CLIPR-59 is a lipid raft-associated protein containing a cytoskeleton-associated protein glycine-rich domain (CAP-Gly) that perturbs microtubule dynamics

Thermoplasmonic Vesicle Fusion Reveals Membrane Phase Segregation of Influenza Spike Proteins

Many cellular processes involve the lateral organization of integral and peripheral membrane proteins into nanoscale domains. Despite the biological significance, the mechanisms that facilitate membrane protein clustering into nanoscale lipid domains remain enigmatic. In cells, the analysis of membrane protein phase affinity is complicated by the size and temporal nature of ordered and disordered lipid domains. To overcome these limitations, we developed a method for delivering membrane proteins from transfected cells into phase-separated model membranes that combines optical trapping with thermoplasmonic-mediated membrane fusion and confocal imaging. Using this approach, we observed clear phase partitioning into the liquid disordered phase following the transfer of GFP-tagged influenza hemagglutinin and neuraminidase from transfected cell membranes to giant unilamellar vesicles. The generic platform presented here allows investigation of the phase affinity of any plasma membrane protein which can be labeled or tagged with a fluorescent marker.

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

Selected Golgi-Localized Proteins and Carcinogenesis: What Do We Know?

The role of the Golgi apparatus in carcinogenesis still remains unclear. A number of structural and functional cis-, medial-, and trans-Golgi proteins as well as a complexity of metabolic pathways which they mediate may indicate a central role of the Golgi apparatus in the development and progression of cancer. Pleiotropy of cellular function of the Golgi apparatus makes it a "metabolic heart" or a relay station of a cell, which combines multiple signaling pathways involved in carcinogenesis. Therefore, any damage to or structural abnormality of the Golgi apparatus, causing its fragmentation and/or biochemical dysregulation, results in an up- or downregulation of signaling pathways and may in turn promote tumor progression, as well as local nodal and distant metastases. Three alternative or parallel models of spatial and functional Golgi organization within tumor cells were proposed: (1) compacted Golgi structure, (2) normal Golgi structure with its increased activity, and (3) the Golgi fragmentation with ministacks formation. Regardless of the assumed model, the increased activity of oncogenesis initiators and promoters with inhibition of suppressor proteins results in an increased cell motility and migration, increased angiogenesis, significantly activated trafficking kinetics, proliferation, EMT induction, decreased susceptibility to apoptosis-inducing factors, and modulating immune response to tumor cell antigens. Eventually, this will lead to the increased metastatic potential of cancer cells and an increased risk of lymph node and distant metastases. This chapter provided an overview of the current state of knowledge of selected Golgi proteins, their role in cytophysiology as well as potential involvement in tumorigenesis.

Effect of CLIP3 Upregulation on Astrocyte Proliferation and Subsequent Glial Scar Formation in the Rat Spinal Cord via STAT3 Pathway After Injury

Spinal cord injury (SCI) is a devastating event resulting in neuron degeneration and permanent paralysis through inflammatory cytokine overproduction and glial scar formation. Presently, the endogenous molecular mechanisms coordinating glial scar formation in the injured spinal cord remain elusive. Signal transducer and activator of transcription 3 (STAT3) is a well-known transcription factor particularly involving in cell proliferation and inflammation in the lesion site following SCI. Meanwhile, CAP-Gly domain containing linker protein 3(CLIP3), a vital cytoplasmic protein, has been confirmed to providing an optimal conduit for intracellular signal transduction and interacting with STAT3 with mass spectrometry analysis. In this study, we aimed to identify the expression of CLIP3 in the spinal cord as well as its role in mediating astrocyte activation and glial scar formation after SCI by establishing an acute traumatic SCI model in male adult rats. Western blot analysis revealed that CLIP3 increased gradually after injury, reached a peak at day 3. The immunohistochemistry staining showed the same result in white matter. With double immunofluorescence staining, we found that CLIP3 was expressed in glial cells and significant changes of CLIP3 expression occurred in astrocytes during the pathological process. Statistical analysis demonstrated there was a correlation between the number of positive cells stained by CLIP3 and STAT3 in the spinal cord after SCI. Co-immunoprecipitation further indicated that CLIP3 interacted with STAT3 in the injured spinal cord. Taken together, our study clearly suggested that CLIP3 played an essential role in astrocyte activation, associating with the STAT3 pathway activation induced by SCI.

