GRIF-1 and OIP106, members of a novel gene family of coiled-coil domain proteins: association in vivo and in vitro with kinesin

A mitotic GlcNAcylation/phosphorylation signaling complex alters the posttranslational state of the cytoskeletal protein vimentin

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic intracellular protein modification responsive to stress, hormones, nutrients, and cell cycle stage. Alterations in O-GlcNAc addition or removal (cycling) impair cell cycle progression and cytokinesis, but the mechanisms are not well understood. Here, we demonstrate that the enzymes responsible for O-GlcNAc cycling, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are in a transient complex at M phase with the mitotic kinase Aurora B and protein phosphatase 1. OGT colocalized to the midbody during telophase with Aurora B. Furthermore, these proteins coprecipitated with each other in a late mitotic extract. The complex was stable under Aurora inhibition; however, the total cellular levels of O-GlcNAc were increased and the localization of OGT was decreased at the midbody after Aurora inhibition. Vimentin, an intermediate filament protein, is an M phase substrate for both Aurora B and OGT. Overexpression of OGT or OGA led to defects in mitotic phosphorylation on multiple sites, whereas OGT overexpression increased mitotic GlcNAcylation of vimentin. OGA inhibition caused a decrease in vimentin late mitotic phosphorylation but increased GlcNAcylation. Together, these data demonstrate that the O-GlcNAc cycling enzymes associate with kinases and phosphatases at M phase to regulate the posttranslational status of vimentin.

GABA(A) Receptor Dynamics and Constructing GABAergic Synapses

GABA_A receptors are located on the majority of neurons in the central and peripheral nervous system, where they mediate important actions of the neurotransmitter gamma-aminobutyric acid. Early in development the trophic properties of GABA allow a healthy development of the nervous system. Most neurons have a high intracellular Cl-concentration early in life due to the late functional expression of the Cl-pump KCC2, therefore GABA has excitatory effects at this stage. Upon higher expression and activation of KCC2 GABA takes on its inhibitory effects while glutamate functions as the major excitatory neurotransmitter. Like all multisubunit membrane proteins the GABA_A receptor is assembled in the ER and travels through the Golgi and remaining secretory pathway to the cell surface, where it mediates GABA actions either directly at the synapses or at extrasynaptic sites responding to ambient GABA to provide a basal tonic inhibitory state. In order to adapt to changing needs and information states, the GABAergic system is highly dynamic. That includes subtype specific trafficking to different locations in the cell, regulation of mobility by interaction with scaffold molecules, posttranslational modifications, that either directly affect channel function or the interaction with other proteins and finally the dynamic exchange between surface and intracellular receptor pools, that either prepare receptors for recycling to the surface or degradation. Here we give an overview of the current understanding of GABA_A receptor functional and molecular dynamics that play a major part in maintaining the balance between excitation and inhibition and in changes in network activity.

GABA(A) receptor trafficking and its role in the dynamic modulation of neuronal inhibition

GABA (gamma-aminobutyric acid) type A receptors (GABA(A)Rs) mediate most fast synaptic inhibition in the mammalian brain, controlling activity at both the network and the cellular levels. The diverse functions of GABA in the CNS are matched not just by the heterogeneity of GABA(A)Rs, but also by the complex trafficking mechanisms and protein-protein interactions that generate and maintain an appropriate receptor cell-surface localization. In this Review, we discuss recent progress in our understanding of the dynamic regulation of GABA(A)R composition, trafficking to and from the neuronal surface, and lateral movement of receptors between synaptic and extrasynaptic locations. Finally, we highlight a number of neurological disorders, including epilepsy and schizophrenia, in which alterations in GABA(A)R trafficking occur.

EMB2473/MIRO1, an Arabidopsis Miro GTPase, is required for embryogenesis and influences mitochondrial morphology in pollen

The regulation of mitochondrial biogenesis, subcellular distribution, morphology, and metabolism are essential for all aspects of plant growth and development. However, the molecular mechanisms involved are still unclear. Here, we describe an analysis of the three Arabidopsis thaliana orthologs of the evolutionarily conserved Miro GTPases. Two of the genes, MIRO1 and MIRO2, are transcribed ubiquitously throughout the plant tissues, and their gene products localize to mitochondria via their C-terminal transmembrane domains. While insertional mutations in the MIRO2 gene do not have any visible impact on plant development, an insertional mutation in the MIRO1 gene is lethal during embryogenesis at the zygote to four-terminal-cell embryo stage. It also substantially impairs pollen germination and tube growth. Laser confocal and transmission electron microscopy revealed that the miro1 mutant pollen exhibits abnormally enlarged or tube-like mitochondrial morphology, leading to the disruption of continuous streaming of mitochondria in the growing pollen tube. Our findings suggest that mitochondrial morphology is influenced by MIRO1 and plays a vital role during embryogenesis and pollen tube growth.

