trafficking of GLUT1 and GLUT4 in adipocytes. In this study, PACSIN3 was observed to be the only PACSIN isoform that increases in expression during 3T3-L1 adipocyte differentiation. Overexpression of PACSIN3 in 3T3-L1 adipocytes caused an elevation of glucose uptake. Subcellular fractionation revealed that PACSIN3 overexpression elevated GLUT1 plasma membrane localization without effecting GLUT4 distribution. In agreement with this result, examination of GLUT exofacial presentation at the cell surface by photoaffinity labeling revealed significantly increased GLUT1, but not GLUT4, after overexpression of PACSIN3. These results establish a role for PACSIN3 in regulating glucose uptake in adipocytes via its preferential participation in GLUT1 trafficking. They are consistent with the proposal, which is supported by a recent study, that GLUT1, but not GLUT4, is predominantly endocytosed via the coated pit pathway in unstimulated 3T3-L1 adipocytes.

Interaction of SPIN90 with syndapin is implicated in clathrin-mediated endocytic pathway in fibroblasts

SPIN90, a 90-kDa Nck-interacting protein with a SH3 domain, plays a role in sarcomere formation and myofibril assembly, and its phosphorylation is modulated by cell adhesion and Erk activation. Here we demonstrate that SPIN90 participates in receptor-mediated endocytic pathway in fibroblasts. We identified syndapin (synaptic dynamin-binding protein) as a SPIN90 interacting protein using yeast two-hybrid screening. SPIN90 directly binds the SH3 domain of syndapin via its proline rich domain in vitro and in vivo and also associates with clathrin. Over-expression of SPIN90-full length in COS-7 cells inhibited transferrin uptake, a marker of endocytosis. Interestingly, SPIN90-PRD, a syndapin-binding domain, significantly inhibited endocytosis, and the inhibition was reversed by co-expression of syndapin. Depleting SPIN90 through antibody microinjection or Knocking it down using siRNAs also significantly inhibited transferrin internalization. Moreover, early endosomal marker proteins (EEA1 and Rab5) appeared to closely associate or partially co-localize with SPIN90 in endosomes and an internalized FITC-dextran and Texas Red-EGF were found on the endosomes in association with SPIN90. Time-lapse video showed that GFP-SPIN90 travels with moving vesicles within living cells. Taken together, these findings suggest that SPIN90 is implicated in receptor-mediated endocytic pathway in fibroblasts.

Pombe Cdc15 homology proteins: regulators of membrane dynamics and the actin cytoskeleton

Pombe Cdc15 homology (PCH) proteins have emerged in many species as important coordinators of signalling pathways that regulate actomyosin assembly and membrane dynamics. For example, the prototype PCH protein, Cdc15p of Schizosaccharomyces pombe, has a role in assembly of the contractile ring, which is needed to separate dividing cells. Recently, mammalian PCH proteins have been found to bind phospholipids and to participate in membrane deformation. These findings suggest that PCH proteins are crucial linkers of membrane dynamics and actin polymerization, for example, during the internalization of transmembrane receptors. Intriguingly, some members of the PCH protein family are mutated in neurodegenerative and inflammatory diseases, which has implications for the identification of cures for such disorders.

Bar domain proteins: a role in tubulation, scission and actin assembly in clathrin-mediated endocytosis

Endocytosis is an important way for cells to take up liquids and particles from their environment. It requires membrane bending to be coupled with membrane fission, and the actin cytoskeleton has an active role in membrane remodelling. Here, we review recent research into the function of Bin-Amphiphysin-Rvs (BAR) domain proteins, which can sense membrane curvature and recruit actin to membranes. BAR proteins interact with the endocytic and cytoskeletal machinery, including the GTPase dynamin (which mediates vesicle fission), N-WASP (an Arp2/3 complex regulator) and synaptojanin (a phosphoinositide phosphatase). We describe three classes of BAR domains, BAR, N-BAR and F-BAR, providing examples of each discussing and how they function in linking membranes to the actin cytoskeleton in endocytosis.

Regulation of FasL expression: A SH3 domain containing protein family involved in the lysosomal association of FasL

As a death factor of T cells and Natural Killer (NK) cells, Fas Ligand (FasL) is stored in association with secretory lysosomes. Upon stimulation, these cytotoxic granules are transported to the cell membrane where FasL is exposed on the cell surface, shed or secreted. It has been noted before that the proline-rich domain within the cytosolic part of FasL is required for its vesicular association. However, the molecular interactions involved in targeting FasL to secretory lysosomes or to the plasma membrane have not been elucidated. We now identified a family of structurally related proteins that upon co-expression with FasL reallocate the death factor from a membrane to an intracellular localization. Members of this protein family are characterized by a similar domain structure and include FBP17, PACSIN1-3, CD2BP1, CIP4, Rho-GAP C1 and several hypothetical proteins. We show that all tested members of this "FCH/SH3-family" co-precipitate FasL from transfectants. The interactions strictly depend on functional SH3 domains within the FCH/SH3 proteins. Since co-expression of FasL with individual FCH/SH3 proteins dramatically alters the intracellular localization of FasL especially in non-hematopoietic cells, our data suggest that FCH/SH3 proteins might play an important role for the subcellular distribution and lysosomal association of FasL.

