Arf proteins bind to mitotic kinesin-like protein 1 (MKLP1) in a GTP-dependent fashion

The congenital dyserythropoieitic anemias: genetics and pathophysiology

PURPOSE OF REVIEW

The congenital dyserythropoietic anemias (CDA) are hereditary disorders characterized by ineffective erythropoiesis. This review evaluates newly developed CDA disease models, the latest advances in understanding the pathogenesis of the CDAs, and recently identified CDA genes.

RECENT FINDINGS

Mice exhibiting features of CDAI were recently generated, demonstrating that Codanin-1 (encoded by Cdan1) is essential for primitive erythropoiesis. Additionally, Codanin-1 was found to physically interact with CDIN1, suggesting that mutations in CDAN1 and CDIN1 result in CDAI via a common mechanism. Recent advances in CDAII (which results from SEC23B mutations) have also been made. SEC23B was found to functionally overlap with its paralogous protein, SEC23A, likely explaining the absence of CDAII in SEC23B-deficient mice. In contrast, mice with erythroid-specific deletion of 3 or 4 of the Sec23 alleles exhibited features of CDAII. Increased SEC23A expression rescued the CDAII erythroid defect, suggesting a novel therapeutic strategy for the disease. Additional recent advances included the identification of new CDA genes, RACGAP1 and VPS4A, in CDAIII and a syndromic CDA type, respectively.

SUMMARY

Establishing cellular and animal models of CDA is expected to result in improved understanding of the pathogenesis of these disorders, which may ultimately lead to the development of new therapies.

Septin Remodeling During Mammalian Cytokinesis

Cytokinesis mediates the final separation of a mother cell into two daughter cells. Septins are recruited to the cleavage furrow at an early stage. During cytokinetic progression the septin cytoskeleton is constantly rearranged, ultimately leading to a concentration of septins within the intercellular bridge (ICB), and to the formation of two rings adjacent to the midbody that aid ESCRT-dependent abscission. The molecular mechanisms underlying this behavior are poorly understood. Based on observations that septins can associate with actin, microtubules and associated motors, we review here established roles of septins in mammalian cytokinesis, and discuss, how septins may support cytokinetic progression by exerting their functions at particular sites. Finally, we discuss how this might be assisted by phosphoinositide-metabolizing enzymes.

The ARF GAP ELMOD2 acts with different GTPases to regulate centrosomal microtubule nucleation and cytokinesis

ELMOD2 is a ∼32 kDa protein first purified by its GTPase-activating protein (GAP) activity toward ARL2 and later shown to have uniquely broad specificity toward ARF family GTPases in in vitro assays. To begin the task of defining its functions in cells, we deleted ELMOD2 in immortalized mouse embryonic fibroblasts and discovered a number of cellular defects, which are reversed upon expression of ELMOD2-myc. We show that these defects, resulting from the loss of ELMOD2, are linked to two different pathways and two different GTPases: with ARL2 and TBCD to support microtubule nucleation from centrosomes and with ARF6 in cytokinesis. These data highlight key aspects of signaling by ARF family GAPs that contribute to previously underappreciated sources of complexity, including GAPs acting from multiple sites in cells, working with multiple GTPases, and contributing to the spatial and temporal control of regulatory GTPases by serving as both GAPs and effectors.

Biochemical characterization of purified mammalian ARL13B protein indicates that it is an atypical GTPase and ARL3 guanine nucleotide exchange factor (GEF)

Primary cilia play central roles in signaling during metazoan development. Several key regulators of ciliogenesis and ciliary signaling are mutated in humans, resulting in a number of ciliopathies, including Joubert syndrome (JS). ARL13B is a ciliary GTPase with at least three missense mutations identified in JS patients. ARL13B is a member of the ADP ribosylation factor family of regulatory GTPases, but is atypical in having a non-homologous, C-terminal domain of ∼20 kDa and at least one key residue difference in the consensus GTP-binding motifs. For these reasons, and to establish a solid biochemical basis on which to begin to model its actions in cells and animals, we developed preparations of purified, recombinant, murine Arl13b protein. We report results from assays for solution-based nucleotide binding, intrinsic and GTPase-activating protein-stimulated GTPase, and ARL3 guanine nucleotide exchange factor activities. Biochemical analyses of three human missense mutations found in JS and of two consensus GTPase motifs reinforce the atypical properties of this regulatory GTPase. We also discovered that murine Arl13b is a substrate for casein kinase 2, a contaminant in our preparation from human embryonic kidney cells. This activity, and the ability of casein kinase 2 to use GTP as a phosphate donor, may be a source of differences between our data and previously published results. These results provide a solid framework for further research into ARL13B on which to develop models for the actions of this clinically important cell regulator.

Arf GAPs and molecular motors

Arf GTPase-activating proteins (Arf GAPs) were first identified as regulators of the small GTP-binding proteins ADP-ribosylation factors (Arfs). The Arf GAPs are a large family of proteins in metazoans, outnumbering the Arfs that they regulate. The members of the Arf GAP family have complex domain structures and some have been implicated in particular cellular functions, such as cell migration, or with particular pathologies, such as tumor invasion and metastasis. The specific effects of Arfs sometimes depend on the Arf GAP involved in their regulation. These observations have led to speculation that the Arf GAPs themselves may affect cellular activities in capacities beyond the regulation of Arfs. Recently, 2 Arf GAPs, ASAP1 and AGAP1, have been found to bind directly to and influence the activity of myosins and kinesins, motor proteins associated with filamentous actin and microtubules, respectively. The Arf GAP-motor protein interaction is critical for cellular behaviors involving the actin cytoskeleton and microtubules, such as cell migration and other cell movements. Arfs, then, may function with molecular motors through Arf GAPs to regulate microtubule and actin remodeling.

The Function of Arf-like Proteins ARL2 and ARL3 in Photoreceptors

Arf-like proteins (ARLs) are ubiquitously expressed small G proteins of the RAS superfamily. In photoreceptors, ARL2 and ARL3 participate in the trafficking of lipidated membrane-associated proteins and colocalize in the inner segment with UNC119A and PDEδ. UNC119A and PDEδ are acyl- and prenyl-binding proteins, respectively, involved in trafficking of acylated (transducin-α subunit, nephrocystin NPHP3) and prenylated proteins (GRK1, PDE6). Germline Arl3 knockout mice do not survive beyond postnatal day 21 and display ciliary defects in multiple organs (kidney, liver and pancreas) as well as retinal degeneration. Conditional knockouts will be necessary to delineate mechanisms of protein transport in retina disease.