MicroRNA-593-3p regulates insulin-promoted glucose consumption by targeting Slc38a1 and CLIP3

Insulin plays an important role in the regulation of glucose metabolism. However, the molecular mechanisms involved are not entirely clarified. In this context, we found that miR-593-3p negatively regulates insulin-regulated glucose metabolism in hepatocellular carcinoma HepG2 and Bel7402 cells. We then identified Slc38a1 and CLIP3 as novel targets of miR-593-3p. Further studies demonstrated that Slc38a1 and CLIP3 mediate insulin-regulated glucose metabolism. Interestingly, we also demonstrated that miR-593-3p expression was negatively associated with Slc38a1 and CLIP3 expression in insulin-treated HepG2 cells, and insulin-induced Slc38a1 and CLIP3 expression via downregulation of miR-593-3p. Taken together, this study indicates that inhibition of miRNA-593-3p by insulin promotes glucose metabolism through the regulation of Slc38a1 and CLIP3 expression, and provides a new insight into the role and mechanism of insulin-induced glycolysis.

Role of mitochondrial raft-like microdomains in the regulation of cell apoptosis

Lipid rafts are envisaged as lateral assemblies of specific lipids and proteins that dissociate and associate rapidly and form functional clusters in cell membranes. These structural platforms are not confined to the plasma membrane; indeed lipid microdomains are similarly formed at subcellular organelles, which include endoplasmic reticulum, Golgi and mitochondria, named raft-like microdomains. In addition, some components of raft-like microdomains are present within ER-mitochondria associated membranes. This review is focused on the role of mitochondrial raft-like microdomains in the regulation of cell apoptosis, since these microdomains may represent preferential sites where key reactions take place, regulating mitochondria hyperpolarization, fission-associated changes, megapore formation and release of apoptogenic factors. These structural platforms appear to modulate cytoplasmic pathways switching cell fate towards cell survival or death. Main insights on this issue derive from some pathological conditions in which alterations of microdomains structure or function can lead to severe alterations of cell activity and life span. In the light of the role played by raft-like microdomains to integrate apoptotic signals and in regulating mitochondrial dynamics, it is conceivable that these membrane structures may play a role in the mitochondrial alterations observed in some of the most common human neurodegenerative diseases, such as Amyotrophic lateral sclerosis, Huntington's chorea and prion-related diseases. These findings introduce an additional task for identifying new molecular target(s) of pharmacological agents in these pathologies.

ClipR-59 plays a critical role in the regulation of body glucose homeostasis

By regulating Akt membrane compartmentalization, ClipR-59 modulates adipocyte glucose transport. To elucidate the role of ClipR-59 in the regulation of whole body glucose homeostasis, we have generated adipose tissue specific transgenic mice and examined how forcing expression of ClipR-59 in adipose tissue affects body glucose homeostasis. We found that ClipR-59 adipose transgenic mice showed lower blood glucose level with increased glucose tolerance and enhanced insulin sensitivity. Moreover, ClipR-59 adipose transgenic mice were lean with reduced fat mass and against diet induced obesity. Finally, we examined the potential impact of ClipR-59 on adipose endocrine function and found that ClipR-59 expression enhanced adiponectin secretion in both 3T3-L1 adipocytes and adipose tissue, accompanied with increased circulating adiponectin and enhanced AMPKα phosphorylation at Thr172 in adipose tissue and skeletal muscle. Overall, these studies demonstrate that ClipR-59 is likely an important regulator of body glucose homeostasis and adipocyte function.