Chemical approaches to understanding O-GlcNAc glycosylation in the brain

O-GlcNAc glycosylation is a unique, dynamic form of glycosylation found on intracellular proteins of all multicellular organisms. Studies suggest that O-GlcNAc represents a key regulatory modification in the brain, contributing to transcriptional regulation, neuronal communication and neurodegenerative disease. Recently, several new chemical tools have been developed to detect and study the modification, including chemoenzymatic tagging methods, quantitative proteomics strategies and small-molecule inhibitors of O-GlcNAc enzymes. Here we highlight some of the emerging roles for O-GlcNAc in the nervous system and describe how chemical tools have significantly advanced our understanding of the scope, functional significance and cellular dynamics of this modification.

Nucleocytoplasmic shuttling of the Golgi phosphatidylinositol 4-kinase Pik1 is regulated by 14-3-3 proteins and coordinates Golgi function with cell growth

The yeast phosphatidylinositol 4-kinase Pik1p is essential for proliferation, and it controls Golgi homeostasis and transport of newly synthesized proteins from this compartment. At the Golgi, phosphatidylinositol 4-phosphate recruits multiple cytosolic effectors involved in formation of post-Golgi transport vesicles. A second pool of catalytically active Pik1p localizes to the nucleus. The physiological significance and regulation of this dual localization of the lipid kinase remains unknown. Here, we show that Pik1p binds to the redundant 14-3-3 proteins Bmh1p and Bmh2p. We provide evidence that nucleocytoplasmic shuttling of Pik1p involves phosphorylation and that 14-3-3 proteins bind Pik1p in the cytoplasm. Nutrient deprivation results in relocation of Pik1p from the Golgi to the nucleus and increases the amount of Pik1p-14-3-3 complex, a process reversed upon restored nutrient supply. These data suggest a role of Pik1p nucleocytoplasmic shuttling in coordination of biosynthetic transport from the Golgi with nutrient signaling.

Moving mitochondria: establishing distribution of an essential organelle

Mitochondria form a dynamic network responsible for energy production, calcium homeostasis and cell signaling. Appropriate distribution of the mitochondrial network contributes to organelle function and is essential for cell survival. Highly polarized cells, including neurons and budding yeast, are particularly sensitive to defects in mitochondrial movement and have emerged as model systems for studying mechanisms that regulate organelle distribution. Mitochondria in multicellular eukaryotes move along microtubule tracks. Actin, the primary cytoskeletal component used for transport in yeast, has more subtle functions in other organisms. Kinesin, dynein and myosin isoforms drive motor-based movement along cytoskeletal tracks. Milton and syntabulin have recently been identified as potential organelle-specific adaptor molecules for microtubule-based motors. Miro, a conserved GTPase, may function with Milton to regulate transport. In yeast, Mmr1p and Ypt11p, a Rab GTPase, are implicated in myosin V-based mitochondrial movement. These potential adaptors could regulate motor activity and therefore determine individual organelle movements. Anchoring of stationary mitochondria also contributes to organelle retention at specific sites in the cell. Together, movement and anchoring ultimately determine mitochondrial distribution throughout the cell.

Mitochondria on the move

Interactions of mitochondria with the cytoskeleton are crucial for normal mitochondrial function and for localization of the organelle at its sites of action within cells. Early studies revealed a role for microtubule motors in mitochondrial motility in neurons and other cell types. Here, we describe advances in our understanding of mitochondrial movement and distribution. Specifically, we review recent studies on proteins that mediate or regulate the interaction between motor molecules and the organelle, motor-independent mechanisms for mitochondrial motility, anchorage of mitochondria at cortical sites within cells and links between mitochondria-cytoskeleton interactions and mitochondrial plasticity.

Marlin-1 and conventional kinesin link GABAB receptors to the cytoskeleton and regulate receptor transport

The cytoskeleton and cytoskeletal motors play a fundamental role in neurotransmitter receptor trafficking, but proteins that link GABA(B) receptors (GABA(B)Rs) to the cytoskeleton have not been described. We recently identified Marlin-1, a protein that interacts with GABA(B)R1. Here, we explore the association of GABA(B)Rs and Marlin-1 to the cytoskeleton using a combination of biochemistry, microscopy and live cell imaging. Our results indicate that Marlin-1 is associated to microtubules and the molecular motor kinesin-I. We demonstrate that a fraction of Marlin-1 is mobile in dendrites of cultured hippocampal neurons and that mobility is microtubule-dependent. We also show that GABA(B)Rs interact robustly with kinesin-I and that intracellular membranes containing GABA(B)Rs are sensitive to treatments that disrupt a protein complex containing Marlin-1, kinesin-I and tubulin. Finally, we report that a kinesin-I mutant severely impairs receptor transport. We conclude that Marlin-1 and kinesin-1 link GABA(B)Rs to the tubulin cytoskeleton in neurons.