Overexpression of KIF23 predicts clinical outcome in primary lung cancer patients

OBJECTIVE

High-level expression of kinesin family member 23 (KIF23), a member of microtubule-dependent molecular motors that transport organelles within cells and move chromosomes during cell division, has been observed in a variety of human malignancies. The aims of the present study were to observe the expression of KIF23 in lung cancer, examine the role of KIF23 in lung cancer cell growth and/or survival by small interfering RNA experiments, and explore its clinicopathologic significance and evaluate KIF23 expression as a prognostic marker.

MATERIALS AND METHODS

Quantitative reverse transcription-polymerase chain reaction analysis was performed to detect the expression of KIF23 mRNA using metastatic lymph nodes from patients with advanced lung cancer obtained by endobronchial ultrasonography-guided transbronchial needle aspiration (EBUS-TBNA) and primary lung tumors through surgical sample. The role of KIF23 in cancer cell growth was examined by small interfering RNA experiments. A total of 339 lung cancers were analyzed immunohistochemically on tissue microarrays to examine the expression of KIF23 protein and its clinicopathologic significance.

RESULTS

KIF23 transcript was found to be overexpressed in the great majority of metastatic lymph nodes from advanced lung cancers and primary lung tumors. Inhibiting KIF23 expression effectively suppressed lung cancer cell growth. High-level KIF23 expression was observed in 67.8% of the 339 cases. Lung adenocarcinoma patients with tumors displaying a high-level of KIF23 expression was also identified as an independent prognostic factor by multivariate analysis (P=0.0064).

CONCLUSION

KIF23 not only provides additional prognostic information for surgical treatment of lung cancer, but may also be a novel therapeutic target for these patients.

Binding of Kif23-iso1/CHO1 to 14-3-3 is regulated by sequential phosphorylations at two LATS kinase consensus sites

Kif23 kinesin is an essential actor of cytokinesis in animals. It exists as two major isoforms, known as MKLP1 and CHO1, the longest of which, CHO1, contains two HXRXXS/T NDR/LATS kinase consensus sites. We demonstrate that these two sites are readily phosphorylated by NDR and LATS kinases in vitro, and this requires the presence of an upstream -5 histidine residue. We further show that these sites are phosphorylated in vivo and provide evidence revealing that LATS1,2 participate in the phosphorylation of the most C-terminal S814 site, present on both isoforms. This S814 phosphosite was previously reported to constitute a 14-3-3 binding site, which plays a role in Kif23 clustering during cytokinesis. Surprisingly, we found that phosphorylation of the upstream S716 NDR/LATS consensus site, present only in the longest Kif23 isoform, is required for efficient phosphorylation at S814, thus revealing sequential phosphorylation at these two sites, and differential regulation of Kif23-14-3-3 interaction for the two Kif23 isoforms. Finally, we provide evidence that Kif23 is largely unphosphorylated on S814 in post-abscission midbodies, making this Kif23 post-translational modification a potential marker to probe these structures.

Reverse two-hybrid techniques in the yeast Saccharomyces cerevisiae

Use of the yeast two-hybrid system has provided definition to many previously uncharacterized pathways through the identification and characterization of novel protein-protein interactions. The two-hybrid system uses the bifunctional nature of transcription factors, such as the yeast enhancer Gal4, to allow protein-protein interactions to be monitored through changes in transcription of reporter genes. Once a positive interaction has been identified, either of the interacting proteins can be mutated by site-specific or randomly introduced changes, to produce proteins with a decreased ability to interact. Mutants generated using this strategy are very powerful reagents in tests of the biological significance of the interaction and in defining the residues involved in the interaction. Such techniques are termed reverse two-hybrid methods. We describe a reverse two-hybrid method that generates loss-of-interaction mutations of the catalytic subunit of the Escherichia coli heat-labile toxin (LTA1) with decreased binding to the active (GTP-bound) form of human ARF3, its protein cofactor. While newer methods are emerging for performing interaction screens in mammalian cells, instead of yeast, the use of reverse two-hybrid in yeast remains a robust and powerful means of identifying loss-of-interaction point mutants and compensating changes that remain among the most powerful tools of testing the biological significance of a protein-protein interaction.

LdFlabarin, a new BAR domain membrane protein of Leishmania flagellum

During the Leishmania life cycle, the flagellum undergoes successive assembly and disassembly of hundreds of proteins. Understanding these processes necessitates the study of individual components. Here, we investigated LdFlabarin, an uncharacterized L. donovani flagellar protein. The gene is conserved within the Leishmania genus and orthologous genes only exist in the Trypanosoma genus. LdFlabarin associates with the flagellar plasma membrane, extending from the base to the tip of the flagellum as a helicoidal structure. Site-directed mutagenesis, deletions and chimera constructs showed that LdFlabarin flagellar addressing necessitates three determinants: an N-terminal potential acylation site and a central BAR domain for membrane targeting and the C-terminal domain for flagellar specificity. In vitro, the protein spontaneously associates with liposomes, triggering tubule formation, which suggests a structural/morphogenetic function. LdFlabarin is the first characterized Leishmania BAR domain protein, and the first flagellum-specific BAR domain protein.

Intrinsic Disorder in the Kinesin Superfamily

Kinesin molecular motors perform a myriad of intracellular transport functions. While their mechanochemical mechanisms are well understood and well-conserved throughout the superfamily, the cargo-binding and regulatory mechanisms governing the activity of kinesins are highly diverse and in general, are incompletely characterized. Here we present evidence from bioinformatic predictions indicating that most kinesin superfamily members contain significant regions of intrinsically disordered (ID) residues. ID regions can bind to multiple partners with high specificity, and are highly labile to post-translational modification and degradation signals. In kinesins, the predicted ID regions are primarily found in areas outside the motor domains, where primary sequences diverge by family, suggesting that ID may be a critical structural element for determining the functional specificity of individual kinesins. To support this idea, we present a systematic analysis of the kinesin superfamily, family by family, for predicted regions of ID. We combine this analysis with a comprehensive review of kinesin binding partners and post-translational modifications. We find two key trends across the entire kinesin superfamily. First, ID residues tend to be in the tail regions of kinesins, opposite the superfamily-conserved motor domains. Second, predicted ID regions correlate to regions that are known to bind to cargoes and/or undergo post-translational modifications. We therefore propose that ID is a structural element utilized by the kinesin superfamily in order to impart functional specificity to individual kinesins.