Spy1 induces de-ubiquitinating of RIP1 arrest and confers glioblastoma's resistance to tumor necrosis factor (TNF-α)-induced apoptosis through suppressing the association of CLIPR-59 and CYLD

Glioblastoma multiforme (GBM), a grade-IV glioma, is resistant to TNF-α induced apoptosis. CLIPR-59 modulates ubiquitination of RIP1, thus promoting Caspase-8 activation to induce apoptosis by TNF-α. Here we reported that CLIPR-59 was down-regulated in GBM cells and high-grade glioma tumor samples, which was associated with decreased cancer-free survival. In GBM cells, CLIPR-59 interacts with Spy1, resulting in its decreased association with CYLD, a de-ubiquitinating enzyme. Moreover, experimental reduction of Spy1 levels decreased GBM cells viability, while increased the lysine-63-dependent de-ubiquitinating activity of RIP1 via enhancing the binding ability of CLIPR-59 and CYLD in GBM, thus promoting Caspase-8 and Caspase-3 activation to induce apoptosis by TNF-α. These findings have identified a novel Spy1-CLIPR-59 interplay in GBM cell's resistance to TNF-α-induced apoptosis revealing a potential target in the intervention of malignant brain tumors.

ClipR-59 interacts with Elmo2 and modulates myoblast fusion

Recent studies using ClipR-59 knock-out mice implicated this protein in the regulation of muscle function. In this report, we have examined the role of ClipR-59 in muscle differentiation and found that ClipR-59 knockdown in C2C12 cells suppressed myoblast fusion. To elucidate the molecular mechanism whereby ClipR-59 regulates myoblast fusion, we carried out a yeast two-hybrid screen using ClipR-59 as the bait and identified Elmo2, a member of the Engulfment and cell motility protein family, as a novel ClipR-59-associated protein. We showed that the interaction between ClipR-59 and Elmo2 was mediated by the atypical PH domain of Elmo2 and the Glu-Pro-rich domain of ClipR-59 and regulated by Rho-GTPase. We have examined the impact of ClipR-59 on Elmo2 downstream signaling and found that interaction of ClipR-59 with Elmo2 enhanced Rac1 activation. Collectively, our studies demonstrate that formation of an Elmo2·ClipR-59 complex plays an important role in myoblast fusion.

Proteomic analysis of protein palmitoylation in adipocytes

Protein palmitoylation, by modulating the dynamic interaction between protein and cellular membrane, is involved in a wide range of biological processes, including protein trafficking, sorting, sub-membrane partitioning, protein-protein interaction and cell signaling. To explore the role of protein palmitoylation in adipocytes, we have performed proteomic analysis of palmitoylated proteins in adipose tissue and 3T3-L1 adipocytes and identified more than 800 putative palmitoylated proteins. These include various transporters, enzymes required for lipid and glucose metabolism, regulators of protein trafficking and signaling molecules. Of note, key proteins involved in membrane translocation of the glucose-transporter Glut4 including IRAP, Munc18c, AS160 and Glut4, and signaling proteins in the JAK-STAT pathway including JAK1 and 2, STAT1, 3 and 5A and SHP2 in JAK-STAT, were palmitoylated in cultured adipocytes and primary adipose tissue. Further characterization showed that palmitoylation of Glut4 and IRAP was altered in obesity, and palmitoylation of JAK1 played a regulatory role in JAK1 intracellular localization. Overall, our studies provide evidence to suggest a novel and potentially regulatory role for protein palmitoylation in adipocyte function.

DHHC17 palmitoylates ClipR-59 and modulates ClipR-59 association with the plasma membrane

ClipR-59 interacts with Akt and regulates Akt compartmentalization and Glut4 membrane trafficking in a plasma membrane association-dependent manner. The association of ClipR-59 with plasma membrane is mediated by ClipR-59 palmitoylation at Cys534 and Cys535. To understand the regulation of ClipR-59 palmitoylation, we have examined all known mammalian DHHC palmitoyltransferases with respect to their ability to promote ClipR-59 palmitoylation. We found that, among 23 mammalian DHHC palmitoyltransferases, DHHC17 is the major ClipR-59 palmitoyltransferase, as evidenced by the fact that DHHC17 interacted with ClipR-59 and palmitoylated ClipR-59 at Cys534 and Cys535. By palmitoylating ClipR-59, DHHC17 directly regulates ClipR-59 plasma membrane association, as ectopic expression of DHHC17 increased whereas silencing of DHHC17 reduced the levels of ClipR-59 associated with plasma membrane. We have also examined the role of DHHC17 in Akt signaling and found that silencing of DHHC17 in 3T3-L1 adipocytes decreased the levels of Akt as well as ClipR-59 on the plasma membrane and impaired insulin-dependent Glut4 membrane translocation. We suggest that DHHC17 is a ClipR-59 palmitoyltransferase that modulates ClipR-59 plasma membrane binding, thereby regulating Akt signaling and Glut4 membrane translocation in adipocytes.

