Endofin, an endosomal FYVE domain protein

Melanophilin regulates dendritogenesis in melanocytes for feather pigmentation

Limited studies using animal models with a few natural mutations in melanophilin (Mlph) provided partial functions of Mlph in melanosome trafficking. To investigate cellular functions of Mlph, especially ZnF motif of Mlph, we analyzed all three Mlph knockout (KO) quail lines, one and two base pair (bp) deletions as models for total KO, and three bp deletion causing deletion of one Cysteine (C84del) in the ZnF motif. All quail lines had diluted feather pigmentation with impaired dendritogenesis and melanosome transport in melanocytes. In vitro studies revealed capability of binding of the ZnF motif to PIP3, and impairment of PI3P binding and mislocalization of MLPH proteins with ZnF motif mutations. The shortened melanocyte dendrites by the C84del mutation were rescued by introducing WT Mlph in vitro. These results revealed the diluted feather pigmentation by Mlph mutations resulted from congregation of melanosomes in the cell bodies with impairment of the dendritogenesis and the transport of melanosomes to the cell periphery.

Profiling of basal and ligand-dependent GPCR activities by means of a polyvalent cell-based high-throughput platform

Representing the most attractive and successful druggable receptors of the proteome, GPCRs regulate a myriad of physiological and pathophysiological functions. Although over half of present pharmaceuticals target GPCRs, the advancement of drug discovery is hampered by a lack of adequate screening tools, the majority of which are limited to probing agonist-induced G-protein and β-arrestin-2-mediated events as a measure of receptor activation. Here, we develop Tango-Trio, a comprehensive cell-based high-throughput platform comprising cumate-inducible expression of transducers, capable of the parallelized profiling of both basal and agonist-dependent GPCR activities. We capture the functional diversity of GPCRs, reporting β-arrestin-1/2 couplings, selectivities, and receptor internalization signatures across the GPCRome. Moreover, we present the construction of cumate-induced basal activation curves at approximately 200 receptors, including over 50 orphans. Overall, Tango-Trio's robustness is well-suited for the functional characterization and screening of GPCRs, especially for parallel interrogation, and is a valuable addition to the pharmacological toolbox.

Arl15 upregulates the TGFβ family signaling by promoting the assembly of the Smad-complex

The hallmark event of the canonical transforming growth factor β (TGFβ) family signaling is the assembly of the Smad-complex, consisting of the common Smad, Smad4, and phosphorylated receptor-regulated Smads. How the Smad-complex is assembled and regulated is still unclear. Here, we report that active Arl15, an Arf-like small G protein, specifically binds to the MH2 domain of Smad4 and colocalizes with Smad4 at the endolysosome. The binding relieves the autoinhibition of Smad4, which is imposed by the intramolecular interaction between its MH1 and MH2 domains. Activated Smad4 subsequently interacts with phosphorylated receptor-regulated Smads, forming the Smad-complex. Our observations suggest that Smad4 functions as an effector and a GTPase activating protein (GAP) of Arl15. Assembly of the Smad-complex enhances the GAP activity of Smad4 toward Arl15, therefore dissociating Arl15 before the nuclear translocation of the Smad-complex. Our data further demonstrate that Arl15 positively regulates the TGFβ family signaling.

RABENOSYN separation-of-function mutations uncouple endosomal recycling from lysosomal degradation, causing a distinct Mendelian Disorder

Rabenosyn (RBSN) is a conserved endosomal protein necessary for regulating internalized cargo. Here, we present clinical, genetic, cellular and biochemical evidence that two distinct RBSN missense variants are responsible for a novel Mendelian disorder consisting of progressive muscle weakness, facial dysmorphisms, ophthalmoplegia and intellectual disability. Using exome sequencing, we identified recessively-acting germline alleles p.Arg180Gly and p.Gly183Arg which are both situated in the FYVE domain of RBSN. We find that these variants abrogate binding to its cognate substrate PI3P and thus prevent its translocation to early endosomes. Although the endosomal recycling pathway was unaltered, mutant p.Gly183Arg patient fibroblasts exhibit accumulation of cargo tagged for lysosomal degradation. Our results suggest that these variants are separation-of-function alleles, which cause a delay in endosomal maturation without affecting cargo recycling. We conclude that distinct germline mutations in RBSN cause non-overlapping phenotypes with specific and discrete endolysosomal cellular defects.

Denis N, Gingras AC, Lukacs GL, Pause A. Endofin is required for HD-PTP and ESCRT-0 interdependent endosomal sorting of ubiquitinated transmembrane cargoes

Internalized and ubiquitinated signaling receptors are silenced by their intraluminal budding into multivesicular bodies aided by the endosomal sorting complexes required for transport (ESCRT) machinery. HD-PTP, an ESCRT protein, forms complexes with ESCRT-0, -I and -III proteins, and binds to Endofin, a FYVE-domain protein confined to endosomes with poorly understood roles. Using proximity biotinylation, we showed that Endofin forms a complex with ESCRT constituents and Endofin depletion increased integrin α5-and EGF-receptor plasma membrane density and stability by hampering their lysosomal delivery. This coincided with sustained receptor signaling and increased cell migration. Complementation of Endofin- or HD-PTP-depleted cells with wild-type Endofin or HD-PTP, but not with mutants harboring impaired Endofin/HD-PTP association or cytosolic Endofin, restored EGFR lysosomal delivery. Endofin also promoted Hrs indirect interaction with HD-PTP. Jointly, our results indicate that Endofin is required for HD-PTP and ESCRT-0 interdependent sorting of ubiquitinated transmembrane cargoes to ensure efficient receptor desensitization and lysosomal delivery.

Protein Trafficking or Cell Signaling: A Dilemma for the Adaptor Protein TOM1

Lysosomal degradation of ubiquitinated transmembrane protein receptors (cargo) relies on the function of Endosomal Sorting Complex Required for Transport (ESCRT) protein complexes. The ESCRT machinery is comprised of five unique oligomeric complexes with distinct functions. Target of Myb1 (TOM1) is an ESCRT protein involved in the initial steps of endosomal cargo sorting. To exert its function, TOM1 associates with ubiquitin moieties on the cargo via its VHS and GAT domains. Several ESCRT proteins, including TOLLIP, Endofin, and Hrs, have been reported to form a complex with TOM1 at early endosomal membrane surfaces, which may potentiate the role of TOM1 in cargo sorting. More recently, it was found that TOM1 is involved in other physiological processes, including autophagy, immune responses, and neuroinflammation, which crosstalk with its endosomal cargo sorting function. Alteration of TOM1 function has emerged as a phosphoinositide-dependent survival mechanism for bacterial infections and cancer progression. Based on current knowledge of TOM1-dependent cellular processes, this review illustrates how TOM1 functions in coordination with an array of protein partners under physiological and pathological scenarios.