The diaphanous-related formin DAAM1 collaborates with the Rho GTPases RhoA and Cdc42, CIP4 and Src in regulating cell morphogenesis and actin dynamics

Binding partners for the Cdc42 effector CIP4 were identified by the yeast two-hybrid system, as well as by testing potential CIP4-binding proteins in coimmunoprecipitation experiments. One of the CIP4-binding proteins, DAAM1, was characterised in more detail. DAAM1 is a ubiquitously expressed member of the mammalian diaphanous-related formins, which include proteins such as mDia1 and mDia2. DAAM1 was shown to bind to the SH3 domain of CIP4 in vivo. Ectopically expressed DAAM1 localised in dotted pattern at the dorsal side of transfected cells and the protein was accumulated in the proximity to the microtubule organising centre. Moreover, ectopic expression of DAAM1 induced a marked alteration of the cell morphology, seen as rounding up of the cells, the formation of branched protrusions as well as a reduction of stress-fibres in the transfected cells. Coimmunoprecipitation experiments demonstrated that DAAM1 bound to RhoA and Cdc42 in a GTP-dependent manner. Moreover, DAAM1 was found to interact and collaborate with the non-receptor tyrosine kinase Src in the formation of branched protrusions. Taken together, our data indicate that DAAM1 communicates with Rho GTPases, CIP4 and Src in the regulation of the signalling pathways that co-ordinate the dynamics of the actin filament system.

PACSINs bind to the TRPV4 cation channel. PACSIN 3 modulates the subcellular localization of TRPV4

TRPV4 is a cation channel that responds to a variety of stimuli including mechanical forces, temperature, and ligand binding. We set out to identify TRPV4-interacting proteins by performing yeast two-hybrid screens, and we isolated with the avian TRPV4 amino terminus the chicken orthologues of mammalian PACSINs 1 and 3. The PACSINs are a protein family consisting of three members that have been implicated in synaptic vesicular membrane trafficking and regulation of dynamin-mediated endocytotic processes. In biochemical interaction assays we found that all three murine PACSIN isoforms can bind to the amino terminus of rodent TRPV4. No member of the PACSIN protein family was able to biochemically interact with TRPV1 and TRPV2. Co-expression of PACSIN 3, but not PACSINs 1 and 2, shifted the ratio of plasma membrane-associated versus cytosolic TRPV4 toward an apparent increase of plasma membrane-associated TRPV4 protein. A similar shift was also observable when we blocked dynamin-mediated endocytotic processes, suggesting that PACSIN 3 specifically affects the endocytosis of TRPV4, thereby modulating the subcellular localization of the ion channel. Mutational analysis shows that the interaction of the two proteins requires both a TRPV4-specific proline-rich domain upstream of the ankyrin repeats of the channel and the carboxyl-terminal Src homology 3 domain of PACSIN 3. Such a functional interaction could be important in cell types that show distribution of both proteins to the same subcellular regions such as renal tubule cells where the proteins are associated with the luminal plasma membrane.

Coordination between the actin cytoskeleton and membrane deformation by a novel membrane tubulation domain of PCH proteins is involved in endocytosis

The conserved FER-CIP4 homology (FCH) domain is found in the pombe Cdc15 homology (PCH) protein family members, including formin-binding protein 17 (FBP17). However, the amino acid sequence homology extends beyond the FCH domain. We have termed this region the extended FC (EFC) domain. We found that FBP17 coordinated membrane deformation with actin cytoskeleton reorganization during endocytosis. The EFC domains of FBP17, CIP4, and other PCH protein family members show weak homology to the Bin-amphiphysin-Rvs (BAR) domain. The EFC domains bound strongly to phosphatidylserine and phosphatidylinositol 4,5-bisphosphate and deformed the plasma membrane and liposomes into narrow tubules. Most PCH proteins possess an SH3 domain that is known to bind to dynamin and that recruited and activated neural Wiskott-Aldrich syndrome protein (N-WASP) at the plasma membrane. FBP17 and/or CIP4 contributed to the formation of the protein complex, including N-WASP and dynamin-2, in the early stage of endocytosis. Furthermore, knockdown of endogenous FBP17 and CIP4 impaired endocytosis. Our data indicate that PCH protein family members couple membrane deformation to actin cytoskeleton reorganization in various cellular processes.

Syndapin oligomers interconnect the machineries for endocytic vesicle formation and actin polymerization

Syndapins were proposed to interconnect the machineries for vesicle formation and actin polymerization, as they interact with dynamin and the Arp2/3 complex activator N-WASP (neural Wiskott-Aldrich syndrome protein). Syndapins, however, have only one Src homology 3 domain mediating both interactions. Here we show that syndapins self-associate via direct syndapin/syndapin interactions, providing a molecular mechanism for the coordinating role of syndapin. Cross-link studies with overexpressed and endogenous syndapins suggest that predominantly dimers form in vivo. Our analyses show that the N-terminal Fes/Cip4 homology domain but not the central coiled-coil domain is sufficient for oligomerization. Additionally, a second interface located further C-terminally mediated interactions with the N terminus. The Src homology 3 domain and the NPF region are not involved and thus available for further interactions interconnecting different syndapin binding partners. Our analyses showed that self-association is crucial for syndapin function. Both syndapin-mediated cytoskeletal rearrangements and endocytosis were disrupted by a self-association-deficient mutant. Consistent with a role of syndapins in linking actin polymerization bursts with endocytic vesicle formation, syndapin-containing complexes had a size of 300-500 kDa in gel filtration analysis and contained both dynamin and N-WASP. The existence of an interconnection of the GTPase dynamin with N-WASP via syndapin oligomers was demonstrated both by coimmunoprecipitations and by reconstitution at membranes in intact cells. The interconnection was disrupted by coexpression of syndapin mutants incapable of self-association. Syndapin oligomers may thus act as multivalent organizers spatially and temporally coordinating vesicle fission with local actin polymerization.