CLIPR-59: a protein essential for neuromuscular junction stability during mouse late embryonic development

CLIPR-59 is a new member of the cytoplasmic linker proteins (CLIP) family mainly localized to the trans-Golgi network. We show here that Clipr-59 expression in mice is restricted to specific pools of neurons, in particular motoneurons (MNs), and progressively increases from embryonic day 12.5 (E12.5) until the first postnatal days. We generated a Clipr-59 knockout mouse model that presents perinatal lethality due to respiratory defects. Physiological experiments revealed that this altered innervation prevents the normal nerve-elicited contraction of the mutant diaphragm that is reduced both in amplitude and fatigue-resistance at E18.5, despite unaffected functional muscular contractility. Innervation of the mutant diaphragm is not altered until E15.5, but is then partially lost in the most distal parts of the muscle. Ultrastructural observations of neuromuscular junctions (NMJs) in the distal region of the diaphragm reveal a normal organization, but a lower density of nerve terminals capped by terminal Schwann cells in E18.5 mutant when compared with control embryos. Similar defects in NMJ stability, with a hierarchy of severity along the caudo-rostral axis, are also observed in other muscles innervated by facial and spinal MNs in Clipr-59 mutant mice. Clipr-59 deficiency therefore affects axon maintenance but not axon guidance toward muscle targets. Thus, CLIPR-59 is involved in the stabilization of specific motor axons at the NMJ during mouse late embryogenesis and its role is crucial for mouse perinatal development.

Phospholipases may play multiple roles in anisotropic plant cell growth

Both the cortical microtubule cytoskeleton and cellulose microfibrils are important for the anisotropic growth of plant cells. Although the two systems interact, the details of this interaction are far from clear. It has been shown the inhibitors of phospholipase D, phospholipase A(2) and phospholipase C all cause disorganisation of the microtubule cytoskeleton. Since the phospholipases act on the plasma membrane, which links cortical microtubules to cellulose microfibrils in the cell wall, they may play a key role in the communication between the two structures. This communication may take various forms. Microtubule-linked phospholipase activity may cause the organisation of underlying cellulose microfibril liquid crystals. Alternatively, phospholipases may co-operate in the regulation of plasma membrane fluidity, affecting the movement of cellulose synthase complexes in the underlying plasma membrane. GPI-anchored proteins in the plasma membrane, which are cleaved by phospholipases, may possibly play a role.

Changes in CLIP3 expression after sciatic nerve injury in adult rats

CLIP3 (cytoplasmic linker protein 3) is a 547 amino acid residue cytoplasmic protein that localises to Golgi stacks and tubulovesicular elements juxtaposed to Golgi cisternae. Composed of three Ank (ankyrin) repeats and two CAP-Gly (cytoskeleton-associated protein-glycine) domains, CLIP3 may function as a cytoplasmic linker protein that is involved in TGN-endosome dynamics. To define the expression and role of CLIP3 during peripheral nervous system degeneration and regeneration, we created an acute sciatic nerve injury (SNI) model in adult rats. Western blot analyses revealed prominent up-regulation of CLIP3 and PCNA (proliferating cell nuclear antigen) protein levels at 3 days after SNI. Immunohistochemistry displayed that the expression of CLIP3 was noticeably increased in the injured nerve. Immunofluorescence further revealed that the CLIP3 and PCNA proteins colocalised respectively with S100 in the cytoplasm of Schwann cells. The expression profile of the SC/neuron co-cultures demonstrated that CLIP3 and PCNA protein levels were markedly expressed during the early stage of myelination. These results suggest that CLIP3 is likely associated with the myelination of proliferating Schwann cells, and nerve tissue regeneration after peripheral nerve injury. CLIP3 and PCNA expression during early myelination may be related to the direct uptake and transport of lipids and cholesterol, which were derived from the degenerating myelin, by Schwann cells to prepare for the formation of myelin sheath-like structures around regenerated axons after SNI.