GABAA receptors: properties and trafficking

Fast synaptic inhibition in the brain and spinal cord is mediated largely by ionotropic gamma-aminobutyric acid (GABA) receptors. GABAA receptors play a key role in controlling neuronal activity; thus modulating their function will have important consequences for neuronal excitation. GABAA receptors are important therapeutic targets for a range of sedative, anxiolytic, and hypnotic agents and are involved in a number of CNS diseases, including sleep disturbances, anxiety, premenstrual syndrome, alcoholism, muscle spasms, Alzheimer's disease, chronic pain, schizophrenia, bipolar affective disorders, and epilepsy. This review focuses on the functional and pharmacological properties of GABAA receptors and trafficking as an essential mechanism underlying the dynamic regulation of synaptic strength.

Targeting and retention of type 1 ryanodine receptors to the endoplasmic reticulum

Most ryanodine receptors and their relatives, inositol 1,4,5-trisphosphate receptors, are expressed in the sarcoplasmic or endoplasmic reticulum (ER), where they mediate Ca(2+) release. We expressed fragments of ryanodine receptor type 1 (RyR1) in COS cells alone or fused to intercellular adhesion molecule-1 (ICAM-1), each tagged with yellow fluorescent protein, and used confocal imaging and glycoprotein analysis to identify the determinants of ER targeting and retention. Single transmembrane domains (TMD) of RyR1 taken from the first (TMD1-TMD2) or last (TMD5-TMD6) pair were expressed in the ER membrane. TMD3-TMD4 was expressed in the outer mitochondrial membrane. The TMD outer pairs (TMD1-TMD2 and TMD5-TMD6) retained ICAM-1, a plasma membrane-targeted protein, within the ER membrane. TMD1 alone provided a strong ER retention signal and TMD6 a weaker signal, but the other single TMD were unable to retain ICAM-1 in the ER. We conclude that TMD1 provides the first and sufficient signal for ER targeting of RyR1. The TMD outer pairs include redundant ER retention signals, with TMD1 providing the strongest signal.

A cell biological perspective on mitochondrial dysfunction in Parkinson disease and other neurodegenerative diseases

Dysfunction of mitochondria is frequently proposed to be involved in neurodegenerative disease. Deficiencies in energy supply, free radical generation, Ca2+ buffering or control of apoptosis, could all theoretically contribute to progressive decline of the central nervous system. Parkinson disease illustrates how mutations in very different genes finally impinge directly or indirectly on mitochondrial function, causing subtle but finally fatal dysfunction of dopaminergic neurons. Neurons in general appear more sensitive than other cells to mutations in genes encoding mitochondrial proteins. Particularly interesting are mutations in genes such as Opa1, Mfn1 and Dnm1l, whose products are involved in the dynamic morphological alterations and subcellular trafficking of mitochondria. These indicate that mitochondrial dynamics are especially important for the long-term maintenance of the nervous system. The emerging evidence clearly demonstrates the crucial role of specific mitochondrial functions in maintaining neuronal circuit integrity.

Dystrobrevins in muscle and non-muscle tissues

The alpha- and beta-dystrobrevins belong to the family of dystrophin-related and dystrophin-associated proteins. As constituents of the dystrophin-associated protein complex, alpha-dystrobrevin was believed to have a role predominantly in muscles and beta-dystrobrevin in non-muscle tissues. Recent reports described novel localisations and molecular characteristics of alpha-dystrobrevin isoforms in non-muscle tissues (developing and adult). While single and double knockout studies have revealed distinct functions of dystrobrevin in some tissues, these also suggested a strong compensatory mechanism, where dystrobrevins displaying overlapping tissue expression pattern and structure/function similarity can substitute each other. No human disease has been unequivocally associated within mutations of dystrobrevin genes. However, some significant exceptions to these overlapping expression patterns, mainly in the brain, suggest that dystrobrevin mutations might underlie some specific motor, behavioural or cognitive defects. Dystrobrevin binding partner DTNBP1 (dysbindin) is a probable susceptibility gene for schizophrenia and bipolar affective disorder in some populations. As dysbindin abnormality is linked to Hermansky-Pudlak syndrome, dystrobrevins and/or their binding partners may also be required for proper function of other non-muscle tissues.