Dynamin and the actin cytoskeleton cooperatively regulate plasma membrane invagination by BAR and F-BAR proteins

Cell membranes undergo continuous curvature changes as a result of membrane trafficking and cell motility. Deformations are achieved both by forces extrinsic to the membrane as well as by structural modifications in the bilayer or at the bilayer surface that favor the acquisition of curvature. We report here that a family of proteins previously implicated in the regulation of the actin cytoskeleton also have powerful lipid bilayer-deforming properties via an N-terminal module (F-BAR) similar to the BAR domain. Several such proteins, like a subset of BAR domain proteins, bind to dynamin, a GTPase implicated in endocytosis and actin dynamics, via SH3 domains. The ability of BAR and F-BAR domain proteins to induce tubular invaginations of the plasma membrane is enhanced by disruption of the actin cytoskeleton and is antagonized by dynamin. These results suggest a close interplay between the mechanisms that control actin dynamics and those that mediate plasma membrane invagination and fission.

FCH/Cdc15 domain determines distinct subcellular localization of NOSTRIN

NOSTRIN, an NO synthase binding protein, belongs to the PCH family of proteins, exposing a typical domain structure. While its SH3 domain and the C-terminal coiled-coil region cc2 have been studied earlier, the function of the N-terminal half comprising a Cdc15 domain with an FCH (Fes/CIP homology) region followed by a coiled-coil stretch cc1 is unknown. Here, we show that the FCH region is necessary and sufficient for membrane association of NOSTRIN, whereas the Cdc15 domain further specifies subcellular distribution of the protein. Thus, the FCH region and the Cdc15 domain fulfill complementary functions in subcellular targeting of NOSTRIN.

NOSTRIN functions as a homotrimeric adaptor protein facilitating internalization of eNOS

Intracellular trafficking of endothelial nitric oxide synthase (eNOS) between different compartments is incompletely understood. Recently, we described a novel eNOS-interacting protein, NOSTRIN, which upon overexpression drives eNOS away from the plasma membrane towards intracellular compartments. Sequence similarity of NOSTRIN and pacsins/syndapins suggested a role for NOSTRIN in endocytosis. Accordingly, we show here that NOSTRIN interacts with the large GTPase dynamin and the actin nucleation promoting factor N-WASP by means of its SH3 domain, which also represents the docking site for eNOS. Via a coiled-coil region in the C-terminal portion of the protein, NOSTRIN oligomerizes, mainly forming trimers, which would allow simultaneous interaction with multiple binding partners of the SH3 domain. Consistent with this notion, expression of dynamin-2-GFP in CHO cells stably expressing eNOS (CHO-eNOS) results in recruitment of eNOS to dynamin-positive structures, only when NOSTRIN is present as well. Similarly, when N-WASP-GFP and NOSTRIN are co-expressed in CHO-eNOS cells, both proteins strongly co-localize with eNOS and are recruited to structures running along actin filaments. If, however, the actin cytoskeleton is depolymerized by cytochalasin D, NOSTRIN and eNOS are associated with extended structures in the cell periphery, possibly being unable to leave the plasma membrane. Together, these results indicate that NOSTRIN may facilitate endocytosis of eNOS by coordinating the function of dynamin and N-WASP.

Proteomic analysis of synaptosomes using isotope-coded affinity tags and mass spectrometry

Synaptosomes are isolated synapses produced by subcellular fractionation of brain tissue. They contain the complete presynaptic terminal, including mitochondria and synaptic vesicles, and portions of the postsynaptic side, including the postsynaptic membrane and the postsynaptic density (PSyD). A proteomic characterisation of synaptosomes isolated from mouse brain was performed employing the isotope-coded affinity tag (ICAT) method and tandem mass spectrometry (MS/MS). After isotopic labelling and tryptic digestion, peptides were fractionated by cation exchange chromatography and cysteine-containing peptides were isolated by affinity chromatography. The peptides were identified by microcapillary liquid chromatography-electrospray ionisation MS/MS (muLC-ESI MS/MS). In two experiments, peptides representing a total of 1131 database entries were identified. They are involved in different presynaptic and postsynaptic functions, including synaptic vesicle exocytosis for neurotransmitter release, vesicle endocytosis for synaptic vesicle recycling, as well as postsynaptic receptors and proteins constituting the PSyD. Moreover, a large number of soluble and membrane-bound molecules serving functions in synaptic signal transduction and metabolism were detected. The results provide an inventory of the synaptic proteome and confirm the suitability of the ICAT method for the assessment of synaptic structure, function and plasticity.

Mutually exclusive interactions of EHD1 with GS32 and syndapin II

GS32/SNAP-29 is a SNAP-25-like SNARE and has been shown to interact with syntaxin 6. Using immobilized recombinant GS32, we have recovered EHD1 as a major GS32-interacting protein from total HeLa cell extracts. This interaction is mediated by the EH domain of EHD1 and the N-terminal NPF-containing 17-residue region of GS32. Co-immunoprecipitation suggests that GS32 could also interact with EHD1 in intact cells. When immobilized GST-EHD1 was used to fish out interacting proteins from total brain extracts, syndapin II was identified as a major interacting partner. Similar to the GS32-EHD1 interaction, syndapin II also interacts with the EH domain of EHD1 via its NPF repeat region. Interaction of endogenous EHD1 and syndapin II was also established by co-immunoprecipitation. Furthermore, interaction of GS32 and syndapin II with EHD1 was shown to be mutually exclusive, suggesting that EHD1 may regulate/participate in the functional pathways of both GS32 and syndapin II in a mutual exclusive manner. Opposing roles of GS32 and syndapin II in regulating the surface level of transferrin receptor (TfR) were observed.