TRPV1-tubulin complex: involvement of membrane tubulin in the regulation of chemotherapy-induced peripheral neuropathy

Existence of microtubule cytoskeleton at the membrane and submembranous regions, referred as 'membrane tubulin' has remained controversial for a long time. Since we reported physical and functional interaction of Transient Receptor Potential Vanilloid Sub Type 1 (TRPV1) with microtubules and linked the importance of TRPV1-tubulin complex in the context of chemotherapy-induced peripheral neuropathy, a few more reports have characterized this interaction in in vitro and in in vivo condition. However, the cross-talk between TRPs with microtubule cytoskeleton, and the complex feedback regulations are not well understood. Sequence analysis suggests that other than TRPV1, few TRPs can potentially interact with microtubules. The microtubule interaction with TRPs has evolutionary origin and has a functional significance. Biochemical evidence, Fluorescence Resonance Energy Transfer analysis along with correlation spectroscopy and fluorescence anisotropy measurements have confirmed that TRPV1 interacts with microtubules in live cell and this interaction has regulatory roles. Apart from the transport of TRPs and maintaining the cellular structure, microtubules regulate signaling and functionality of TRPs at the single channel level. Thus, TRPV1-tubulin interaction sets a stage where concept and parameters of 'membrane tubulin' can be tested in more details. In this review, I critically analyze the advancements made in biochemical, pharmacological, behavioral as well as cell-biological observations and summarize the limitations that need to be overcome in the future.

The association of ClipR-59 protein with AS160 modulates AS160 protein phosphorylation and adipocyte Glut4 protein membrane translocation

ClipR-59 is a membrane-associated protein and has been implicated in membrane signaling and vesicle trafficking. Recently, we have identified ClipR-59 as an Akt-interacting protein, and we have found that, by interacting with Akt, ClipR-59 modulates Akt subcellular compartmentalization and Akt substrate AS160 phosphorylation, thereby promoting Glut4 membrane translocation. Here, we have further investigated the regulatory effects of ClipR-59 on AS160 phosphorylation and subsequent adipocyte glucose transport. Our data showed that ClipR-59 interacted with AS160, which was mediated by the ankyrin repeats of ClipR-59 and regulated by insulin signaling. Moreover, the data also demonstrated that the interaction of ClipR-59 with AS160 was required for ClipR-59 to modulate Glut4 membrane translocation as ΔANK-ClipR-59, an AS160 interaction-defective mutant, failed to promote AS160 phosphorylation, Glut4 membrane translocation, and glucose transport induced by insulin in 3T3-L1 adipocytes. Because ClipR-59 also interacts with Akt and enhances the interaction between Akt and AS160, we suggest that ClipR-59 functions as a scaffold protein to facilitate Akt-mediated AS160 phosphorylation, thereby regulating glucose transport.

Interplay between microtubule dynamics and intracellular organization

Microtubules are hollow tubes essential for many cellular functions such as cell polarization and migration, intracellular trafficking and cell division. They are polarized polymers composed of α and β tubulin that are, in most cells, nucleated at the centrosome at the center of the cell. Microtubule plus-ends are oriented towards the periphery of the cell and explore the cytoplasm in a very dynamic manner. Microtubule alternate between phases of growth and shrinkage in a manner described as dynamic instability. Their dynamics is highly regulated by multiple factors: tubulin post-translational modifications such as detyrosination or acetylation, and microtubule-associated proteins, among them the plus-tip tracking proteins. This regulation is necessary for microtubule functions in the cell. In this review, we will focus on the role of microtubules in intracellular organization. After an overview of the mechanisms responsible for the regulation of microtubule dynamics, the major roles of microtubules dynamics in organelle positioning and organization in interphase cells will be discussed. Conversely, the role of certain organelles, like the nucleus and the Golgi apparatus as microtubule organizing centers will be reviewed. We will then consider the role of microtubules in the establishment and maintenance of cell polarity using few examples of cell polarization: epithelial cells, neurons and migrating cells. In these cells, the microtubule network is reorganized and undergoes specific and local regulation events; microtubules also participate in the intracellular reorganization of different organelles to ensure proper cell differentiation.