ARF6 GTPase protects the post-mitotic midbody from 14-3-3-mediated disintegration

In cytokinesis, there is a lengthy interval between cleavage furrow ingression and abscission, during which the midbody microtubule bundle provides both structural support for a narrow intercellular bridge and a platform that orchestrates the biochemical preparations for abscission. It is currently unclear how the midbody structure is stably maintained during this period. Here, we report a novel role for the ADP-ribosylation factor 6 (ARF6) GTPase in the post-mitotic stabilisation of midbody. Centralspindlin kinesin-6/RhoGAP complex, a midbody component critical for both the formation and function of the midbody, assembles in a sharp band at the centre of the structure in a manner antagonised by 14-3-3 protein. We show that ARF6 competes with 14-3-3 for binding to centralspindlin such that midbodies formed by centralspindlin mutants that can bind 14-3-3 but not ARF6 frequently collapse before abscission. These data indicate a novel mechanism for the regulation of midbody dynamics in which ARF6 protects the compacted centralspindlin assembly from dissipation by 14-3-3.

Structural basis for Arf6-MKLP1 complex formation on the Flemming body responsible for cytokinesis

A small GTPase, Arf6, is involved in cytokinesis by localizing to the Flemming body (the midbody). However, it remains unknown how Arf6 contributes to cytokinesis. Here, we demonstrate that Arf6 directly interacts with mitotic kinesin-like protein 1 (MKLP1), a Flemming body-localizing protein essential for cytokinesis. The crystal structure of the Arf6-MKLP1 complex reveals that MKLP1 forms a homodimer flanked by two Arf6 molecules, forming a 2:2 heterotetramer containing an extended β-sheet composed of 22 β-strands that spans the entire heterotetramer, suitable for interaction with a concave membrane surface at the cleavage furrow. We show that, during cytokinesis, Arf6 is first accumulated around the cleavage furrow and, prior to abscission, recruited onto the Flemming body via interaction with MKLP1. We also show by structure-based mutagenesis and siRNA-mediated knockdowns that the complex formation is required for completion of cytokinesis. A model based on these results suggests that the Arf6-MKLP1 complex plays a crucial role in cytokinesis by connecting the microtubule bundle and membranes at the cleavage plane.

Midbody assembly and its regulation during cytokinesis

The midbody is a transient structure that connects two daughter cells at the end of cytokinesis, with the principal function being to localize the site of abscission, which physically separates two daughter cells. Despite its importance, understanding of midbody assembly and its regulation is still limited. Here we describe how the structural composition of the midbody changes during progression throughout cytokinesis and explore the functional implications of these changes. Deriving from midzones, midbodies are organized by a set of microtubule interacting proteins that colocalize to a zone of microtubule overlap in the center. We found that these proteins split into three subgroups that relocalize to different parts of the midbody: the bulge, the dark zone, and the flanking zone. We characterized these relocalizations and defined domain requirements for three key proteins: MKLP1, KIF4, and PRC1. Two cortical proteins-anillin and RhoA-localized to presumptive abscission sites in mature midbodies, where they may regulate the endosomal sorting complex required for transport machinery. Finally, we characterized the role of Plk1, a key regulator of cytokinesis, in midbody assembly. Our findings represent the most detailed description of midbody assembly and maturation to date and may help elucidate how abscission sites are positioned and regulated.

Distinct roles of Rab11 and Arf6 in the regulation of Rab11-FIP3/arfophilin-1 localization in mitotic cells

Rab11 family interacting protein 3/arfophilin-1 is a dual effector of Rab11 and Arf6 and exhibits Rab11-dependent localization to recycling endosomes in interphase. Furthermore, FIP3 undergoes dynamic redistribution to the intercellular bridge during cytokinesis. However, regulation of FIP3 redistribution and its local function by Rab11 and Arf6 has remained controversial. In this study, we developed a procedure for detecting endogenous FIP3, Arf6, and Rab11 and determined that FIP3 is localized near the intercellular bridge during cytokinesis, and to the Flemming body (the midbody) immediately before abscission; Rab11 is localized near the intercellular bridge, but not to the Flemming body; and Arf6 is localized to the Flemming body. Time-lapse analyses showed that FIP3 is transported to the intercellular bridge during cytokinesis, together with Rab11; before abscission, FIP3 becomes localized to the Flemming body, where Arf6 is already present. After abscission, FIP3 and Arf6 are incorporated into one of the daughter cells as a Flemming body remnant. Based on these observations, we propose that FIP3 localization to recycling endosomes in interphase and their transport to the intercellular bridge during cytokinesis depend on Rab11, and targeting of FIP3-positive endosomal vesicles to the Flemming body in the abscission phase depends on Arf6.

Arfaptins are localized to the trans-Golgi by interaction with Arl1, but not Arfs

Arfaptins (arfaptin-1 and arfaptin-2/POR1) were originally identified as binding partners of the Arf small GTPases. Both proteins contain a BAR (Bin/Amphiphysin/Rvs) domain, which participates in membrane deformation. Here we show that arfaptins associate with trans-Golgi membranes. Unexpectedly, Arl1 (Arf-like 1), but not Arfs, determines the trans-Golgi association of arfaptins. We also demonstrate that arfaptins interact with Arl1 through their BAR domain-containing region and compete for Arl1 binding with golgin-97 and golgin-245/p230, both of which also bind to Arl1 through their GRIP (golgin-97/RanBP2/Imh1p/p230) domains. However, arfaptins and these golgins show only limited colocalization at the trans-Golgi. Time-lapse imaging of cells overexpressing fluorescent protein-tagged arfaptins and golgin-97 reveals that arfaptins, but not golgin-97, are included in vesicular and tubular structures emanating from the Golgi region. These observations indicate that arfaptins are recruited onto trans-Golgi membranes by interacting with Arl1, and capable of inducing membrane deformation via their BAR domains.

Still entangled: assembly of the central spindle by multiple microtubule modulators

The central spindle is a microtubule-based structure that assembles during anaphase of mitosis in animal cells and is essential for multiple steps of cytokinesis. Central spindle assembly occurs by the cooperative action of multiple microtubule motors and modulators. Here, we review the mechanism by which the central spindle is formed, the role of several key proteins in this process and how central spindle assembly is temporally and spatially coordinated with mitosis.

Arf3 is activated uniquely at the trans-Golgi network by brefeldin A-inhibited guanine nucleotide exchange factors

Arf3 associates with the TGN in a manner that is both temperature-sensitive and uniquely dependent on BIGs. TGN localization and release at 20°C are readily separated and depend on pairs of residues absolutely conserved and unique to Arf3 present at opposite ends of the protein. These results suggest that Arf3 plays a unique function at the TGN.