The PCH family member MAYP/PSTPIP2 directly regulates F-actin bundling and enhances filopodia formation and motility in macrophages

Macrophage actin-associated tyrosine phosphorylated protein (MAYP) belongs to the Pombe Cdc15 homology (PCH) family of proteins involved in the regulation of actin-based functions including cell adhesion and motility. In mouse macrophages, MAYP is tyrosine phosphorylated after activation of the colony-stimulating factor-1 receptor (CSF-1R), which also induces actin reorganization, membrane ruffling, cell spreading, polarization, and migration. Because MAYP associates with F-actin, we investigated the function of MAYP in regulating actin organization in macrophages. Overexpression of MAYP decreased CSF-1-induced membrane ruffling and increased filopodia formation, motility and CSF-1-mediated chemotaxis. The opposite phenotype was observed with reduced expression of MAYP, indicating that MAYP is a negative regulator of CSF-1-induced membrane ruffling and positively regulates formation of filopodia and directional migration. Overexpression of MAYP led to a reduction in total macrophage F-actin content but was associated with increased actin bundling. Consistent with this, purified MAYP bundled F-actin and regulated its turnover in vitro. In addition, MAYP colocalized with cortical and filopodial F-actin in vivo. Because filopodia are postulated to increase directional motility by acting as environmental sensors, the MAYP-stimulated increase in directional movement may be at least partly explained by enhancement of filopodia formation.

Neural Wiskott Aldrich Syndrome Protein (N-WASP) and the Arp2/3 complex are recruited to sites of clathrin-mediated endocytosis in cultured fibroblasts

Several findings suggest that actin-mediated motility can play a role in clathrin-mediated endocytosis but it remains unclear whether and when key proteins required for this process are recruited to endocytic sites. Here we investigate this question in live Swiss 3T3 cells using two-colour evanescent field (EF) microscopy. We find that Arp3, a component of the Arp2/3 complex, appears transiently while single clathrin-coated pits internalize. There is also additional recruitment of Neural-Wiskott Aldrich Syndrome Protein (N-WASP), a known activator of the Arp2/3 complex. Both proteins appear at about the same time as actin. We suggest that N-WASP and the Arp2/3 complex trigger actin polymerization during a late step in clathrin-mediated endocytosis, and propel clathrin-coated pits or vesicles from the plasma membrane into the cytoplasm.

The stimulatory action of amphiphysin on dynamin function is dependent on lipid bilayer curvature

Amphiphysin is a major dynamin-binding partner at the synapse; however, its function in fission is unclear. Incubation of large unilamellar liposomes with mice brain cytosol led to massive formation of small vesicles, whereas cytosol of amphiphysin 1 knockout mice was much less efficient in this reaction. Vesicle formation from large liposomes by purified dynamin was also strongly enhanced by amphiphysin. In the presence of liposomes, amphiphysin strongly affected dynamin GTPase activity and the recruitment of dynamin to the liposomes, but this activity was highly dependent on liposome size. Deletion from amphiphysin of its central proline-rich stretch dramatically potentiated its effect on dynamin, possibly by relieving an inhibitory intramolecular interaction. These results suggest a model in which maturation of endocytic pits correlates with the oligomerization of dynamin with either amphiphysin or other proteins with similar domain structure. Formation of these complexes is coupled to the activation of dynamin GTPase activity, thus explaining how deep invagination of the pit leads to fission.

A novel dynamin-associating molecule, formin-binding protein 17, induces tubular membrane invaginations and participates in endocytosis

Dynamin associates with a variety of SH3 domain-containing molecules via a C-terminal proline-rich motif and takes part, with them, in endocytic processes. Here, we have investigated a new dynamin-associating molecule, formin-binding protein 17 (FBP17), involved in deforming the plasma membrane and in endocytosis. FBP17 formed tubular invaginations originating from the plasma membrane. Its N-terminal Fer/CIP4 homology domain, a coiled-coil domain, and a proline-rich motif were required for tubular invagination and self-assembly, by which tubular invagination might be induced. Using anti-FBP17 antibody, we detected positive immunoreactions in the testis that were restricted to the germ cells. We also detected FBP17 in the brain by immunoblotting and in situ hybridization. When COS cells expressing enhanced green fluorescent protein-tagged FBP17 were incubated with fluorescently labeled transferrin, epidermal growth factor, and cholera toxin, these molecules co-localized with FBP17-induced tubular invaginations, suggesting that FBP17 is involved in dynamin-mediated endocytosis in both a clathrin-dependent and -independent manner. These observations therefore indicate that FBP17 interacts with dynamin and regulates endocytosis by forming vesicotubular structures.

The syndapin protein family: linking membrane trafficking with the cytoskeleton

Syndapins – also called PACSINs – are highly conserved Src-homology 3 (SH3)-domain-containing proteins that seem to exist in all multicellular eukaryotes. They interact with the large GTPase dynamin and several other proteins implicated in vesicle trafficking. Syndapin-dynamin complexes appear to play an important role in vesicle fission at different donor membranes, including the plasma membrane (endocytosis) and Golgi membranes. In addition, syndapins are implicated in later steps of vesicle cycling in neuronal and non-neuronal cells. Syndapins also interact with N-WASP, a potent activator of the Arp2/3 complex that forms a critical part of the actin polymerization machinery. Syndapin oligomers can thereby couple bursts of actin polymerization with the vesicle fission step involving dynamins. This allows newly formed vesicles to move away from the donor membrane driven by actin polymerization. Syndapins also engage in additional interactions with molecules involved in several signal transduction pathways, producing crosstalk at the interface between membrane trafficking and the cytoskeleton. Given the distinct expression patterns of the different syndapins and their splice forms, these proteins could have isoform-specific functions.