Mitochondrial morphology and distribution in mammalian cells

It is now appreciated that mitochondria form tubular networks that adapt to the requirements of the cell by undergoing changes in their shape through fission and fusion. Proper mitochondrial distribution also appears to be required for ATP delivery and calcium regulation, and, in some cases, for cell development. While we now realise the great importance of mitochondria for the cell, we are only beginning to work out how these organelles undergo the drastic morphological changes that are essential for cellular function. Of the few known components involved in shaping mitochondria, some have been found to be essential to life and their gene mutations are linked to neurological disorders, while others appear to be recruited in the activation of cell death pathways. Here we review our current understanding of the functions of the main players involved in mitochondrial fission, fusion and distribution in mammalian cells.

GABAAreceptor associated proteins: a key factor regulating GABAAreceptor function

gamma-Aminobutyric acid (GABA), an important inhibitory neurotransmitter in both vertebrates and invertebrates, acts on GABA receptors that are ubiquitously expressed in the CNS. GABA(A) receptors also represent a major site of action of clinically relevant drugs, such as benzodiazepines, barbiturates, ethanol, and general anesthetics. It has been shown that the intracellular M3-M4 loop of GABA(A) receptors plays an important role in regulating GABA(A) receptor function. Therefore, studies of the function of receptor intracellular loop associated proteins become important for understanding mechanisms of regulating receptor activity. Recently, several labs have used the yeast two-hybrid assay to identify proteins interacting with GABA(A) receptors, for example, the interaction of GABA(A) receptor associated protein (GABARAP) and Golgi-specific DHHC zinc finger protein (GODZ) with gamma subunits, PRIP, phospholipase C-related, catalytically inactive proteins (PRIP-1) and (PRIP-2) with GABARAP and receptor gamma2 and beta subunits, Plic-1 with some alpha and beta subunits, radixin with the alpha5 subunit, HAP1 with the beta1 subunit, GABA(A) receptor interacting factor-1 (GRIF-1) with the beta2 subunit, and brefeldin A-inhibited GDP/GTP exchange factor 2 (BIG2) with the beta3 subunit. These proteins have been shown to play important roles in modulating the activities of GABA(A) receptors ranging from enhancing trafficking, to stabilizing surface and internalized receptors, to regulating modification of GABA(A) receptors. This article reviews the current studies of GABA(A) receptor intracellular loop-associated proteins.

Kidins220/ARMS is transported by a kinesin-1-based mechanism likely to be involved in neuronal differentiation

Kinase D-interacting substrate of 220 kDa/ankyrin repeat-rich membrane spanning (Kidins220/ARMS) is a conserved membrane protein mainly expressed in brain and neuroendocrine cells, which is a downstream target of the signaling cascades initiated by neurotrophins and ephrins. We identified kinesin light chain 1 (KLC1) as a binding partner for Kidins220/ARMS by a yeast two-hybrid screen. The interaction between Kidins220/ARMS and the kinesin-1 motor complex was confirmed by glutathione S-transferase-pull-down and coimmunoprecipitation experiments. In addition, Kidins220/ARMS and kinesin-1 were shown to colocalize in nerve growth factor (NGF)-differentiated PC12 cells. Using Kidins220/ARMS and KLC1 mutants, we mapped the regions responsible for the binding to a short sequence of Kidins220/ARMS, termed KLC-interacting motif (KIM), which is sufficient for the interaction with KLC1. Optimal binding of KIM requires a region of KLC1 spanning both the tetratricopeptide repeats and the heptad repeats, previously not involved in cargo recognition. Overexpression of KIM in differentiating PC12 cells impairs the formation and transport of EGFP-Kidins220/ARMS carriers to the tips of growing neurites, leaving other kinesin-1 dependent processes unaffected. Furthermore, KIM overexpression interferes with the activation of the mitogen-activated protein kinase signaling and neurite outgrowth in NGF-treated PC12 cells. Our results suggest that Kidins220/ARMS-positive carriers undergo a kinesin-1-dependent transport linked to neurotrophin action.

Structure and trafficking of NMDA and GABAA receptors

The fidelity of synaptic function is dependent on the expression of the appropriate neurotransmitter receptor subtype, the targeting and trafficking of receptors to synapses as well as the regulation of the actual number of receptors at synapses. GABAA (γ-aminobutyric acid type A) receptors and NMDA (N-methyl-D-aspartate) receptors are both examples of ligand-gated, heteromeric neurotransmitter receptors whose cell-surface expression is dynamic and tightly regulated. NMDA receptors are localized at excitatory synapses. These synapses are highly structured but dynamic, with the interplay between NMDA receptors and NMDA receptor-associated scaffolding proteins regulating the expression of functional cell-surface synaptic and extrasynaptic receptors. Based on current information, inhibitory synapses seem to be less ordered, and a GABAA receptor equivalent of PSD-95 (postsynaptic density-95), the scaffolding molecule pivotal to the organization of NMDA receptor complexes at synapses, is yet to be validated. In the present paper, processes regulating the trafficking, assembly and molecular organization of both NMDA receptors and GABAA receptors will be discussed.