It is widely assumed that class I and II Arfs function interchangeably throughout the Golgi complex. However, we report here that in vivo, Arf3 displays several unexpected properties. Unlike other Golgi-localized Arfs, Arf3 associates selectively with membranes of the trans-Golgi network (TGN) in a manner that is both temperature-sensitive and uniquely dependent on guanine nucleotide exchange factors of the BIGs family. For example, BIGs knockdown redistributed Arf3 but not Arf1 from Golgi membranes. Furthermore, shifting temperature to 20°C, a temperature known to block cargo in the TGN, selectively redistributed Arf3 from Golgi membranes. Arf3 redistribution occurred slowly, suggesting it resulted from a change in membrane composition. Arf3 knockdown and overexpression experiments suggest that redistribution is not responsible for the 20°C block. To investigate in more detail the mechanism for Arf3 recruitment and temperature-dependent release, we characterized several mutant forms of Arf3. This analysis demonstrated that those properties are readily separated and depend on pairs of residues present at opposite ends of the protein. Furthermore, phylogenetic analysis established that all four critical residues were absolutely conserved and unique to Arf3. These results suggest that Arf3 plays a unique function at the TGN that likely involves recruitment by a specific receptor.

Understanding cytokinesis failure

Cytokinesis is the final step in cell division. The process begins during chromosome segregation, when the ingressing cleavage furrow begins to partition the cytoplasm between the nascent daughter cells. The process is not completed until much later, however, when the final cytoplasmic bridge connecting the two daughter cells is severed. Cytokinesis is a highly ordered process, requiring an intricate interplay between cytoskeletal, chromosomal and cell cycle regulatory pathways. A surprisingly broad range of additional cellular processes are also important for cytokinesis, including protein and membrane trafficking, lipid metabolism, protein synthesis and signaling pathways. As a highly regulated, complex process, it is not surprising that cytokinesis can sometimes fail. Cytokinesis failure leads to both centrosome amplification and production of tetraploid cells, which may set the stage for the development of tumor cells. However, tetraploid cells are abundant components of some normal tissues including liver and heart, indicating that cytokinesis is physiologically regulated. In this chapter, we summarize our current understanding of the mechanisms of cytokinesis, emphasizing steps in the pathway that may be regulated or prone to failure. Our discussion emphasizes findings in vertebrate cells although we have attempted to highlight important contributions from other model systems.

IQ-ArfGEF/BRAG1 is associated with synaptic ribbons in the mouse retina

IQ-ArfGEF/BRAG1 is a guanine nucleotide exchange factor for ADP ribosylation factors (Arfs), which are implicated in membrane trafficking and actin cytoskeleton dynamics. In this study, we examined the immunohistochemical localization of IQ-ArfGEF/BRAG1 in the adult mouse retina using light and electron microscopy. IQ-ArfGEF/BRAG1 was distributed in a punctate manner and colocalized well with RIBEYE in both the outer and inner plexiform layers. Immunoelectron microscopic analysis showed that IQ-ArfGEF/BRAG1 was localized at the synaptic ribbons of photoreceptors. When heterologously expressed in HeLa cells, IQ-ArfGEF/BRAG1 was recruited to RIBEYE-containing clusters and formed an immunoprecipitable complex with RIBEYE. Furthermore, immunoprecipitation analysis showed that anti-IQ-ArfGEF/BRAG1 antibody efficiently pulled down RIBEYE from retinal lysates. These findings indicate that IQ-ArfGEF/BRAG1 is a novel component of retinal synaptic ribbons and forms a protein complex with RIBEYE.

Spermatocyte cytokinesis requires rapid membrane addition mediated by ARF6 on central spindle recycling endosomes

The dramatic cell shape changes during cytokinesis require the interplay between microtubules and the actomyosin contractile ring, and addition of membrane to the plasma membrane. Numerous membrane-trafficking components localize to the central spindle during cytokinesis, but it is still unclear how this machinery is targeted there and how membrane trafficking is coordinated with cleavage furrow ingression. Here we use an arf6 null mutant to show that the endosomal GTPase ARF6 is required for cytokinesis in Drosophila spermatocytes. ARF6 is enriched on recycling endosomes at the central spindle, but it is required neither for central spindle nor actomyosin contractile ring assembly, nor for targeting of recycling endosomes to the central spindle. However, in arf6 mutants the cleavage furrow regresses because of a failure in rapid membrane addition to the plasma membrane. We propose that ARF6 promotes rapid recycling of endosomal membrane stores during cytokinesis, which is critical for rapid cleavage furrow ingression.

MKLP1 requires specific domains for its dendritic targeting

The mitotic kinesin-like protein 1 (MKLP1) is specifically localized to the dendrite of the developed neuron, but its targeting mechanism is still unclear. In this study, the role of distinct domains of MKLP1 in dendritic targeting was investigated by producing a series of enhanced green fluorescent protein (eGFP)-tagged MKLP1 and its variant mutations, and studying the distribution of these molecules in cultured primary hippocampal neurons using fluorescence microscopy. We have found that: (a) full-length MKLP1(1-856)-eGFP was distributed to the dendrite of hippocampal neurons, not the axon; (b) deletion of the ;motor' domain prevented the dendritic distribution; (c) deletion of the ;tail' domain caused axonal appearance; (d) mutants devoid of the ;stalk' domain were still specifically distributed to the dendrite. The results indicate that the motor and tail regions of MKLP1 are important and significant for its localization to the dendrites. We also discuss the difference between the targeting of membrane-anchoring proteins and the kinesin-like protein.

Centaurin-α1 interacts directly with kinesin motor protein KIF13B

Centaurin-α1 is a phosphatidylinositol 3,4,5-trisphosphate binding protein as well as a GTPase activating protein (GAP) for the ADP-ribosylation factor (ARF) family of small GTPases. To further understand its cellular function, we screened a rat brain cDNA library using centaurin-α1 as bait to identify centaurin-α1 interacting proteins. The yeast two-hybrid screen identified a novel kinesin motor protein as a centaurin-α1 binding partner. The motor protein, termed KIF13B, encoded by a single ∼9.5-kb transcript, is widely expressed with high levels observed in brain and kidney. Yeast two-hybrid and GST pull-down assays showed that the interaction between centaurin-α1 and KIF13B is direct and mediated by the GAP domain of centaurin-α1 and the stalk domain of KIF13B. Centaurin-α1 and KIF13B form a complex in vivo and the KIF13B interaction appears to be specific to centaurin-α1 as other members of the ARF GAP family did not show any binding activity. We also show that KIF13B and centaurin-α1 colocalize at the leading edges of the cell periphery whereas a deletion mutant of centaurin-α1 that lacks the KIF13B binding site, failed to colocalize with KIF13B in vivo. Finally, we demonstrate that KIF13B binding suppresses the ARF6 GAP activity of centaurin-α1 in intact cells. Together, our data suggest a mechanism where direct binding between centaurin-α1 and KIF13B could concentrate centaurin-α1 at the leading edges of cells, thus modulating ARF6 function.