Whole Exome Sequencing Identifies Novel De Novo Variants Interacting with Six Gene Networks in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a highly heritable condition caused by a combination of environmental and genetic factors such as de novo and inherited variants, as well as rare or common variants among hundreds of related genes. Previous genome-wide association studies have identified susceptibility genes; however, most ASD-associated genes remain undiscovered. This study aimed to examine rare de novo variants to identify genetic risk factors of ASD using whole exome sequencing (WES), functional characterization, and genetic network analyses of identified variants using Korean familial dataset. We recruited children with ASD and their biological parents. The clinical best estimate diagnosis of ASD was made according to the Diagnostic and Statistical Manual of Mental Disorders (DSM-5^TM), using comprehensive diagnostic instruments. The final analyses included a total of 151 individuals from 51 families. Variants were identified and filtered using the GATK Best Practices for bioinformatics analysis, followed by genome alignments and annotation to the reference genome assembly GRCh37 (liftover to GRCh38), and further annotated using dbSNP 154 build databases. To evaluate allele frequencies of de novo variants, we used the dbSNP, gnomAD exome v2.1.1, and genome v3.0. We used Ingenuity Pathway Analysis (IPA, Qiagen) software to construct networks using all identified de novo variants with known autism-related genes to find probable relationships. We identified 36 de novo variants with potential relations to ASD; 27 missense, two silent, one nonsense, one splice region, one splice site, one 5′ UTR, and one intronic SNV and two frameshift deletions. We identified six networks with functional relationships. Among the interactions between de novo variants, the IPA assay found that the NF-κB signaling pathway and its interacting genes were commonly observed at two networks. The relatively small cohort size may affect the results of novel ASD genes with de novo variants described in our findings. We did not conduct functional experiments in this study. Because of the diversity and heterogeneity of ASD, the primary purpose of this study was to investigate probable causative relationships between novel de novo variants and known autism genes. Additionally, we based functional relationships with known genes on network analysis rather than on statistical analysis. We identified new variants that may underlie genetic factors contributing to ASD in Korean families using WES and genetic network analyses. We observed novel de novo variants that might be functionally linked to ASD, of which the variants interact with six genetic networks.

Zfyve16 regulates the proliferation of B-lymphoid cells

Zfyve16 (a.k.a. endofin or endosome-associated FYVE-domain protein), a member of the FYVE-domain protein family, is involved in endosomal trafficking and in TGF-β, BMP, and EGFR signaling. The FYVE protein SARA regulates the TGF-β signaling pathway by recruiting Smad2/3 and accelerating their phosphorylation, thereby altering their susceptibility to TGF-β-mediated T cell suppression. Zfyve16 binds to Smad4 and their binding affects the formation of Smad2/3-Smad4 complex in TGF-β signaling. However, the in vivo function of Zfyve16 remains unknown. In this study, we generated a Zfyve16 knockout mouse strain (Zfyve16^KO) and examined its hematopoietic phenotypes and hematopoietic reconstruction ability. The proportion of Tcells in the peripheral blood of Zfyve16^KO mice increases compared with that in wild-type mice. This finding is consistent with the role of Zfyve16 in facilitating TGF-β signaling. Unpredictably, B cell proliferation is inhibited in Zfyve16^KO mice. The proliferation potential of Zfyve16^KO B-lymphoid cells also significantly decreases in vitro. These results suggest that Zfyve16 inhibits the proliferation of T cells, possibly through the TGF-β signaling, but upregulates the proliferation of B-lymphoid cells.

Structural Basis for Specific Interaction of TGFβ Signaling Regulators SARA/Endofin with HD-PTP

SARA and endofin are endosomal adaptor proteins that drive Smad phosphorylation by ligand-activated transforming growth factor β/bone morphogenetic protein (TGFβ/BMP) receptors. We show in this study that SARA and endofin also recruit the tumor supressor HD-PTP, a master regulator of endosomal sorting and ESCRT-dependent receptor downregulation. High-affinity interactions occur between the SARA/endofin N termini, and the conserved hydrophobic region in the HD-PTP Bro1 domain that binds CHMP4/ESCRT-III. CHMP4 engagement is a universal feature of Bro1 proteins, but SARA/endofin binding is specific to HD-PTP. Crystallographic structures of HD-PTP_Bro1 in complex with SARA, endofin, and three CHMP4 isoforms revealed that all ligands bind similarly to the conserved site but, critically, only SARA/endofin interact at a neighboring pocket unique to HD-PTP. The structures, together with mutagenesis and binding analysis, explain the high affinity and specific binding of SARA/endofin, and why they compete so effectively with CHMP4. Our data invoke models for how endocytic regulation of TGFβ/BMP signaling is controlled.

Nervous system development relies on endosomal trafficking

Accumulating findings have begun to unveil the important role of the endosomal machinery in the nervous system development. Endosomes have been linked to the differential segregation of cell fate determining molecules in asymmetrically dividing progenitors during neurogenesis. Additionally, the precise removal and reinsertion of membrane components through endocytic trafficking regulates the spatial and temporal distribution of signaling receptors and adhesion molecules, which determine the morphology and motility of migrating neurons. Emerging evidence suggests that the role of the endosomal sorting adaptors is dependent upon cell type and developmental stage. The repertoire of the signaling receptors and/or adhesion molecules sorted by the endosome during these processes remains to be explored. In this commentary, we will briefly address the progress in this research field.