GRIF1 binds Hrs and is a new regulator of endosomal trafficking

Endosomal sorting of internalized cell surface receptors to the lysosomal pathway plays a crucial role in the control of cell signaling and function. Here we report the identification of GABA(A) receptor interacting factor-1 (GRIF1), a recently discovered protein of unknown function, as a new regulator of endosome-to-lysosome trafficking. Yeast two-hybrid screen and co-immunoprecipitation analysis reveal that GRIF1 interacts with hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), an essential component of the endosomal sorting machinery. We have mapped the binding domains of GRIF1 and Hrs that mediate their association and shown the colocalization of GRIF1 with Hrs on early endosomes. Like Hrs, both overexpression and siRNA-mediated depletion of GRIF1 inhibit the degradation of internalized epidermal growth factor receptors and block the trafficking of the receptors from early endosomes to the lysosomal pathway. Our results indicate, for the first time, a functional role for GRIF1 in the regulation of endosomal trafficking. Interestingly, overexpression of full-length GRIF1, but not the Hrs- or kinesin-interacting GRIF1 deletion mutants, causes a perinuclear clustering of early endosomes. Our findings suggest that GRIF1 may also participate in microtubule-based transport of early endosomes by acting as an adaptor linking Hrs-containing endosomes to kinesin.

Microtubule motors at the intersection of trafficking and transport

Molecular motors drive the transport of vesicles and organelles within the cell. Traditionally, these transport processes have been considered separately from membrane trafficking events, such as regulated budding and fusion. However, recent progress has revealed mechanistic links that integrate these processes within the cell. Rab proteins, which function as key regulators of intracellular trafficking, have now been shown to recruit specific motors to organelle membranes. Rab-independent recruitment of motors by adaptor or scaffolding proteins is also a key mechanism. Once recruited to vesicles and organelles, these motors can then drive directed transport; this directed transport could in turn affect the efficiency of trafficking events. Here, we discuss this coordinated regulation of trafficking and transport, which provides a powerful mechanism for temporal and spatial control of cellular dynamics.

Identification of gamma-aminobutyric acid receptor-interacting factor 1 (TRAK2) as a trafficking factor for the K+ channel Kir2.1

To identify proteins that regulate potassium channel activity and expression, we performed functional screening of mammalian cDNA libraries in yeast that express the mammalian K(+) channel Kir2.1. Growth of Kir2.1-expressing yeast in media with low K(+) concentration is a function of K(+) uptake via Kir2.1 channels. Therefore, the host strain was transformed with a human cDNA library, and cDNA clones that rescued growth at low K(+) concentration were selected. One of these clones was identical to the protein of unknown function isolated previously as gamma-aminobutyric acid receptor-interacting factor 1 (GRIF-1) (Beck, M., Brickley, K., Wilkinson, H., Sharma, S., Smith, M., Chazot, P., Pollard, S., and Stephenson, F. (2002) J. Biol. Chem. 277, 30079-30090). GRIF-1 specifically enhanced Kir2.1-dependent growth in yeast and Kir2.1-mediated (86)Rb(+) efflux in HEK293 cells. Quantitative microscopy and flow cytometry analysis of immunolabeled surface Kir2.1 channel showed that GRIF-1 significantly increased the number of Kir2.1 channels in the plasma membrane of COS and HEK293 cells. Physical interaction of Kir2.1 channel and GRIF-1 was demonstrated by co-immunoprecipitation from HEK293 lysates and yeast two-hybrid assay. In vivo association of Kir2.1 and GRIF-1 was demonstrated by co-immunoprecipitation from brain lysate. Yeast two-hybrid assays showed that an N-terminal region of GRIF-1 interacts with a C-terminal region of Kir2.1. These results indicate that GRIF-1 binds to Kir2.1 and facilitates trafficking of this channel to the cell surface.

Milton controls the early acquisition of mitochondria by Drosophila oocytes

Mitochondria in many species enter the young oocyte en mass from interconnected germ cells to generate the large aggregate known as the Balbiani body. Organelles and germ plasm components frequently associate with this structure. Balbiani body mitochondria are thought to populate the germ line, ensuring that their genomes will be inherited preferentially. We find that milton, a gene whose product was previously shown to associate with Kinesin and to mediate axonal transport of mitochondria, is needed to form a normal Balbiani body. In addition, germ cells mutant for some milton or Kinesin heavy chain (Khc) alleles transport mitochondria to the oocyte prematurely and excessively, without disturbing Balbiani body-associated components. Our observations show that the oocyte acquires the majority of its mitochondria by competitive bidirectional transport along microtubules mediated by the Milton adaptor. These experiments provide a molecular explanation for Balbiani body formation and, surprisingly, show that viable fertile offspring can be obtained from eggs in which the normal program of mitochondrial acquisition has been severely perturbed.