Annexin 11 is required for midbody formation and completion of the terminal phase of cytokinesis

Annexins are Ca(2+)-binding, membrane-fusogenic proteins with diverse but poorly understood functions. Here, we show that during cell cycle progression annexin 11 translocates from the nucleus to the spindle poles in metaphase and to the spindle midzone in anaphase. Annexin 11 is recruited to the midbody in late telophase, where it forms part of the detergent-resistant matrix that also contains CHO1. To investigate the significance of these observations, we used RNA interference to deplete cells of annexin 11. A combination of confocal and video time-lapse microscopy revealed that cells lacking annexin 11 fail to establish a functional midbody. Instead, daughter cells remain connected by intercellular bridges that contain bundled microtubules and cytoplasmic organelles but exclude normal midbody components such as MKLP1 and Aurora B. Annexin 11-depleted cells failed to complete cytokinesis and died by apoptosis. These findings demonstrate an essential role for annexin 11 in the terminal phase of cytokinesis.

Role of the midbody matrix in cytokinesis: RNAi and genetic rescue analysis of the mammalian motor protein CHO1

CHO1 is a kinesin-like motor protein essential for cytokinesis in mammalian cells. To analyze how CHO1 functions, we established RNAi and genetic rescue assays. CHO1-depleted cells reached a late stage of cytokinesis but fused back to form binucleate cells because of the absence of the midbody matrix in the middle of the intercellular bridge. Expression of exogenous CHO1 restored the formation of the midbody matrix and rescued cytokinesis in siRNA-treated cells. By analyzing phenotypes rescued with different constructs, it was shown that both motor and stalk domains function in midbody formation, whereas the tail is essential for completion of cytokinesis after the midbody matrix has formed. During the terminal stage of cytokinesis, different subregions of the tail play distinctive roles in stabilizing the midbody matrix and maintaining an association between the midbody and cell cortex. These results demonstrate that CHO1 consists of functionally differentiated subregions that act in concert to ensure complete cell separation.

Small GTPases and the evolution of the eukaryotic cell

The origin of eukaryotes is one of the major challenges of evolutionary cell biology. Other than the endosymbiotic origin of mitochondria and chloroplasts, the steps leading to eukaryotic endomembranes and endoskeleton are poorly understood. Ras-family small GTPases are key regulators of cytoskeleton dynamics, vesicular trafficking and nuclear function. They are specific for eukaryotes and their expansion probably traces the evolution of core eukaryote features. The phylogeny of small GTPases suggests that the first endomembranes to evolve during eukaryote evolution had secretory, and not phagocytic, function. Based on the reconstruction of putative roles for ancestral small GTPases, a hypothetical scenario on the origins of the first endomembranes, the nucleus, and phagocytosis is presented.

Munc18 interacting proteins: ADP-ribosylation factor-dependent coat proteins that regulate the traffic of beta-Alzheimer's precursor protein

Coat proteins cycle between soluble and membrane-bound locations at the time of vesicle biogenesis and act to regulate the assembly of the vesicle coat that determines the specificity in cargo selection and the destination of the vesicle. A transmembrane cargo protein, an Arf GTPase, and a coat protein (e.g. COPs, APs, or GGAs) are minimal components required for budding of vesicles. Munc18 interacting proteins (MINTs) are a family of three proteins implicated in the localization of receptors to the plasma membrane. We show that MINTs bind Arfs directly, co-localize with Arf and the Alzheimer's precursor protein (beta-APP) to regions of the Golgi/trans-Golgi network, and can co-immunoprecipitate clathrin. We demonstrate that MINTs bind Arfs through a region of the PTB domain and the PDZ2 domain, and Arf-MINT interaction is necessary for the increased cellular levels of beta-APP produced by MINT overexpression. Knockdown (small interference RNA) experiments implicate beta-APP as a transmembrane cargo protein that works together with MINTs. We propose that MINTs are a family of Arf-dependent, vesicle-coat proteins that can regulate the traffic of beta-APP.

Arf and its many interactors

Arf GTP-binding proteins regulate membrane traffic and actin remodeling. Similar to other GTP-binding proteins, a complex of Arf-GTP with an effector protein mediates Arf function. Arf interacts with at least three qualitatively different types of effectors. First, it interacts with structural proteins, the vesicle coat proteins. The second type of effector is lipid-metabolizing enzymes, and the third comprises those proteins that bind to Arf-GTP but whose biochemical or biological functions are not yet clearly defined. Arf interacts with two other families of proteins, the exchange factors and the GTPase-activating proteins. Recent work examining the functional relationships among the diverse Arf interactors has led to reconsideration of the prevailing paradigms for Arf action.

M phase phosphoprotein 1 is a human plus-end-directed kinesin-related protein required for cytokinesis

The human M phase phosphoprotein 1 (MPP1), previously identified through a screening of a subset of proteins specifically phosphorylated at the G2/M transition (Matsumoto-Taniura, N., Pirollet, F., Monroe, R., Gerace, L., and Westendorf, J. M. (1996) Mol. Biol. Cell 7, 1455-1469), is characterized as a plus-end-directed kinesin-related protein. Recombinant MPP1 exhibits in vitro microtubule-binding and microtubule-bundling properties as well as microtubule-stimulated ATPase activity. In gliding experiments using polarity-marked microtubules, MPP1 is a slow molecular motor that moves toward the microtubule plus-end at a 0.07 microm/s speed. In cycling cells, MPP1 localizes mainly to the nuclei in interphase. During mitosis, MPP1 is diffuse throughout the cytoplasm in metaphase and subsequently localizes to the midzone to further concentrate on the midbody. MPP1 suppression by RNA interference induces failure of cell division late in cytokinesis. We conclude that MPP1 is a new mitotic molecular motor required for completion of cytokinesis.