TGF-β Signaling from Receptors to Smads

Transforming growth factor β (TGF-β) and related growth factors are secreted pleiotropic factors that play critical roles in embryogenesis and adult tissue homeostasis by regulating cell proliferation, differentiation, death, and migration. The TGF-β family members signal via heteromeric complexes of type I and type II receptors, which activate members of the Smad family of signal transducers. The main attribute of the TGF-β signaling pathway is context-dependence. Depending on the concentration and type of ligand, target tissue, and developmental stage, TGF-β family members transmit distinct signals. Deregulation of TGF-β signaling contributes to developmental defects and human diseases. More than a decade of studies have revealed the framework by which TGF-βs encode a context-dependent signal, which includes various positive and negative modifiers of the principal elements of the signaling pathway, the receptors, and the Smad proteins. In this review, we first introduce some basic components of the TGF-β signaling pathways and their actions, and then discuss posttranslational modifications and modulatory partners that modify the outcome of the signaling and contribute to its context-dependence, including small noncoding RNAs.

SARA regulates neuronal migration during neocortical development through L1 trafficking

Emerging evidence suggests that endocytic trafficking of adhesion proteins plays a crucial role in neuronal migration during neocortical development. However, molecular insights into these processes remain elusive. Here, we study the early endosomal protein Smad anchor for receptor activation (SARA) in the developing mouse brain. SARA is enriched at the apical endfeet of radial glia of the neocortex. Although SARA knockdown did not lead to detectable neurogenic phenotypes, SARA-suppressed neurons exhibited impaired orientation and migration across the intermediate zone. Mechanistically, we show that SARA knockdown neurons exhibit increased surface expression of the L1 cell adhesion molecule. Neurons ectopically expressing L1 phenocopy the migration and orientation defects caused by SARA knockdown and display increased contact with neighboring neurites. L1 knockdown effectively rescues SARA suppression-induced phenotypes. SARA knockdown neurons eventually overcome their migration defect and enter later into the cortical plate. Nevertheless, these neurons localize at more superficial cortical layers than their control counterparts. These results suggest that SARA regulates the orientation, multipolar-to-bipolar transition and the positioning of cortical neurons via modulating surface L1 expression.

Involvement of SARA in Axon and Dendrite Growth

SARA (Smad Anchor for Receptor Activation) plays a crucial role in Rab5-mediated endocytosis in cell lines localizing to early endosomes where it regulates morphology and function. Here, we analyzed the role of SARA during neuronal development and tested whether it functions as a regulator of endocytic trafficking of selected axonal and membrane proteins. Suppression of SARA perturbs the appearance of juxtanuclear endocytic recycling compartments and the neurons show long axons with large growth cones. Furthermore, surface distribution of the cell adhesion molecule L1 in axons and the fusion of vesicles containing transferring receptor (TfR) in dendrites were increased in neurons where SARA was silenced. Conversely, SARA overexpression generated large early endosomes and reduced neurite outgrowth. Taken together, our findings suggest a significant contribution of SARA to key aspects of neuronal development, including neurite formation.

Endofin, a novel BMP-SMAD regulator of the iron-regulatory hormone, hepcidin

BMP-SMAD signalling plays a crucial role in numerous biological processes including embryonic development and iron homeostasis. Dysregulation of the iron-regulatory hormone hepcidin is associated with many clinical iron-related disorders. We hypothesised that molecules which mediate BMP-SMAD signalling play important roles in the regulation of iron homeostasis and variants in these proteins may be potential genetic modifiers of iron-related diseases. We examined the role of endofin, a SMAD anchor, and show that knockdown of endofin in liver cells inhibits basal and BMP-induced hepcidin expression along with other BMP-regulated genes, ID1 and SMAD7. We show for the first time, the in situ interaction of endofin with SMAD proteins and significantly reduced SMAD phosphorylation with endofin knockdown, suggesting that endofin modulates hepcidin through BMP-SMAD signalling. Characterisation of naturally occurring SNPs show that mutations in the conserved FYVE domain result in mislocalisation of endofin, potentially affecting downstream signalling and modulating hepcidin expression. In conclusion, we have identified a hitherto unrecognised link, endofin, between the BMP-SMAD signalling pathway, and the regulation of hepcidin expression and iron homeostasis. This study further defines the molecular network involved in iron regulation and provides potential targets for the treatment of iron-related disorders.

TOM1 is a PI5P effector involved in the regulation of endosomal maturation

Phosphoinositides represent a major class of lipids specifically involved in the organisation of signaling cascades, maintenance of the identity of organelles and regulation of multiple intracellular trafficking steps. We previously described that phosphatidylinositol 5-monophosphate (PI5P), produced by the Shigella flexneri phosphatase IpgD, is implicated in the endosomal sorting of the EGFR. Here, we show that the adaptor protein TOM1 is a new PI5P direct binding partner. We identify the domain of TOM1 involved in this interaction and characterize the binding motif. Finally, we demonstrate that the recruitment of TOM1 by PI5P on signaling endosomes is responsible for the delay in EGFR degradation and fluid-phase bulk endocytosis. Taken together, our data strongly suggest that PI5P-enrichment in signaling endosomes prevents endosomal maturation through the recruitment of TOM1, and point out to a new function of PI5P in regulating discrete maturation steps in the endosomal system.

Phosphoinositides differentially regulate protrudin localization through the FYVE domain

Protrudin is a FYVE (Fab 1, YOTB, Vac 1, and EEA1) domain-containing protein involved in transport of neuronal cargoes and implicated in the onset of hereditary spastic paraplegia. Our image-based screening of the lipid binding domain library revealed novel plasma membrane localization of the FYVE domain of protrudin unlike canonical FYVE domains that are localized to early endosomes. The membrane binding study by surface plasmon resonance analysis showed that this FYVE domain preferentially binds phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2)), and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) unlike canonical FYVE domains that specifically bind phosphatidylinositol 3-phosphate (PtdIns(3)P). Furthermore, we found that these phosphoinositides (PtdInsP) differentially regulate shuttling of protrudin between endosomes and plasma membrane via its FYVE domain. Protrudin mutants with reduced PtdInsP-binding affinity failed to promote neurite outgrowth in primary cultured hippocampal neurons. These results suggest that novel PtdInsP selectivity of the protrudin-FYVE domain is critical for its cellular localization and its role in neurite outgrowth.