Actin assembly and endocytosis: from yeast to mammals

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

A novel proteomic screen for peptide-protein interactions

Regulated interactions between short, unstructured amino acid sequences and modular protein domains are central to cell signaling. Here we use synthetic peptides in "active" (e.g. phosphorylated) and "control" (e.g. non-phosphorylated) forms as baits in affinity pull-down experiments to determine such interactions by quantitative proteomics. Stable isotope labeling by amino acids in cell culture distinguishes specific binders directly by the isotope ratios determined by mass spectrometry (Blagoev, B., Kratchmarova, I., Ong, S.-E., Nielsen, M., Foster, L. J., and Mann, M. (2003) Nat. Biotechnol. 21, 315-318). A tyrosine-phosphorylated peptide of the epidermal growth factor receptor specifically retrieved the Src homology domain (SH) 2- and SH3 domain-containing adapter protein Grb2. A proline-rich sequence of Son of Sevenless also specifically bound Grb2, demonstrating that the screen maintains specificity with low affinity interactions. The proline-rich Sos peptide retrieved only SH3 domain containing proteins as specific binding partners. Two of these, Pacsin 3 and Sorting Nexin 9, were confirmed by immunoprecipitation. Our data are consistent with a change in the role of Sos from Ras-dependent signaling to actin remodeling/endocytic signaling events by a proline-SH3 domain switch.

PACSIN3 binds ADAM12/meltrin alpha and up-regulates ectodomain shedding of heparin-binding epidermal growth factor-like growth factor

A disintegrin and metalloprotease 12 (ADAM12/meltrin alpha) is a key enzyme implicated in the ectodomain shedding of membrane-anchored heparin-binding epidermal growth factor (EGF)-like growth factor (proHB-EGF)-dependent epidermal growth factor receptor (EGFR) transactivation. However, the activation mechanisms of ADAM12 are obscure. To determine how ADAM12 is activated, we screened proteins that bind to the cytoplasmic domain of ADAM12 using a yeast two-hybrid system and identified a protein called PACSIN3 that contains a Src homology 3 domain. An analysis of interactions between ADAM12 and PACSIN3 using glutathione S-transferase fusion protein revealed that a proline-rich region (amino acid residues 829-840) of ADAM12 was required to bind PACSIN3. Furthermore, co-immunoprecipitation and co-localization analyses of ADAM12 and PACSIN3 proteins also revealed their interaction in mammalian cells expressing both of them. The overexpression of PACSIN3 in HT1080 cells enhanced 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced proHB-EGF shedding. Furthermore, knockdown of endogenous PACSIN3 by small interfering RNA in HT1080 cells significantly attenuated the shedding of proHB-EGF induced by TPA and angiotensin II. Our data indicate that PACSIN3 has a novel function as an up-regulator in the signaling of proHB-EGF shedding induced by TPA and angiotensin II.

Actin microfilaments et al.--the many components, effectors and regulators of epithelial cell endocytosis

The aim of this review is to introduce the advances made over the past several years regarding the participation of actin and actin-associated proteins in clathrin-mediated endocytosis in simple cell models, and then to consider the evidence for the involvement of these effectors in apical clathrin-mediated endocytosis in epithelial cells. Basic mechanisms of clathrin-mediated endocytosis are initially addressed, followed by a detailed description of the actin cytoskeleton: its organization, function and, most importantly, the essential role played by proteins and signaling pathways responsible for the regulation of actin filament dynamics. Our focus then shifts to the GTPase, dynamin and its pivotal role as a bridge between various components of the clathrin endocytic machinery and the actin cytoskeleton. Mechanisms and effectors of dynamin-dependent endocytosis are then described, with a particular emphasis on novel proteins, which link dynamin to actin filaments. We consider additional effectors proposed to interact with actin to facilitate clathrin-mediated endocytosis in a dynamin-independent manner. The multiple roles which actin filaments are thought to play in endocytosis are addressed followed by a more detailed characterization of actin filament participation specifically in apical endocytosis. We conclude by discussing how these concepts may be integrated to improve drug internalization at the apical plasma membrane of epithelial cells.

Slowly getting a clue on CD95 ligand biology

Since the ligand for the death factor CD95 (CD95L) was identified almost a decade ago, it has been established that this molecule (CD95L, FasL, Apo-1L, CD178, TNFSF6, APT1LG1) has multiple immunoregulatory and pathophysiologically relevant functions. CD95L does not only act as a death factor when externalized with secretory lysosomes on cytotoxic T and NK cells or when expressed on CD4(+) T cells in the course of activation-induced cell death, it is also a key molecule for the establishment of immune privilege or tumor cell survival and may serve as a costimulatory molecule during T cell activation. Moreover, alterations of expression or shedding of different forms of CD95L are associated with many diseases including various malignancies, HIV infection, autoimmune disorders (systemic lupus erythematodes, rheumatoid arthritis), acute myocardial infarction, traumatic injury and many others. In most cases, however, the physiological link between altered CD95L expression and pathophysiology is unknown. Given the potency of the molecule to regulate death and survival of many different cell types, the control of CD95L production, transport, storage, shedding and release is of tremendous biological and clinical interest. This commentary aims at briefly summarizing the current knowledge, hypotheses and controversies about CD95L as a multifunctional ligand and receptor. It touches upon the complex networks of intracellular dynamics of protein transport and trafficking and the potential bidirectional signal transduction capacity of CD95L with a focus on molecular interactions that have been worked out over the past years.