Localization of Pavarotti-KLP in living Drosophila embryos suggests roles in reorganizing the cortical cytoskeleton during the mitotic cycle

Pav-KLP is the Drosophila member of the MKLP1 family essential for cytokinesis. In the syncytial blastoderm embryo, GFP-Pav-KLP cyclically associates with astral, spindle, and midzone microtubules and also to actomyosin pseudocleavage furrows. As the embryo cellularizes, GFP-Pav-KLP also localizes to the leading edge of the furrows that form cells. In mononucleate cells, nuclear localization of GFP-Pav-KLP is mediated through NLS elements in its C-terminal domain. Mutants in these elements that delocalize Pav-KLP to the cytoplasm in interphase do not affect cell division. In mitotic cells, one population of wild-type GFP-Pav-KLP associates with the spindle and concentrates in the midzone at anaphase B. A second is at the cell cortex on mitotic entry and later concentrates in the region of the cleavage furrow. An ATP binding mutant does not localize to the cortex and spindle midzone but accumulates on spindle pole microtubules to which actin is recruited. This leads either to failure of the cleavage furrow to form or later defects in which daughter cells remain connected by a microtubule bridge. Together, this suggests Pav-KLP transports elements of the actomyosin cytoskeleton to plus ends of astral microtubules in the equatorial region of the cell to permit cleavage ring formation.

ADP-ribosylation factor (ARF) interaction is not sufficient for yeast GGA protein function or localization

Golgi-localized gamma-ear homology domain, ADP-ribosylation factor (ARF)-binding proteins (GGAs) facilitate distinct steps of post-Golgi traffic. Human and yeast GGA proteins are only ~25% identical, but all GGA proteins have four similar domains based on function and sequence homology. GGA proteins are most conserved in the region that interacts with ARF proteins. To analyze the role of ARF in GGA protein localization and function, we performed mutational analyses of both human and yeast GGAs. To our surprise, yeast and human GGAs differ in their requirement for ARF interaction. We describe a point mutation in both yeast and mammalian GGA proteins that eliminates binding to ARFs. In mammalian cells, this mutation disrupts the localization of human GGA proteins. Yeast Gga function was studied using an assay for carboxypeptidase Y missorting and synthetic temperature-sensitive lethality between GGAs and VPS27. Based on these assays, we conclude that non-Arf-binding yeast Gga mutants can function normally in membrane trafficking. Using green fluorescent protein-tagged Gga1p, we show that Arf interaction is not required for Gga localization to the Golgi. Truncation analysis of Gga1p and Gga2p suggests that the N-terminal VHS domain and C-terminal hinge and ear domains play significant roles in yeast Gga protein localization and function. Together, our data suggest that yeast Gga proteins function to assemble a protein complex at the late Golgi to initiate proper sorting and transport of specific cargo. Whereas mammalian GGAs must interact with ARF to localize to and function at the Golgi, interaction between yeast Ggas and Arf plays a minor role in Gga localization and function.

Heregulin promotes expression and subcellular redistribution of ADP-ribosylation factor 3

To identify genes whose expression is modulated by heregulin-beta1 (HRG), a regulatory polypeptide for mammary epithelial cells, we performed differential display screening of MCF-7 cell mRNA. One cDNA clone upregulated by HRG was identical to human ADP-ribosylation factor 3 (ARF3), a guanine nucleotide-binding protein functioning in vesicular trafficking, phospholipase D activation and intracellular transport. HRG treatment increased expression of ARF3 mRNA and protein. Also, HRG triggered a rapid redistribution of ARF3, first to cell membranes and then to the nuclear compartment, where ARF3 colocalized with acetylated histone H3 in discrete regions. In addition, the ARF3 protein was developmentally regulated in the mammary gland with the highest levels in virgin and post-weaning glands. Together, these findings suggest for the first time that stimulation of ARF3 expression, subcellular redistribution and interaction with acetylated histone H3 may play a role in the action of HRG in mammary epithelial cells.

Kinesin-like protein CHO1 is required for the formation of midbody matrix and the completion of cytokinesis in mammalian cells

CHO1 is a mammalian kinesin-like motor protein of the MKLP1 subfamily. It associates with the spindle midzone during anaphase and concentrates to a midbody matrix during cytokinesis. CHO1 was originally implicated in karyokinesis, but the invertebrate homologues of CHO1 were shown to function in the midzone formation and cytokinesis. To analyze the role of the protein in mammalian cells, we mutated the ATP-binding site of CHO1 and expressed it in CHO cells. Mutant protein (CHO1F') was able to interact with microtubules via ATP-independent microtubule-binding site(s) but failed to accumulate at the midline of the central spindle and affected the localization of endogenous CHO1. Although the segregation of chromosomes, the bundling of midzone microtubules, and the initiation of cytokinesis proceeded normally in CHO1F'-expressing cells, the completion of cytokinesis was inhibited. Daughter cells were frequently entering interphase while connected by a microtubule-containing cytoplasmic bridge from which the dense midbody matrix was missing. Depletion of endogenous CHO1 via RNA-mediated interference also affected the formation of midbody matrix in dividing cells, caused the disorganization of midzone microtubules, and resulted in abortive cytokinesis. Thus, CHO1 may not be required for karyokinesis, but it is essential for the proper midzone/midbody formation and cytokinesis in mammalian cells.

CHO1, a mammalian kinesin-like protein, interacts with F-actin and is involved in the terminal phase of cytokinesis

CHO1 is a kinesin-like protein of the mitotic kinesin-like protein (MKLP)1 subfamily present in central spindles and midbodies in mammalian cells. It is different from other subfamily members in that it contains an extra approximately 300 bp in the COOH-terminal tail. Analysis of the chicken genomic sequence showed that heterogeneity is derived from alternative splicing, and exon 18 is expressed in only the CHO1 isoform. CHO1 and its truncated isoform MKLP1 are coexpressed in a single cell. Surprisingly, the sequence encoded by exon 18 possesses a capability to interact with F-actin, suggesting that CHO1 can associate with both microtubule and actin cytoskeletons. Microinjection of exon 18-specific antibodies did not result in any inhibitory effects on karyokinesis and early stages of cytokinesis. However, almost completely separated daughter cells became reunited to form a binulceate cell, suggesting that the exon 18 protein may not have a role in the formation and ingression of the contractile ring in the cortex. Rather, it might be involved directly or indirectly in the membrane events necessary for completion of the terminal phase of cytokinesis.

Rabs grab motors: defining the connections between Rab GTPases and motor proteins

Rab GTPases and their effectors regulate membrane traffic by determining, along with cognate SNAREs, the specificity of transport vesicle docking and fusion steps. Recent studies have also implicated Rabs in the movement of these transport vesicles from their site of formation to their site of fusion, and several Rabs have been linked to specific microtubule- or actin-based motor proteins. Analyses of Rab and motor protein mutants, coupled with advanced imaging techniques, have led to the suggestion that certain Rabs function as essential components of the vesicle receptor for specific motor proteins.