SARA is dispensable for functional TGF-β signaling

Smad anchor for receptor activation (SARA or ZFYVE9) has been proposed to mediate transforming growth factor β (TGF-β) signaling by direct interaction with the non-activated Smad proteins and the TGF-β receptors; however, these findings are controversial. We demonstrate no correlation between SARA expression and the levels of TGF-β-induced phosphorylation of Smads in various B-cell lymphomas. Moreover, knockdown of SARA in HeLa cells did not interfere with TGF-β-induced Smad activation, Smad nuclear translocation, or induction of TGF-β target genes. Various R-Smads and TGF-β receptors did not co-immunoprecipitate with SARA. Collectively, our results demonstrate that SARA is dispensable for functional TGF-β-mediated signaling.

Deficiency of sorting nexin 27 (SNX27) leads to growth retardation and elevated levels of N-methyl-D-aspartate receptor 2C (NR2C)

Phox (PX) domain-containing sorting nexins (SNXs) are emerging as important regulators of endocytic trafficking. Sorting nexin 27 (SNX27) is unique, as it contains a PDZ (Psd-95/Dlg/ZO1) domain. We show here that SNX27 is primarily targeted to the early endosome by interaction of its PX domain with PtdIns(3)P. Although targeted ablation of the SNX27 gene in mice did not significantly affect growth and survival during embryonic development, SNX27 plays an essential role in postnatal growth and survival. N-Methyl-d-aspartate (NMDA) receptor 2C (NR2C) was identified as a novel SNX27-interacting protein, and this interaction is mediated by the PDZ domain of SNX27 and the C-terminal PDZ-binding motif of NR2C. Increased NR2C expression levels, together with impaired NR2C endocytosis in SNX27(-/-) neurons, indicate that SNX27 may function to regulate endocytosis and/or endosomal sorting of NR2C. This is consistent with a role of SNX27 as a general regulator for sorting of membrane proteins containing a PDZ-binding motif, and its absence may alter the trafficking of these proteins, leading to growth and survival defects.

Gene expression profile of multiple myeloma cell line treated by arsenic trioxide

cDNA microarray was used to compare the gene expression profiles of multiple myeloma cell line RPMI8226 24 h before and after treatment with arsenic trioxide. Two cDNA probes were prepared by mRNA reverse transcription of both arsenic trioxide-treated and untreated RPMI8226 cells. The probes were labeled with Cy3 and Cy5 fluorescence dyes separately, hybridized with cDNA microarray representing 4096 different human genes, and scanned for fluorescence intensity. The differences in gene expression were calculated on the basis of the ratios of signal intensity of treated and untreated samples. The up-and down-regulated genes were screened through the analysis of gene expression ratios. The results showed that 273 genes were differentially altered at mRNA level, 121 genes were up-regulated and 152 were down-regulated. It is concluded that the treatment with arsenic trioxide can induce a variety of gene changes in RPMI8226 cell line. Many genes may be involved in the pathogenesis of multiple myeloma. ALK-1 and TXNIP genes may play an important role in the apoptosis and partial differentiation of RPMI8226 cells.

Identification and structural characterization of FYVE domain-containing proteins of Arabidopsis thaliana

BACKGROUND

FYVE domains have emerged as membrane-targeting domains highly specific for phosphatidylinositol 3-phosphate (PtdIns(3)P). They are predominantly found in proteins involved in various trafficking pathways. Although FYVE domains may function as individual modules, dimers or in partnership with other proteins, structurally, all FYVE domains share a fold comprising two small characteristic double-stranded beta-sheets, and a C-terminal alpha-helix, which houses eight conserved Zn2+ ion-binding cysteines. To date, the structural, biochemical, and biophysical mechanisms for subcellular targeting of FYVE domains for proteins from various model organisms have been worked out but plant FYVE domains remain noticeably under-investigated.

RESULTS

We carried out an extensive examination of all Arabidopsis FYVE domains, including their identification, classification, molecular modeling and biophysical characterization using computational approaches. Our classification of fifteen Arabidopsis FYVE proteins at the outset reveals unique domain architectures for FYVE containing proteins, which are not paralleled in other organisms. Detailed sequence analysis and biophysical characterization of the structural models are used to predict membrane interaction mechanisms previously described for other FYVE domains and their subtle variations as well as novel mechanisms that seem to be specific to plants.

CONCLUSIONS

Our study contributes to the understanding of the molecular basis of FYVE-based membrane targeting in plants on a genomic scale. The results show that FYVE domain containing proteins in plants have evolved to incorporate significant differences from those in other organisms implying that they play a unique role in plant signaling pathways and/or play similar/parallel roles in signaling to other organisms but use different protein players/signaling mechanisms.

Recent advances in BMP receptor signaling

Bone Morphogenetic Proteins (BMPs) play an important role during organ development and during regeneration after tissue damage. BMPs signal via transmembrane serine/threonine kinase receptors. From our current understanding heteromeric complexes of type I and type II receptors are required for signal propagation. Presently, three type I and three type II receptors are known to bind BMPs with different affinities. Ligands and receptors eventually oligomerize via defined modes into signaling complexes. Co-receptors recruit into these complexes to either inhibit or to promote signaling. The Smad pathway, initiated by phosphorylation through the activated type I receptors, results in transcriptional regulation of early target genes. However, on its way to the nucleus, Smads represent signaling platforms for other pathways, which eventually finetune BMP signal transduction. We also describe BMP-induced signaling cascades leading to cytoskeletal rearrangements, non-transcriptional and non-Smad pathways. BMPs induce a plethora of different cellular effects ranging from stem cell maintenance, migration, differentiation, proliferation to apoptosis. The molecular mechanism, by which the same ligand induces these manifold effects, depends on the cellular context. Here we try to give a current picture of the most important players in regulating and directing BMP signaling towards the desired cellular outcome. Examples of BMP action during development, but also physiological and pathophysiological conditions in the adult organism are presented.