Huntington's disease: a synaptopathy?

Huntington's disease (HD) is caused by a polyglutamine expansion in the protein huntingtin. In its terminal stage, HD is characterized by widespread neuronal death in the neocortex and the striatum. Classically, this neuronal death has been thought to underlie most of the symptoms of the disease. Accumulating evidence suggests, however, that cellular dysfunction is important in the pathogenesis of HD. We propose that specific impairment of the exocytosis and endocytosis machinery contributes to the development of HD. We also suggest that abnormal synaptic transmission underlies the early symptoms of HD and can contribute to the triggering of cell death in later stages of the disease.

Tuba, a novel protein containing bin/amphiphysin/Rvs and Dbl homology domains, links dynamin to regulation of the actin cytoskeleton

Tuba is a novel scaffold protein that functions to bring together dynamin with actin regulatory proteins. It is concentrated at synapses in brain and binds dynamin selectively through four N-terminal Src homology-3 (SH3) domains. Tuba binds a variety of actin regulatory proteins, including N-WASP, CR16, WAVE1, WIRE, PIR121, NAP1, and Ena/VASP proteins, via a C-terminal SH3 domain. Direct binding partners include N-WASP and Ena/VASP proteins. Forced targeting of the C-terminal SH3 domain to the mitochondrial surface can promote accumulation of F-actin around mitochondria. A Dbl homology domain present in the middle of Tuba upstream of a Bin/amphiphysin/Rvs (BAR) domain activates Cdc42, but not Rac and Rho, and may thus cooperate with the C terminus of the protein in regulating actin assembly. The BAR domain, a lipid-binding module, may functionally replace the pleckstrin homology domain that typically follows a Dbl homology domain. The properties of Tuba provide new evidence for a close functional link between dynamin, Rho GTPase signaling, and the actin cytoskeleton.

Expression profiling of osteosarcoma cells transfected with MDR1 and NEO genes: regulation of cell adhesion, apoptosis, and tumor suppression-related genes

The expression patterns of the osteosarcoma cell line U-2 OS, and three derived subclones containing stably transfected MDR1, NEO and MDR1/NEO genes were compared using cDNA microarrays comprising 8976 known genes and expressed sequenced tags. Data provided new insights into three critical issues. First, MDR1 overexpression was associated with altered expression of genes related to several cellular pathways, including (a). drug influx/efflux (eg, dynamin 3), (b). metabolic enzymes (eg, monoamine oxidase A), (c). cell adhesion (eg, EPCAM), (d). apoptotic signaling (eg, I-TRAF), (e). senescence (eg, telomerase RNA binding protein staufen), (f). tumor suppression-related genes (eg, KISS-1 and ephrin B3), and (g). immune system receptors (eg, LENG2). MDR1, EPCAM, and ephrin B3 expression was confirmed by immunohistochemistry. Second, MDR1 transfected cells selected with either doxorubicin or neomycin showed distinct expression profiles that could be related to differential selection. Moreover, hierarchical clustering indicated that cells transfected with MDR1 alone, or cotransfected with NEO, displayed more closely related expression profiles than cells transfected only with NEO. Third, transfection with NEO and selection with neomycin produced a considerable number of expression changes within the cell. This study further elucidates the genetic events associated with MDR1 expression and identifies novel targets associated with multidrug resistance.

Neural Wiskott-Aldrich syndrome protein is recruited to rafts and associates with endophilin A in response to epidermal growth factor

Neural Wiskott-Aldrich syndrome protein (N-WASP) has been implicated in endocytosis; however, little is known about how it interacts functionally with the endocytic machinery. Sucrose gradient fractionation experiments and immunofluorescence studies with anti-N-WASP antibody revealed that N-WASP is recruited together with clathrin and dynamin, which play essential roles in clathrin-mediated endocytosis, to lipid rafts in an epidermal growth factor (EGF)-dependent manner. Endophilin A (EA) binds to dynamin and plays an essential role in the fission step of clathrin-mediated endocytosis. In the present study, we show that the Src homology 3 (SH3) domain of EA associates with the proline-rich domain of N-WASP and dynamin in vitro. Co-immunoprecipitation assays with anti-N-WASP antibody revealed that EGF induces association of N-WASP with EA. In addition, EA enhances N-WASP-induced actin-related protein 2/3 (Arp2/3) complex activation in vitro. Immunofluorescence studies revealed that actin accumulates at sites where N-WASP and EA are co-localized after EGF stimulation. Furthermore, studies of overexpression of the SH3 domain of EA indicate that EA may regulate EGF-induced recruitment of N-WASP to lipid rafts. These results suggest that, upon EGF stimulation, N-WASP interacts with EA through its proline-rich domain to induce the fission step of clathrin-mediated endocytosis.