Regulation of Golgi structure and function by ARF-like protein 1 (Arl1)

Arl1 is a member of the ARF-like protein (Arl) subfamily of small GTPases. Nothing is known about the function of Arl1 except for the fact that it is essential for normal development in Drosophila and that it is associated with the Golgi apparatus. In this study, we first demonstrate that Arl1 is enriched at the trans side of the Golgi, marked by AP-1. Association of Arl1 with the Golgi is saturable in intact cells and depends on N-terminal myristoylation. Over-expression of Arl1(T31N), which is expected to be restricted to the GDP-bound form and thus function as a dominant-negative mutant, causes the disappearance of the Golgi apparatus (marked by Golgi SNARE GS28), suggesting that Arl1 is necessary for maintaining normal Golgi structure. Overexpression of Arl1(Q71L), a mutant restricted primarily to the activated GTP-bound form, causes an expansion of the Golgi apparatus with massive and stable Golgi association of COPI and AP-1 coats. Interestingly, Golgi ARFs also become stably associated with the expanded Golgi. Transport of the envelope protein of vesicular stomatitis virus (VSV-G) along the secretory pathway is arrested at the expanded Golgi upon expression of Arl1(Q71L). The structure of stacked cisternae of the Golgi is disrupted in cells expressing Arl1(Q71L), resulting in the transformation of the Golgi into an extensive vesicule-tubule network. In addition, the GTP form of Arl1 interacts with arfaptin-2/POR1 but not GGA1, both of which interact with GTP-restricted ARF1, suggesting that Arl1 and ARF1 share some common effectors in regulating cellular events. On the basis of these observations, we propose that one of the mechanisms for the cell to regulate the structure and function of the Golgi apparatus is through the action of Arl1.

Functional and physical interactions of the adaptor protein complex AP-4 with ADP-ribosylation factors (ARFs)

AP-4 is a member of the family of heterotetrameric adaptor protein (AP) complexes that mediate the sorting of integral membrane proteins in post-Golgi compartments. This complex consists of four subunits (epsilon, beta4, mu4 and sigma4) and localizes to the cytoplasmic face of the trans-Golgi network (TGN). Here, we show that the recruitment of endogenous AP-4 to the TGN in vivo is regulated by the small GTP-binding protein ARF1. In addition, we demonstrate a direct interaction of the epsilon and mu4 subunits of AP-4 with ARF1. epsilon binds only to ARF1-GTP and requires residues in the switch I and switch II regions of ARF1. In contrast, mu4 binds equally well to the GTP- and GDP-bound forms of ARF1 and is less dependent on switch I and switch II residues. These observations establish AP-4 as an ARF1 effector and suggest a novel mode of interaction between ARF1 and an AP complex involving both constitutive and regulated interactions.

Interactions between adaptor protein-1 of the clathrin coat and microtubules via type 1a microtubule-associated proteins

The classical view suggests that adaptor proteins of the clathrin coat mediate the sorting of cargo protein passengers into clathrin-coated pits and the recruitment of clathrin into budding areas in the donor membrane. In the present study, we provide biochemical and morphological evidence that the adaptor protein 1 (AP-1) adaptor of the trans-Golgi network clathrin interacts with microtubules. AP-1 in cytosolic extracts interacted with in vitro assembled microtubules, and these interactions were inhibited by ATP depletion of the extracts or in the presence of 5'-adenylylimidodiphosphate. An overexpressed gamma-subunit of the AP-1 complex associated with microtubules, suggesting that this subunit may mediate the interaction of AP-1 with the cytoskeleton. Purified AP-1 did not interact with purified microtubules, but interaction occurred when an isolated microtubule-associated protein fraction was added to the reaction mix. The gamma-adaptin subunit of AP-1 specifically co-immunoprecipitated with a microtubule-associated protein of type 1a from rat brain cytosol. This suggests that type 1a microtubule-associated protein may mediate the association of AP-1 with microtubules in the cytoplasm. The microtubule binding activity of AP-1 was markedly inhibited in cytosol of mitotic cells. By means of its interaction with microtubule-associated proteins, we propose novel roles for AP-1 adaptors in modulating the dynamics of the cytoskeleton, the stability and shape of coated organelles, and the loading of nascent AP-1-coated vesicles onto appropriate microtubular tracks.

ADP-ribosylation factors (ARFs) and ARF-like 1 (ARL1) have both specific and shared effectors: characterizing ARL1-binding proteins

Despite the 40-60% identity between ADP-ribosylation factors (ARFs) and ARF-like (ARL) proteins, distinct functional roles have been inferred from findings that ARLs lack the biochemical or genetic activities characteristic of ARFs. The potential for functional overlap between ARFs and ARLs was examined by comparing effects of expression on intact cells and the ability to bind effectors. Expression of [Q71L]ARL1 in mammalian cells led to altered Golgi structure similar to, but less dramatic than, that reported previously for [Q71L]ARF1. Two previously identified partners of ARFs, MKLP1 and Arfaptin2/POR1, also bind ARL1 but not ARL2 or ARL3. Two-hybrid screens of human cDNA libraries with dominant active mutants of human ARL1, ARL2, and ARL3 identified eight different but overlapping sets of binding partners. Specific interactions between ARL1 and two binding proteins, SCOCO and Golgin-245, are defined and characterized in more detail. Like ARFs and ARL1, the binding of SCOCO to Golgi membranes is rapidly reversed by brefeldin A, suggesting the presence of a brefeldin A-sensitive ARL1 exchange factor. These data reveal a complex network of interactions between GTPases in the ARF family and their effectors and reveal a potential for cross-talk not demonstrated previously.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

Yeast GGA proteins interact with GTP-bound Arf and facilitate transport through the Golgi

ARF proteins regulate the formation of transport vesicles at many steps of the secretory and endocytic pathways. A recently identified family of ARF effectors, named GGAs, appears to regulate membrane traffic exiting the trans-Golgi network in mammalian cells (Boman et al., 2000). We have identified two GGA homologues in the yeast S. cerevisiae. These previously uncharacterized open reading frames, YDR358w and YHR108w, have been named GGA1 and GGA2, respectively. Using the two-hybrid assay and GST-affinity chromatography, we show that Gga1p and Gga2p interact with Arf1p and Arf2p in a GTP-dependent manner, suggesting that both are functional homologues of the human GGA proteins. The Arf-binding domain resides in the amino-terminal half of Gga1p (amino acids 170-330), and the carboxy-terminal 100 amino acids resemble the gamma-adaptin 'ear domain'. Gene deletion experiments indicate that GGA1 and GGA2 are not essential genes, as single and double knockouts are viable at both 30 degrees C and 37 degrees C. However, cells lacking GGA1 and GGA2 exhibit defects in invertase processing and CPY sorting, but not endocytosis. We conclude that yeast Gga proteins are effectors of Arf in yeast that facilitate traffic through the late Golgi.