Structural organization and expression of human MTUS1, a candidate 8p22 tumor suppressor gene encoding a family of angiotensin II AT2 receptor-interacting proteins, ATIP

The Mitochondrial Tumor suppressor 1 (MTUS1) gene is a newly identified candidate tumor suppressor gene at chromosomal position 8p22. We report here that MTUS1 encodes a family of proteins whose leader member (ATIP1) was previously isolated in our laboratory as a novel interacting partner of the angiotensin II AT2 receptor involved in growth inhibition (Nouet, JBC 279: 28989-97, 2004). The MTUS1 gene contains 17 coding exons distributed over 112 kb of genomic DNA. Alternative exon usage generates three major transcripts (ATIP1, ATIP3 and ATIP4), each showing different tissue distribution. ATIP polypeptides are identical in their carboxy-terminal region carrying four coiled-coil domains. In their amino-terminal portion, ATIP polypeptides exhibit distinct motifs for localisation in the cytosol, nucleus or cell membrane, suggesting that MTUS1 gene products may be involved in a variety of intracellular functions in an AT2-dependent and independent manner. ATIP1 is ubiquitous and highly expressed in the brain. ATIP3 is the major transcript in tissues (prostate, bladder, breast, ovary, colon) corresponding to cancer types with frequent loss of heterozygosity at 8p22. Interestingly, ATIP4 is a brain-specific transcript highly abundant in the cerebellum and fetal brain. High evolutionary conservation of ATIP amino-acid sequences suggests important biological roles for this new family of proteins in tumor suppression and/or brain function.

Axonal transport of mitochondria requires milton to recruit kinesin heavy chain and is light chain independent

Mitochondria are distributed within cells to match local energy demands. We report that the microtubule-dependent transport of mitochondria depends on the ability of milton to act as an adaptor protein that can recruit the heavy chain of conventional kinesin-1 (kinesin heavy chain [KHC]) to mitochondria. Biochemical and genetic evidence demonstrate that kinesin recruitment and mitochondrial transport are independent of kinesin light chain (KLC); KLC antagonizes milton's association with KHC and is absent from milton–KHC complexes, and mitochondria are present in klc^−/− photoreceptor axons. The recruitment of KHC to mitochondria is, in part, determined by the NH₂ terminus–splicing variant of milton. A direct interaction occurs between milton and miro, which is a mitochondrial Rho-like GTPase, and this interaction can influence the recruitment of milton to mitochondria. Thus, milton and miro are likely to form an essential protein complex that links KHC to mitochondria for light chain–independent, anterograde transport of mitochondria.

Mapping the GRIF-1 binding domain of the kinesin, KIF5C, substantiates a role for GRIF-1 as an adaptor protein in the anterograde trafficking of cargoes

Gamma-aminobutyric acid, type A (GABAA) receptor interacting factor-1 (GRIF-1) and N-acetylglucosamine transferase interacting protein (OIP) 106 are both members of a newly identified coiled-coil family of proteins. They are kinesin-associated proteins proposed to function as adaptors in the anterograde trafficking of organelles to synapses. Here we have studied in more detail the interaction between the prototypic kinesin heavy chain, KIF5C, kinesin light chain, and GRIF-1. The GRIF-1 binding site of KIF5C was mapped using truncation constructs in yeast two-hybrid interaction assays, co-immunoprecipitations, and co-localization studies following expression in mammalian cells. Using these approaches, it was shown that GRIF-1 and the KIF5C binding domain of GRIF-1, GRIF-1-(124-283), associated with the KIF5C non-motor domain. Refined studies using yeast two-hybrid interactions and co-immunoprecipitations showed that GRIF-1 and GRIF-1-(124-283) associated with the cargo binding region within the KIF5C non-motor domain. Substantiation that the GRIF-1-KIF5C interaction was direct was shown by fluorescence resonance energy transfer analyses using fluorescently tagged GRIF-1 and KIF5C constructs. A significant fluorescence resonance energy transfer value was found between the C-terminal EYFP-tagged KIF5C and ECFP-GRIF-1, the C-terminal EYFP-tagged KIF5C non-motor domain and ECFP-GRIF-1, but not between the N-terminal EYFP-tagged KIF5C nor the EYFP-KIF5C motor domain and ECFP-GRIF-1, thus confirming direct association between the two proteins at the KIF5C C-terminal and GRIF-1 N-terminal regions. Co-immunoprecipitation and confocal imaging strategies further showed that GRIF-1 can bind to the tetrameric kinesin light-chain/kinesin heavy-chain complex. These findings support a role for GRIF-1 as a kinesin adaptor molecule requisite for the anterograde delivery of defined cargoes such as mitochondria and/or vesicles incorporating beta2 subunit-containing GABAA receptors, in the brain.