Characterization of Endophilin B1b, a brain-specific membrane-associated lysophosphatidic acid acyl transferase with properties distinct from endophilin A1

We have characterized mammalian endophilin B1, a novel member of the endophilins and a representative of their B subgroup. The endophilins B show the same domain organization as the endophilins A, which contain an N-terminal domain responsible for lipid binding and lysophosphatidic acid acyl transferase activity, a central coiled-coil domain for oligomerization, a less conserved linker region, and a C-terminal Src homology 3 (SH3) domain. The endophilin B1 gene gives rise to at least three splice variants, endophilin B1a, which shows a widespread tissue distribution, and endophilins B1b and B1c, which appear to be brain-specific. Endophilin B1, like endophilins A, binds to palmitoyl-CoA, exhibits lysophosphatidic acid acyl transferase activity, and interacts with dynamin, amphiphysins 1 and 2, and huntingtin. However, in contrast to endophilins A, endophilin B1 does not bind to synaptojanin 1 and synapsin 1, and overexpression of its SH3 domain does not inhibit transferrin endocytosis. Consistent with this, immunofluorescence analysis of endophilin B1b transfected into fibroblasts shows an intracellular reticular staining, which in part overlaps with that of endogenous dynamin. Upon subcellular fractionation of brain and transfected fibroblasts, endophilin B1 is largely recovered in association with membranes. Together, our results suggest that the action of the endophilins is not confined to the formation of endocytic vesicles from the plasma membrane, with endophilin B1 being associated with, and presumably exerting a functional role at, intracellular membranes.

Dynamin at the actin-membrane interface

Many important cellular processes such as phagocytosis, cell motility and endocytosis require the participation of a dynamic and interactive actin cytoskeleton that acts to deform cellular membranes. The extensive family of non-traditional myosins has been implicated in linking the cortical actin gel with the plasma membrane. Recently, however, the dynamins have also been included in these cell processes as a second family of mechanochemical enzymes that self-associate and hydrolyze nucleotides to perform 'work' while linking cellular membranes to the actin cytoskeleton. The dynamins are believed to form large helical polymers from which extend many interactive proline-rich tail domains, and these domains bind to a variety of SH3-domain-containing proteins, many of which appear to be actin-binding proteins. Recent data support the concept that the dynamin family might act as a 'polymeric contractile scaffold' at the interface between biological membranes and filamentous actin.

Delayed reentry of recycling vesicles into the fusion-competent synaptic vesicle pool in synaptojanin 1 knockout mice

Synaptojanin 1 is a polyphosphoinositide phosphatase implicated in synaptic vesicle recycling. We used FM1-43 imaging and electron microscopy in cultured cortical neurons from control and synaptojanin 1 knockout mice to study how the absence of this protein affects specific steps of the synaptic vesicle cycle. Exoendocytosis after a moderate stimulus was unchanged. However, during prolonged stimulation, the regeneration of fusion-competent synaptic vesicles was severely impaired. In stimulated nerve terminals, there was a persistent accumulation of clathrin-coated vesicles and a backup of newly reformed vesicles in the cytomatrix-rich area around the synaptic vesicle cluster. These findings demonstrate that synaptojanin 1 function is needed for the progression of recycling vesicles to the functional synaptic vesicle pool.

The endocytic machinery at an interface with the actin cytoskeleton: a dynamic, hip intersection

Clathrin-mediated endocytosis is the major mechanism by which proteins and membrane lipids gain access into cells. Over the past several years, an array of proteins has been identified that define the molecular machinery regulating the formation of clathrin-coated pits and vesicles. This article focuses on how the identification of this machinery has begun to reveal a molecular basis for a link between endocytosis and the actin cytoskeleton--a link that had long been suspected to exist in mammalian cells but which had remained elusive. In particular, I discuss the relationship between actin and three components of the endocytic machinery--dynamin, HIPs (huntingtin-interacting proteins) and intersectin.

Identification of interaction partners of the cytosolic polyproline region of CD95 ligand (CD178)

The CD95/Fas/Apo-1 ligand (CD95L, CD178) induces apoptosis through the death receptor CD95. CD95L was also described as a co-stimulatory receptor for T-cell activation in mice in vivo. The molecular basis for the bidirectional signaling capacity and directed expression of CD95L is unknown. In the present study we identify proteins that precipitate from T-cell lysates with constructs containing fragments of the CD95L cytosolic tail. The determined peptide mass fingerprints correspond to Grb2, actin, beta-tubulin, formin binding protein 17 (FBP17) and PACSIN2. Grb2 had been identified as a putative mediator of T-cell receptor-to-CD95L signaling before. FBP17 and PACSIN2 may be associated with expression and trafficking of CD95L. When overexpressed, CD95L co-precipitates with FBP17 and PACSIN. Protein-protein interactions are mediated via Src homology 3 (SH3) domain binding to the polyproline region of CD95L and can be abolished by mutation or deletion of the respective SH3 domain.

Focal adhesion protein FAP52 self-associates through a sequence conserved among the members of the PCH family proteins

FAP52 is a recently described focal adhesion-associated protein. It is a member of an emerging PCH (pombe Cdc15 homology) family of proteins characterized by a common domain organization and involvement in actin cytoskeleton organization, cytokinesis, and vesicular trafficking. Using gel filtration, surface plasmon resonance, and native polyacrylamide gel electrophoresis analysis, combined with chemical cross-linking of both native and recombinant protein, we show that FAP52 self-associates in vitro and suggest that it occurs predominantly as a trimer also in vivo. Analysis of the various domains of FAP52 by surface plasmon resonance showed that the highly alpha-helical region in the N-terminal half of the protein provides the self-association interface. Overexpression of the oligomerization domain in cultured cells was accompanied by major alterations in cellular morphology, actin organization, and the structure of focal adhesions, suggesting that an orderly coming together of FAP52 molecules is crucial for a proper actin filament organization and cytoskeletal structure. Comparison of the primary structures shows that all of the members of the PCH family have, in their N-terminal halves, a similar, highly alpha-helical region, suggesting that they all have a capacity to self-associate.