Residues forming a hydrophobic pocket in ARF3 are determinants of GDP dissociation and effector interactions

Three residues of human ADP-ribosylation factor 3 (ARF3) (F51, W66 and Y81) cluster into a hydrophobic pocket in the inactive, GDP-bound protein. Disruption of the hydrophobic pocket with mutations at these residues increased the rate of GDP dissociation and association, but not always that of GTPgammaS. Several of the same mutants were found to be defective, often selectively, in binding different ARF effectors in two-hybrid assays. These results highlight three features of these hydrophobic residues in regulating (1) the rate of GDP dissociation, (2) the conformational changes that promote GTP binding and (3) their role in binding target proteins.

Molecular aspects of the cellular activities of ADP-ribosylation factors

Adenosine diphosphate-ribosylation factor (Arf) proteins are members of the Arf arm of the Ras superfamily of guanosine triphosphate (GTP)-binding proteins. Arfs are named for their activity as cofactors for cholera toxin-catalyzed adenosine diphosphate-ribosylation of the heterotrimeric G protein Gs. Physiologically, Arfs regulate membrane traffic and the actin cytoskeleton. Arfs function both constitutively within the secretory pathway and as targets of signal transduction in the cell periphery. In each case, the controlled binding and hydrolysis of GTP is critical to Arf function. The activities of some guanine nucleotide exchange factors (GEFs) and guanosine triphosphatase (GTPase)-activating proteins (GAPs) are stimulated by phosphoinositides, including phosphatidylinositol 3,4,5-trisphosphate (PIP3) and phosphatidylinositol 4,5-bisphosphate (PIP2), and phosphatidic acid (PA), likely providing both a means to respond to regulatory signals and a mechanism to coordinate GTP binding and hydrolysis. Arfs affect membrane traffic in part by recruiting coat proteins, including COPI and clathrin adaptor complexes, to membranes. However, Arf function likely involves many additional biochemical activities. Arf activates phospholipase D and phosphatidylinositol 4-phosphate 5-kinase with the consequent production of PA and PIP2, respectively. In addition to mediating Arf's effects on membrane traffic and the actin cytoskeleton, PA and PIP2 are involved in the regulation of Arf. Arf also works with Rho family proteins to affect the actin cytoskeleton. Several Arf-binding proteins suspected to be effectors have been identified in two-hybrid screens. Arf-dependent biochemical activities, actin cytoskeleton changes, and membrane trafficking may be integrally related. Understanding Arf's role in complex cellular functions such as protein secretion or cell movement will involve a description of the temporal and spatial coordination of these multiple Arf-dependent events.

The Rab6-binding kinesin, Rab6-KIFL, is required for cytokinesis

The Rab6-binding kinesin, Rab6-KIFL, was identified in a two-hybrid screen for proteins that interact with Rab6, a small GTPase involved in membrane traffic through the Golgi apparatus. We find that Rab6-KIFL accumulates in mitotic cells where it localizes to the midzone of the spindle during anaphase, and to the cleavage furrow and midbody during telophase. Overexpression of Rab6-KIFL causes a cell division defect resulting in cell death. Microinjection of antibodies to Rab6-KIFL results in the cells becoming binucleate after one cell cycle, and time-lapse microscopy reveals that this is due to a defect in cleavage furrow formation and thus cytokinesis. These data show that endogenous Rab6-KIFL functions in cell division during cleavage furrow formation and cytokinesis, in addition to its previously described role in membrane traffic.

Regulators and effectors of the ARF GTPases

The small G proteins of the ARF family are key regulators of membrane dynamics. Many functions of ARF proteins in cells are being revealed by studies of their regulators and effectors. Significant progress has been made over the past year, with the identification of a surprisingly large family of novel ARF GTPase-activating proteins. In addition, two new classes of effectors, the PIP kinases and a novel family of monomeric coat-like proteins have been discovered.

Drivers and passengers wanted! the role of kinesin-associated proteins

Members of the kinesin superfamily of proteins participate in a wide variety of cellular processes. Although much attention has been devoted to the structural and biophysical properties of the force-generating motor domain of kinesins, the factors controlling the functional specificity of each kinesin have only recently been examined. Genetic and biochemical approaches have identified two classes of proteins that associate physically with the diverse non-motor domains of kinesins. These proteins can be divided into two general classes: first, those that form tight complexes with the kinesin and are instrumental in directing the distinct function of the motor (i.e. drivers) and, second, those proteins that might transiently interact with the motor or be an integral part of the motor's cargo (i.e. passengers). Here, we discuss known kinesin-binding proteins, and how they might participate in the activity of their motor partners.

Effectors increase the affinity of ADP-ribosylation factor for GTP to increase binding

The stoichiometry of the binding of GTP to ADP-ribosylation factor (ARF) proteins, normally quite low at approximately 0.05 mol/mol protein, was found to increase to a maximum of 1 mol/mol in the presence of effectors. The mechanism of this action was found to result from the ability of these effectors to increase the affinity of ARF for activating guanine nucleotide triphosphates. The existence of a conformation of ARF with low affinity (>100 micrometer) for GTP is proposed. The actions of effectors to increase the equilibrium binding of GTP is interpreted as evidence that these same effectors interact with and modulate the affinity of the inactive ARF for GTP. A new model for these interactions among ARF, effectors, and GTP is proposed, and a preliminary test in cells is supportive of these observations with relevance to signaling in cells.

A family of ADP-ribosylation factor effectors that can alter membrane transport through the trans-Golgi

A family of three structurally related proteins were cloned from human cDNA libraries by their ability to interact preferentially with the activated form of human ADP-ribosylation factor 3 (ARF3) in two-hybrid assays. The specific and GTP-dependent binding was later confirmed through direct protein binding of recombinant proteins. The three proteins share large ( approximately 300 residues) domains at their N termini that are 60-70% identical to each other and a shorter (73 residues) domain at their C termini with 70% homology to the C-terminal "ear" domain of gamma-adaptin. Although GGA1 is found predominantly as a soluble protein by cell fractionation, all three proteins were found to localize to the trans-Golgi network (TGN) by indirect immunofluorescence. The binding of GGAs to TGN was sensitive to brefeldin A, consistent with this being an ARF-dependent event. Thus, these proteins have been named Golgi-localizing, gamma-adaptin ear homology domain, ARF-binding proteins, or GGAs. The finding that overexpression of GGAs was sufficient to alter the distribution of markers of the TGN (TGN38 and mannose 6-phosphate receptors) led us to propose that GGAs are effectors for ARFs that function in the regulation of membrane traffic through the TGN.