A Trypanosoma cruzi phosphatidylinositol 3-kinase (TcVps34) is involved in osmoregulation and receptor-mediated endocytosis

Trypanosoma cruzi, the etiological agent of Chagas disease, has the ability to respond to a variety of environmental changes during its life cycle both in the insect vector and in the vertebrate host. Because regulation of transcription initiation seems to be nonfunctional in this parasite, it is important to investigate other regulatory mechanisms of adaptation. Regulatory mechanisms at the level of signal transduction pathways involving phosphoinositides are good candidates for this purpose. Here we report the identification of the first phosphatidylinositol 3-kinase (PI3K) in T. cruzi, with similarity with its yeast counterpart, Vps34p. TcVps34 specifically phosphorylates phosphatidylinositol to produce phosphatidylinositol 3-phosphate, thus confirming that it belongs to class III PI3K family. Overexpression of TcVps34 resulted in morphological and functional alterations related to vesicular trafficking. Although inhibition of TcVps34 with specific PI3K inhibitors, such as wortmannin and LY294,000, resulted in reduced regulatory volume decrease after hyposmotic stress, cells overexpressing this enzyme were resistant to these inhibitors. Furthermore, these cells were able to recover their original volume faster than wild type cells when they were submitted to severe hyposmotic stress. In addition, in TcVps34-overexpressing cells, the activities of vacuolar-H+-ATPase and vacuolar H+-pyrophosphatase were altered, suggesting defects in the acidification of intracellular compartments. Furthermore, receptor-mediated endocytosis was partially blocked although fluid phase endocytosis was not affected, confirming a function for TcVps34 in membrane trafficking. Taken together, these results strongly support that TcVps34 plays a prominent role in vital processes for T. cruzi survival such as osmoregulation, acidification, and vesicular trafficking.

Identification of the SPG15 gene, encoding spastizin, as a frequent cause of complicated autosomal-recessive spastic paraplegia, including Kjellin syndrome

Hereditary spastic paraplegias (HSPs) are genetically and phenotypically heterogeneous disorders. Both "uncomplicated" and "complicated" forms have been described with various modes of inheritance. Sixteen loci for autosomal-recessive "complicated" HSP have been mapped. The SPG15 locus was first reported to account for a rare form of spastic paraplegia variably associated with mental impairment, pigmented maculopathy, dysarthria, cerebellar signs, and distal amyotrophy, sometimes designated as Kjellin syndrome. Here, we report the refinement of SPG15 to a 2.64 Mb genetic interval on chromosome 14q23.3-q24.2 and the identification of ZFYVE26, which encodes a zinc-finger protein with a FYVE domain that we named spastizin, as the cause of SPG15. Six different truncating mutations were found to segregate with the disease in eight families with a phenotype that included variable clinical features of Kjellin syndrome. ZFYVE26 mRNA was widely distributed in human tissues, as well as in rat embryos, suggesting a possible role of this gene during embryonic development. In the adult rodent brain, its expression profile closely resembled that of SPG11, another gene responsible for complicated HSP. In cultured cells, spastizin colocalized partially with markers of endoplasmic reticulum and endosomes, suggesting a role in intracellular trafficking.

Defective trafficking and localization of mutated transferrin receptor 2: implications for type 3 hereditary hemochromatosis

Transferrin receptor 2 (TfR2), a homologue of transferrin receptor 1 (TfR1), is a key molecule involved in the regulation of iron homeostasis. Mutations in TfR2 result in iron overload with similar features to HFE-associated hereditary hemochromatosis. The precise role of TfR2 in iron metabolism and the functional consequences of disease-causing mutations have not been fully determined. We have expressed wild-type and various mutant forms of TfR2 that are associated with human disease in a mouse liver cell line. Intracellular and surface analysis shows that all the TfR2 mutations analyzed cause the intracellular retention of the protein in the endoplasmic reticulum, whereas the wild-type protein is expressed in endocytic structures and at the cell surface. Our results indicate that the majority of mutations that cause type 3 hereditary hemochromatosis are a consequence of the defective localization of the protein.

A novel enzyme complementation-based assay for monitoring G-protein-coupled receptor internalization

G-protein-coupled receptor (GPCR) signaling is involved in a wide range of physiological processes and diseases, and around one-half of currently used drugs target GPCRs. Assays for the signaling of GPCRs have suffered from drawbacks, including low signal-to-noise, temporally transient signals, and difficulty in applying a single assay to a wide range of GPCRs. We have developed a set of assays for G-protein-coupled receptor signaling based on beta-galactosidase enzyme complementation in live mammalian cells. We previously described an assay for GPCR activation by monitoring the binding of beta-arrestin to the receptor. Here we describe a second assay that monitors the internalization of GPCRs to endosomes, an event that follows receptor activation and is critical in desensitizing and resensitizing the receptor. We show that both assays display high signal-to-noise ratios with low variability and are quantitative for a wide range of GPCRs. EC50s obtained with these assays closely match results reported in the literature. Finally, we show that these assays are readily adapted to high-throughput chemical screens. Thus, these two assays for monitoring G-protein-coupled receptor activation and internalization should prove valuable in basic biological studies as well as in high-throughput screens.

Phosphoproteomics identified Endofin, DCBLD2, and KIAA0582 as novel tyrosine phosphorylation targets of EGF signaling and Iressa in human cancer cells

With the completion of the human genome project, analysis of enriched phosphotyrosyl proteins from epidermal growth factor (EGF)-induced phosphotyrosine proteome permits the identification of novel downstream substrates of the EGF receptor (EGFR). Using cICAT-based LC-MS/MS method, we identified and relatively quantified the tyrosine phosphorylation levels of 21 proteins between control and EGF-treated A431 human cervical cancer cells. Of these, Endofin, DCBLD2, and KIAA0582 were validated to be novel tyrosine-phosphorylation targets of EGF signaling and Iressa, a highly selective inhibitor of EGFR. In addition, EGFR activity was shown to be necessary for EGF-induced localization of Endofin, an FYVE domain-containing protein regulated by phosphoinositol lipid and engaged in endosome-mediated receptor modulation. Although several groups have conducted phosphoproteomics of EGF signaling in recent years, our study is the first to identify and validate Endofin, DCBLD2, and KIAA0582 as part of a complex EGF phosphotyrosine signaling network. These novel data will provide new insights into the complex EGF signaling and may have implications on target-directed cancer therapeutics.