Closing in on the biological functions of Fps/Fes and Fer

Fps/Fes and Fer are the only known members of a distinct subfamily of the non-receptor protein-tyrosine kinase family. Recent studies indicate that these kinases have roles in regulating cytoskeletal rearrangements and inside out signalling that accompany receptor ligand, cell matrix and cell cell interactions. Genetic analysis using transgenic mouse models also implicates these kinases in the regulation of inflammation and innate immunity.

Enhanced endotoxin sensitivity in fps/fes-null mice with minimal defects in hematopoietic homeostasis

The fps/fes proto-oncogene encodes a cytoplasmic protein tyrosine kinase implicated in growth factor and cytokine receptor signaling and thought to be essential for the survival and terminal differentiation of myeloid progenitors. Fps/Fes-null mice were healthy and fertile, displayed slightly reduced numbers of bone marrow myeloid progenitors and circulating mature myeloid cells, and were more sensitive to lipopolysaccharide (LPS). These phenotypes were rescued using a fps/fes transgene. This confirmed that Fps/Fes is involved in, but not required for, myelopoiesis and that it plays a role in regulating the innate immune response. Bone marrow-derived Fps/Fes-null macrophages showed no defects in granulocyte-macrophage colony-stimulating factor-, interleukin 6 (IL-6)-, or IL-3-induced activation of signal transducer and activator of transcription 3 (Stat3) and Stat5A or LPS-induced degradation of I kappa B or activation of p38, Jnk, Erk, or Akt.

FAP52 regulates actin organization via binding to filamin

FAP52, a focal adhesion-associated phosphoprotein, is a member of a FAP52/PACSIN/syndapin family of proteins. They share a multidomain structure and are implicated in actin-based and endocytotic functions. We show, by using both native and recombinant proteins, that FAP52 selectively binds to the actin cross-linking protein filamin (ABP-280). This was based on an affinity purification followed by a sequence determination by mass spectrometry, co-immunoprecipitation, overlay binding, and surface plasmon resonance analysis. Binding studies with deletion mutants showed that the sites of the interaction map to the highly alpha-helical N-terminal part of FAP52 and to the C-terminal region of filamin, which also contains binding sites to some transmembrane signaling proteins. In immunofluorescence and immunoelectron microscopy of cultured fibroblasts, a different overall subcellular distribution was seen for filamin and FAP52 except for a stress fiber-focal adhesion junction where they showed a notable overlap. Overexpression of the full-length and mutant forms of FAP52 led to an extensive reorganization of actin and filamin in cultured fibroblasts. Thus, the results show that FAP52 interacts with filamin, and we propose that this interaction is important in linking and coordinating the events between focal adhesions and the actin cytoskeleton.

Decreased synaptic vesicle recycling efficiency and cognitive deficits in amphiphysin 1 knockout mice

The function of the clathrin coat in synaptic vesicle endocytosis is assisted by a variety of accessory factors, among which amphiphysin (amphiphysin 1 and 2) is one of the best characterized. A putative endocytic function of amphiphysin was supported by dominant-negative interference studies. We have now generated amphiphysin 1 knockout mice and found that lack of amphiphysin 1 causes a parallel dramatic reduction of amphiphysin 2 selectively in brain. Cell-free assembly of endocytic protein scaffolds is defective in mutant brain extracts. Knockout mice exhibit defects in synaptic vesicle recycling that are unmasked by stimulation and suggest impairments at multiple stages of the cycle. These defects correlate with increased mortality due to rare irreversible seizures and with major learning deficits, suggesting a critical role of amphiphysin for higher brain functions.

PSTPIP is a substrate of PTP-PEST and serves as a scaffold guiding PTP-PEST toward a specific dephosphorylation of WASP

PSTPIP is a tyrosine-phosphorylated protein involved in the organization of the cytoskeleton. Its ectopic expression induces filipodial-like membrane extensions in NIH 3T3 cells. We previously observed a defect in cytokinesis and an increase in the tyrosine phosphorylation of PSTPIP in PTP-PEST-deficient fibroblasts. In this article, we demonstrate that PTP-PEST and PSTPIP are found in the same complexes in vivo and that they interact directly through the CTH domain of PTP-PEST and the coiled-coil domain of PSTPIP. We tested pathways that could regulate the tyrosine phosphorylation of PSTPIP. We found that the activation of the epidermal growth factor and platelet-derived growth factor receptors can induce PSTPIP phosphorylation. With the use of the PP2 inhibitor, we demonstrate that Src kinases are not involved in the epidermal growth factor-mediated phosphorylation of PSTPIP. Together with previous results, this suggests that c-Abl is the critical tyrosine kinase downstream of growth factor receptors responsible for PSTPIP phosphorylation. We also demonstrate that PTP-PEST dephosphorylates PSTPIP at tyrosine 344. Importantly, we identified tyrosine 344 as the main phosphorylation site of PSTPIP by performing tryptic phosphopeptide maps. This is an important finding since tyrosine 367 of PSTPIP was also proposed as a candidate phosphorylation site involved in the negative regulation of the association between PSTPIP and WASP. In this respect, we observed that the PSTPIP.WASP complex is stable in vivo and is not affected by the phosphorylation of PSTPIP. Furthermore, we demonstrate that PSTPIP serves as a scaffold protein between PTP-PEST and WASP and allows PTP-PEST to dephosphorylate WASP. This finding suggests a possible mechanism for PTP-PEST to directly modulate actin remodeling through the PSTPIP-WASP interaction.

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

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