The atypical Rho GTPases Miro-1 and Miro-2 have essential roles in mitochondrial trafficking

We recently described the atypical Rho GTPases Miro-1 and Miro-2. These proteins have tandem GTP-binding domains separated by a linker region with putative calcium-binding motives. In addition, the Miro GTPases have a C-terminal transmembrane domain, which confers targeting to the mitochondria. It was reported previously that a constitutively active mutant of Miro-1 induced a clustering of the mitochondria. This response can be separated into two distinct phenotypes: a formation of aggregated mitochondria and the appearance of thread-like mitochondria probably caused by defects in mitochondrial trafficking. The first GTPase domain is required for the clustering of the mitochondria, but the effect is not dependent on the EF-hands. Miro-2 only induces aggregation and not the formation of thread-like mitochondria. Moreover, we show that Miro interacts with the Kinesin-binding proteins, GRIF-1 and OIP106, suggesting that the Miro GTPases form a link between the mitochondria and the trafficking apparatus of the microtubules.

A dishevelled-1/Smad1 interaction couples WNT and bone morphogenetic protein signaling pathways in uncommitted bone marrow stromal cells

Genetic evidence from both humans and mice suggests that Wnt/beta-catenin and bone morphogenetic protein (BMP) signaling pathways are essential for bone marrow mesenchymal stem cells to differentiate into osteoblasts. Here we describe a mechanism through which BMPs antagonize Wnt signaling and retard bone marrow mesenchymal stem cell proliferation. Treatment with Wnt3a, but not BMP-2, stimulated Lef1-mediated transcriptional activity, whereas co-stimulation with both Wnt3a and BMP-2 markedly reduced Wnt3a-induced reporter activity. Immunoprecipitation assays in 293T cells transfected with individual Smads and Wnt pathway components revealed a specific interaction between Dvl-1 and Smad1 that was dependent on the presence of Wnt3a or BMP-2. Under unstimulated conditions, Dvl-1 and Smad1 are co-immunoprecipitated and form a complex through the linker region of Smad1. Wnt3a treatment transiently disrupted the Dvl-1/Smad1 interaction coincident with nuclear accumulation of beta-catenin. In contrast, when cells were exposed to both Wnt3a and BMP-2, there was an enhanced accumulation of the Dvl-1-Smad1 complex and a decreased nuclear accumulation of beta-catenin. Expression of a mutant Smad1 protein, which cannot be phosphorylated in response to BMP, eliminated the inhibitory effect of BMP on Wnt-inducedbeta-catenin accumulation and transcriptional activity. These results identify a potential mechanism whereby BMP-2 antagonizes Wnt signaling in osteoblast progenitors by promoting an interaction between Smad1 and Dvl-1 that restricts beta-catenin activation.

Review: regulation mechanisms of Kinesin-1

Kinesin-1 microtubule motors are common kinesin motors from protozoa, fungi and animals. They transport vesicular or particle cargo in a strictly regulated manner. The relatively well-studied tail inhibition mechanism is based on a conformational change that leads to an interaction of Kinesin-1's tail with the junction of neck and hinge regions. This folding causes a decrease in microtubule binding and motor activity. In fungal Kinesin-1 motors several lines of evidence suggest that a conserved tyrosine in the neck coiled-coil mediates this inhibition. In the active state, a region surrounding a conserved tryptophan in the hinge stabilises the neck coiled-coil, and prevents the tyrosine from inhibiting. Although animal and fungal Kinesin-1 motors are clearly homologous and function according to the same chemo-mechanical mechanism, they differ in their regulation. Unlike fungal Kinesin-1s, animal kinesins associate with light chains that are important for regulation and cargo interaction. Several proteins interacting with animal Kinesin-1 heavy or light chains are known, among them typical scaffolding proteins that seem to link Kinesin-1 to signalling pathways.