SARA-regulated vesicular targeting underlies formation of the light-sensing organelle in mammalian rods

The light-sensing organelle of the vertebrate rod photoreceptor, the outer segment (OS), is a modified cilium containing approximately 1,000 stacked disc membranes that are densely packed with visual pigment rhodopsin. The mammalian OS is renewed every ten days; new discs are assembled at the base of the OS by a poorly understood mechanism. Our results suggest that discs are formed and matured in a process that involves specific phospholipid-directed vesicular membrane targeting. Rhodopsin-laden vesicles in the OS axonemal cytoplasm fuse with nascent discs that are highly specialized with abundant phosphatidylinositol 3-phosphate (PI3P). This membrane coupling is regulated by the FYVE domain-containing protein, SARA, through its direct interaction with PI3P, rhodopsin, and SNARE protein syntaxin 3. Our model, in contrast to the previously proposed evagination model, suggests that the vesicular delivery of rhodopsin in the OS concentrates rhodopsin into discs, and this process directly participates in disc biogenesis.

The structural basis of novel endosome anchoring activity of KIF16B kinesin

KIF16B is a newly identified kinesin that regulates the intracellular motility of early endosomes. KIF16B is unique among kinesins in that its cargo binding is mediated primarily by the strong interaction of its PX domain with endosomal lipids. To elucidate the structural basis of this unique endosomal anchoring activity of KIF16B-PX, we determined the crystal structure of the PX domain and performed in vitro and cellular membrane binding measurements for KIF16B-PX and mutants. The most salient structural feature of KIF16B-PX is that two neighboring residues, L1248 and F1249, on the membrane-binding surface form a protruding hydrophobic stalk with a large solvent-accessible surface area. This unique structure, arising from the complementary stacking of the two side chains and the local conformation, allows strong hydrophobic membrane interactions and endosome tethering. The presence of similar hydrophobic pairs in the amino-acid sequences of other membrane-binding domains and proteins suggests that the same structural motif may be shared by other membrane-binding proteins, whose physiological functions depend on strong hydrophobic membrane interactions.

Proteomics Identification of Sorting Nexin 27 as a Diacylglycerol Kinase ζ-associated Protein

Diacylglycerol kinase zeta is a member of the diacylglycerol kinase family of enzymes, which generate phosphatidic acid through diacylglycerol phosphorylation. In addition to the catalytic and cysteine-rich domains found in all diacylglycerol kinases, diacylglycerol kinase zeta has a MARCKS domain as well as a C-terminal region containing four ankyrin repeats and a PDZ-binding motif. Previous reports demonstrated that diacylglycerol kinase zeta interaction with several proteins is an important mechanism for modulating the localization and activity of this enzyme. Here we used a proteomics approach to search for novel diacylglycerol kinase zeta-interacting proteins and identified sorting nexin 27 (SNX27), a recently described member of a protein family involved in intracellular trafficking, which has a PDZ domain in addition to the phox homology domain characteristic of SNX proteins. Co-immunoprecipitation studies and two-hybrid analysis confirmed physical, PDZ-dependent association between SNX27 and diacylglycerol kinase zeta. Because diacylglycerol kinase zeta is expressed abundantly in T lymphocytes, we characterized SNX27 expression and subcellular localization in these cells. SNX27 co-localized with transferrin receptor-positive vesicles, pointing to its participation in T cell endocytic recycling. Expression of deletion mutants revealed that in addition to the phox homology domain the SNX27 PDZ domain contributed to vesicle localization of this protein, suggesting that interaction with diacylglycerol kinase zeta regulates SNX27 localization. Analysis of cells with RNA interference-mediated knockdown of diacylglycerol kinase zeta showed accelerated transferrin receptor exit from the lymphocyte endocytic recycling compartment back to the plasma membrane, further confirming diacylglycerol kinase zeta-dependent control of vesicle trafficking. These data support a previously unreported role for diacylglycerol kinase zeta in the modulation of membrane trafficking, which may also help to define SNX27 function.

Endofin acts as a Smad anchor for receptor activation in BMP signaling

Signaling through receptors of the transforming growth factor beta (TGFbeta) superfamily is mediated by cytoplasmic Smad proteins. It has been demonstrated that Smad anchor for receptor activation (SARA) facilitates TGFbeta and activin/nodal signaling by recruiting and presenting Smad2/3 to the receptor complex. SARA does not bind Smad1 and hence does not enhance bone morphogenetic protein (BMP) signaling. Here we report for the first time that the endosome-associated FYVE-domain protein endofin acts as a Smad anchor for receptor activation in BMP signaling. We demonstrate that endofin binds Smad1 preferentially and enhances Smad1 phosphorylation and nuclear localization upon BMP stimulation. Silencing of endofin by RNAi resulted in a reduction in BMP-dependent Smad1 phosphorylation. Moreover, disruption of the membrane-anchoring FYVE motif by point mutation led to a reduction of BMP-responsive gene expression in cell culture and Xenopus ectodermal explants. Furthermore, we demonstrate that endofin contains a protein-phosphatase-binding motif, which functions to negatively modulate BMP signals through receptor dephosphorylation. Taken together, our results suggest that endofin plays an important role in both positive and negative feedback regulation of the BMP signaling pathway.

Endofin, a FYVE domain protein, interacts with Smad4 and facilitates transforming growth factor-beta signaling

Transforming growth factor-beta (TGF-beta) signaling is facilitated by scaffold proteins such as SARA (Smad anchor for receptor activation). Endofin, a member of the FYVE domain protein family, has been suggested to regulate membrane trafficking. In this study, we report that endofin functions as a scaffold protein to facilitate TGF-beta signaling. Overexpression of endofin FYVE domain-deletion mutants inhibited TGF-beta-induced expression of CAGA-luciferase. Knockdown of endogenous endofin expression by RNA interference specifically led to reduction of the transcriptional responses of TGF-beta, but had no effect on BMP- or Wnt1-induced reporter expression. Furthermore, in endofin small interfering RNA-expressing stable cells, TGF-beta-mediated expression of plasminogen activator inhibitor-1 and p21(Cip1) was significantly reduced, and TGF-beta-promoted apoptosis was also impaired. We further showed that endofin could interact with Smad4 and TGF-beta type I receptors. Reduction of endogenous endofin expression resulted in a decrease of TGF-beta-induced Smad2 phosphorylation and Smad2-Smad4 complex formation. Together, our findings suggest that endofin facilitates TGF-beta signaling as a scaffold protein to promote the R-Smad-Smad4 complex formation by bringing Smad4 to the proximity of the receptor complex.