GRIF-1–kinesin-1 interactions: a confocal microscopy study

GRIF-1 [GABAA (γ-aminobutyric acidA) receptor interacting factor-1] is a member of a coiled-coil family of proteins thought to function as adaptors in the anterograde trafficking of organelles utilizing the kinesin-1 motor proteins to synapses. To study in more detail the molecular interaction between GRIF-1 and the kinesin-1 family member KIF5C, fluorescent yellow- and fluorescent cyan-tagged GRIF-1, KIF5C, the KIF5C MD (motor domain) and the KIF5C NMD (non-motor domain) fusion proteins were generated. Each was characterized with respect to size and ability to co-associate by immunoprecipitation following expression in HEK-293 (human embryonic kidney 293) cells. Further, their distribution in transfected HEK-293 and transformed African green monkey kidney (COS-7) cells was analysed by confocal microscopy. The fluorescent GRIF-1 and KIF5C fusion proteins were all found to behave as wild-type. Double GRIF-1/KIF5C transfectants revealed co-localization. The GRIF-1/KIF5C and GRIF-1/KIF5C NMD double transfectants showed different subcellular distributions compared with single GRIF-1, KIF5C or KIF5C NMD transfections. These studies confirm the association between GRIF-1 and kinesin-1 NMDs. Fluorescence resonance energy transfer studies are ongoing to characterize this interaction in more detail.

Trak1 mutation disrupts GABA(A) receptor homeostasis in hypertonic mice

Hypertonia, which results from motor pathway defects in the central nervous system (CNS), is observed in numerous neurological conditions, including cerebral palsy, stroke, spinal cord injury, stiff-person syndrome, spastic paraplegia, dystonia and Parkinson disease. Mice with mutation in the hypertonic (hyrt) gene exhibit severe hypertonia as their primary symptom. Here we show that hyrt mutant mice have much lower levels of gamma-aminobutyric acid type A (GABA(A)) receptors in their CNS, particularly the lower motor neurons, than do wild-type mice, indicating that the hypertonicity of the mutants is likely to be caused by deficits in GABA-mediated motor neuron inhibition. We cloned the responsible gene, trafficking protein, kinesin binding 1 (Trak1), and showed that its protein product interacts with GABA(A) receptors. Our data implicate Trak1 as a crucial regulator of GABA(A) receptor homeostasis and underscore the importance of hyrt mice as a model for studying the molecular etiology of hypertonia associated with human neurological diseases.

Molecular Basis of Dystrobrevin Interaction with Kinesin Heavy Chain: Structural Determinants of their Binding

Dystrobrevins are a family of widely expressed dystrophin-associated proteins that comprises alpha and beta isoforms and displays significant sequence homology with several protein-binding domains of the dystrophin C-terminal region. The complex distribution of the multiple dystrobrevin isoforms suggests that the variability of their composition may be important in mediating their function. We have recently identified kinesin as a novel dystrobrevin-interacting protein and localized the dystrobrevin-binding site on the cargo-binding domain of neuronal kinesin heavy chain (Kif5A). In the present study, we assessed the kinetics of the dystrobrevin-Kif5A interaction by quantitative pull-down assay and surface plasmon resonance (SPR) analysis and found that beta-dystrobrevin binds to kinesin with high affinity (K(D) approximately 40 nM). Comparison of the sensorgrams obtained with alpha and beta-dystrobrevin at the same concentration of analyte showed a lower affinity of alpha compared to that of beta-dystrobrevin, despite their functional domain homology and about 70% sequence identity. Analysis of the contribution of single dystrobrevin domains to the interaction revealed that the deletion of either the ZZ domain or the coiled-coil region decreased the kinetics of the interaction, suggesting that the tertiary structure of dystrobrevin may play a role in regulating the interaction of dystrobrevin with kinesin. In order to understand if structural changes induced by post-translational modifications could affect dystrobrevin affinity for kinesin, we phosphorylated beta-dystrobrevin in vitro and found that it showed reduced binding capacity towards kinesin. The interaction between the adaptor/scaffolding protein dystrobrevin and the motor protein kinesin may play a role in the transport and targeting of components of the dystrophin-associated protein complex to specific sites in the cell, with the differences in the binding properties of dystrobrevin isoforms reflecting their functional diversity within the same cell type. Phosphorylation events could have a regulatory role in this context.

Syntabulin-mediated anterograde transport of mitochondria along neuronal processes

In neurons, proper distribution of mitochondria in axons and at synapses is critical for neurotransmission, synaptic plasticity, and axonal outgrowth. However, mechanisms underlying mitochondrial trafficking throughout the long neuronal processes have remained elusive. Here, we report that syntabulin plays a critical role in mitochondrial trafficking in neurons. Syntabulin is a peripheral membrane-associated protein that targets to mitochondria through its carboxyl-terminal tail. Using real-time imaging in living cultured neurons, we demonstrate that a significant fraction of syntabulin colocalizes and co-migrates with mitochondria along neuronal processes. Knockdown of syntabulin expression with targeted small interfering RNA or interference with the syntabulin–kinesin-1 heavy chain interaction reduces mitochondrial density within axonal processes by impairing anterograde movement of mitochondria. These findings collectively suggest that syntabulin acts as a linker molecule that is capable of attaching mitochondrial organelles to the microtubule-based motor kinesin-1, and in turn, contributes to anterograde trafficking of mitochondria to neuronal processes.