Increased App Expression in a Mouse Model of Down's Syndrome Disrupts NGF Transport and Causes Cholinergic Neuron Degeneration

Degeneration of basal forebrain cholinergic neurons (BFCNs) contributes to cognitive dysfunction in Alzheimer's disease (AD) and Down's syndrome (DS). We used Ts65Dn and Ts1Cje mouse models of DS to show that the increased dose of the amyloid precursor protein gene, App, acts to markedly decrease NGF retrograde transport and cause degeneration of BFCNs. NGF transport was also decreased in mice expressing wild-type human APP or a familial AD-linked mutant APP; while significant, the decreases were less marked and there was no evident degeneration of BFCNs. Because of evidence suggesting that the NGF transport defect was intra-axonal, we explored within cholinergic axons the status of early endosomes (EEs). NGF-containing EEs were enlarged in Ts65Dn mice and their App content was increased. Our study thus provides evidence for a pathogenic mechanism for DS in which increased expression of App, in the context of trisomy, causes abnormal transport of NGF and cholinergic neurodegeneration.

Phosphatidylinositol 3-phosphate recognition and membrane docking by the FYVE domain

The FYVE domain is a small zinc binding module that recognizes phosphatidylinositol 3-phosphate [PtdIns(3)P], a phospholipid enriched in membranes of early endosomes and other endocytic vesicles. It is usually present as a single module or rarely as a tandem repeat in eukaryotic proteins involved in a variety of biological processes including endo- and exocytosis, membrane trafficking and phosphoinositide metabolism. A number of FYVE domain-containing proteins are recruited to endocytic membranes through the specific interaction of their FYVE domains with PtdIns(3)P. Structures and PtdIns(3)P binding modes of several FYVE domains have recently been characterized, shedding light on the molecular basis underlying multiple cellular functions of these proteins. Here, structural and functional aspects and the current mechanism of the multivalent membrane anchoring by monomeric or dimeric FYVE domain are reviewed. This mechanism involves stereospecific recognition of PtdIns(3)P that is facilitated by non-specific electrostatic contacts and modulated by the histidine switch, and is accompanied by hydrophobic insertion. Contributions of each component to the FYVE domain specificity and affinity for PtdIns(3)P-containing membranes are discussed.

Recruitment of clathrin onto endosomes by the Tom1–Tollip complex

Tom1 (target of Myb 1) and its related proteins (Tom1L1/Srcasm and Tom1L2) constitute a protein family and share an N-terminal VHS (Vps27p/Hrs/Stam) domain and a following GAT (GGA and Tom1) domain, both of which are also conserved in the GGA family proteins. However, the C-terminal half is not significantly conserved between the Tom1 and GGA families or even between Tom1 and Tom1L1. We have previously shown that the GAT domain of Tom1 interacts with Tollip (Toll-interacting protein), which is associated with endosomes, to which it recruits Tom1. We here extend the previous data and show that the GAT domains of Tom1L1 and Tom1L2 also interact with Tollip, and the C-terminal regions of all the Tom1 family proteins interact with clathrin. Furthermore, when coexpressed with Tollip, all the Tom1 family proteins recruite clathrin onto endosomes. These results indicate that, in conjunction with Tollip, Tom1 family proteins play an important role in recruiting clathrin onto endosomes and suggest that they modulate endosomal functions.

Membrane-Protein Interactions in Cell Signaling and Membrane Trafficking

Research in the past decade has revealed that many cytosolic proteins are recruited to different cellular membranes to form protein-protein and lipid-protein interactions during cell signaling and membrane trafficking. Membrane recruitment of these peripheral proteins is mediated by a growing number of modular membrane-targeting domains, including C1, C2, PH, FYVE, PX, ENTH, ANTH, BAR, FERM, and tubby domains, that recognize specific lipid molecules in the membranes. Structural studies of these membrane-targeting domains demonstrate how they specifically recognize their cognate lipid ligands. However, the mechanisms by which these domains and their host proteins are recruited to and interact with various cell membranes are only beginning to unravel with recent computational studies, in vitro membrane binding studies using model membranes, and cellular translocation studies using fluorescent protein-tagged proteins. This review summarizes the recent progress in our understanding of how the kinetics and energetics of membrane-protein interactions are regulated during the cellular membrane targeting and activation of peripheral proteins.

Endofin recruits clathrin to early endosomes via TOM1

TOM1 and its related proteins, TOM1-like1 (TOM1-L1) and TOM1-like2 (TOM1-L2), constitute a subfamily of the VHS domain protein family. We have recently shown that endofin, a FYVE domain protein associated with the early endosome, is able to recruit cytosolic TOM1 onto endosomal membranes. To reveal the biological consequence of endofin-mediated endosomal recruitment of TOM1, we have identified the clathrin heavy chain as a major interacting protein for TOM1. Optimal clathrin binding by TOM1 involves three sites: residues 300-321, 321-326 and a putative clathrin-binding box at residues 362-366 (362LEDEF366). Although residues 321-326 could function independently as a weak clathrin-binding motif, deletion of amino acids 300-321 or mutation of 362Leu and 364Asp to Ala residues reduced the binding of clathrin to TOM1. A fragment lacking amino acids 300-322 and containing 362Leu and 364Asp to Ala mutations lost the ability to interact with clathrin. Remarkably, overexpression of endofin led to a massive and specific recruitment of clathrin [but not dynamin, or the adaptor protein (AP) complexes, AP1, AP2 or AP3] onto endofin-positive endosomes. Although SARA is homologous to endofin, it did not interact with the C-terminal region of TOM1. Examination of chimeric proteins of endofin and SARA suggests that the C-terminal half of endofin is responsible for interaction with the C-terminal region of TOM1 and for recruitment of TOM1 and clathrin to endosomes. The correlation between the ability of endofin to interact with the C-terminal domain of TOM1 and clathrin recruitment suggests that endofin may recruit clathrin via TOM1. Indeed, a chimeric protein consisting of TOM1 fused to two FYVE domains derived from endofin has the ability to recruit clathrin onto endosomal structures. Moreover, we show that affinity-purified TOM1 antibody can abolish binding of clathrin to the C-terminal region of TOM1. Upon microinjection into cells, this antibody reduced the membrane association of clathrin. These results, taken together, suggest that TOM1 is an important molecule for membrane recruitment of clathrin, and that endofin is able to exploit this recruitment at the endosome.

Emerging roles of phosphatidylinositol monophosphates in cellular signaling and trafficking

The phosphoinositide metabolism that is highly controlled by a set of kinases, phosphatases and phospholipases leads to the production of several second messengers playing critical roles in intracellular signal transduction mechanisms. Recent discoveries have unraveled unexpected roles for the three phosphatidylinositol monophosphates, PtdIns(3)P, PtdIns(4)P and PtdIns(5)P, that appear now as important lipid messengers able to specifically interact with proteins. The formation of functionally distinct and independently regulated pools of phosphatidylinositol monophosphates probably contributes to the specificity of the interactions with their targets. The relative enrichment of organelles in a particular species of phosphoinositides (i.e. PtdIns(3)P in endosomes, PtdIns(4)P in Golgi and PtdIns(4,5)P2 in plasma membrane) suggests the notion of lipid-defined organelle identity. PtdIns(3)P is now clearly involved in vesicular trafficking by interaction with a set of FYVE domain-containing proteins both in yeast and in mammals. PtdIns(4)P, which until now was only considered as a precursor for PtdIns(4,5)P2, appears as a regulator on its own, by recruiting a set of proteins to the trans-Golgi network. PtdIns(5)P, the most recently discovered inositol lipid, is also emerging as a potentially important signaling molecule.

Human Nischarin/imidazoline receptor antisera-selected protein is targeted to the endosomes by a combined action of a PX domain and a coiled-coil region

Around 50 mammalian and 15 yeast proteins are known to contain the phox (PX) domain, the majority (about 30) of which is classified as sorting nexins (SNXs). The PX domain, a hallmark of these proteins, is a conserved stretch of about 120 amino acids and is recently shown to mediate phosphoinositide binding. A few PX domain proteins (including some SNXs) have been shown to participate in diverse cellular processes such as protein sorting, signal transduction, and vesicle fusion. In this report, we present our results supporting a role of human IRAS to act as a SNX. The mouse homologue, previously identified as Nischarin, has been shown to interact with the alpha(5) subunit of integrin and inhibit cell migration (Alahari, S. K., Lee J. W., and Juliano R. L. (2000) J. Cell Biol. 51, 1141-1154). Its human homologue (imidazoline receptor antisera-selected (IRAS)), on the other hand, contains an NH(2)-terminal extension and is a larger protein of 1504 amino acids consisting of an NH(2)-terminal PX domain, 5 putative leucine-rich repeats, a predicted coiled-coil domain, and a long COOH-terminal region. We show that it has the ability to homo-oligomerize via its coiled-coil region. The PX domain of IRAS is essential for association with phosphatidylinositol 3-phosphate-enriched endosomal membranes. However, the PX domain of IRAS alone is insufficient for its localization to endosomes, unless the coiled-coil domain was included or it is artificially dimerized by glutathione S-transferase. Interaction of human IRAS with alpha(5) integrin is not affected by the NH(2)-terminal extension, and overexpression of IRAS could cause a redistribution of surface alpha(5) integrin to intracellular endosomal structures.

The molecular basis of the differential subcellular localization of FYVE domains

This study systematically analyzed the structural and mechanistic basis of the regulation of subcellular membrane targeting using FYVE domains as a model. FYVE domains, which mediate the recruitment of signaling and membrane-trafficking proteins to phosphatidylinositol 3-phosphate-containing endosomes, exhibit distinct subcellular localization despite minor structural variations within the family. Biophysical measurements, cellular imaging, and computational analysis of various FYVE domains showed that the introduction of a single cationic residue and a hydrophobic loop into the membrane binding region of the FYVE domains dramatically enhanced their membrane interactions. The results indicated that there is a threshold affinity for endosomal localization and that endosomal targeting of FYVE domains is sensitive to small changes in membrane affinity about this threshold. Collectively these studies provide new insight into how subcellular localization of FYVE domains and other membrane targeting domains can be regulated by minimal structural and environmental changes.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

A unique region of RILP distinguishes it from its related proteins in its regulation of lysosomal morphology and interaction with Rab7 and Rab34

Rab7 and Rab34 are implicated in regulation of lysosomal morphology and they share a common effector referred to as the RILP (Rab-interacting lysosomal protein). Two novel proteins related to RILP were identified and are tentatively referred to as RLP1 and RLP2 (for RILP-like protein 1 and 2, respectively). Overexpression of RILP caused enlarged lysosomes that are positioned more centrally in the cell. However, the morphology and distribution of lysosomes were not affected by overexpression of either RLP1 or RLP2. The molecular basis for the effect of RILP on lysosomes was investigated, leading to the demonstration that a 62-residue region (amino acids 272-333) of RILP is necessary for RILP's role in regulating lysosomal morphology. Remarkably, transferring this 62-residue region unique to RILP into corresponding sites in RLP1 rendered the chimeric protein capable of regulating lysosome morphology. A correlation between the interaction with GTP-bound form of both Rab proteins and the capability of regulating lysosomes was established. These results define a unique region in RILP responsible for its specific role in regulating lysosomal morphology as well as in its interaction with Rab7 and Rab34.

Rabip4' is an effector of rab5 and rab4 and regulates transport through early endosomes

We describe the characterization of an 80-kDa protein cross-reacting with a monoclonal antibody against the human La autoantigen. The 80-kDa protein is a variant of rabip4 with an N-terminal extension of 108 amino acids and is expressed in the same cells. For this reason, we named it rabip4'. rabip4' is a peripheral membrane protein, which colocalized with internalized transferrin and EEA1 on early endosomes. Membrane association required the presence of the FYVE domain and was perturbed by the phosphatidylinositol 3-kinase inhibitor wortmannin. Expression of a dominant negative rabip4' mutant reduced internalization and recycling of transferrin from early endosomes, suggesting that it may be functionally linked to rab4 and rab5. In agreement with this, we found that rabip4' colocalized with the two GTPases on early endosomes and bound specifically and simultaneously to the GTP form of both rab4 and rab5. We conclude that rabip4' may coordinate the activities of rab4 and rab5, regulating membrane dynamics in the early endosomal system.