Protein palmitoylation in neuronal development and synaptic plasticity

Protein palmitoylation, a classical and common lipid modification, regulates diverse aspects of neuronal protein trafficking and function. The reversible nature of palmitoylation provides a potential general mechanism for protein shuttling between intracellular compartments. The recent discovery of palmitoylating enzymes--a large DHHC (Asp-His-His-Cys) protein family--and the development of new proteomic and imaging methods have accelerated palmitoylation analysis. It is becoming clear that individual DHHC enzymes generate and maintain the specialized compartmentalization of substrates in polarized neurons. Here, we discuss the regulatory mechanisms for dynamic protein palmitoylation and the emerging roles of protein palmitoylation in various aspects of pathophysiology, including neuronal development and synaptic plasticity.

Role of GD3-CLIPR-59 association in lymphoblastoid T cell apoptosis triggered by CD95/Fas

We previously found that a directional movement of the raft component GD3 towards mitochondria, by its association with microtubules, was mandatory to late apoptogenic events triggered by CD95/Fas. Since CLIPR-59, CLIP-170-related protein, has recently been identified as a microtubule binding protein associated with lipid rafts, we analyzed the role of GD3-CLIPR-59 association in lymphoblastoid T cell apoptosis triggered by CD95/Fas. To test whether CLIPR-59 could play a role at the raft-microtubule junction, we performed a series of experiments by using immunoelectron microscopy, static or flow cytometry and biochemical analyses. We first assessed the presence of CLIPR-59 molecule in lymphoblastoid T cells (CEM). Then, we demonstrated that GD3-microtubule interaction occurs via CLIPR-59 and takes place at early time points after CD95/Fas ligation, preceding the association GD3-tubulin. GD3-CLIPR-59 association was demonstrated by fluorescence resonance energy transfer (FRET) analysis. The key role of CLIPR-59 in this dynamic process was clarified by the observation that silencing CLIPR-59 by siRNA affected the kinetics of GD3-tubulin association, spreading of GD3 towards mitochondria and apoptosis execution. We find that CLIPR-59 may act as a typical chaperone, allowing a prompt interaction between tubulin and the raft component GD3 during cell apoptosis triggered by CD95/Fas. On the basis of the suggested role of lipid rafts in conveying pro-apoptotic signals these results disclose new perspectives in the understanding of the mechanisms by which raft-mediated pro-apoptotic signals can directionally reach their target, i.e. the mitochondria, and trigger apoptosis execution.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Heterotrimeric G-proteins interact directly with cytoskeletal components to modify microtubule-dependent cellular processes

A large percentage of current drugs target G-protein-coupled receptors, which couple to well-known signaling pathways involving cAMP or calcium. G-proteins themselves may subserve a second messenger function. Here, we review the role of tubulin and microtubules in directly mediating effects of heterotrimeric G-proteins on neuronal outgrowth, shape and differentiation. G-protein-tubulin interactions appear to be regulated by neurotransmitter activity, and, in turn, regulate the location of Galpha in membrane microdomains (such as lipid rafts) or the cytosol. Tubulin binds with nanomolar affinity to Gsalpha, Gialpha1 and Gqalpha (but not other Galpha subunits) as well as Gbeta(1)gamma(2) subunits. Galpha subunits destabilize microtubules by stimulating tubulin's GTPase, while Gbetagamma subunits promote microtubule stability. The same region on Gsalpha that binds adenylyl cyclase and Gbetagamma also interacts with tubulin, suggesting that cytoskeletal proteins are novel Galpha effectors. Additionally, intracellular Gialpha-GDP, in concert with other GTPase proteins and Gbetagamma, regulates the position of the mitotic spindle in mitosis. Thus, G-protein activation modulates cell growth and differentiation by directly altering microtubule stability. Further studies are needed to fully establish a structural mechanism of this interaction and its role in synaptic plasticity.

ClipR-59 interacts with Akt and regulates Akt cellular compartmentalization

Akt is activated on the plasma membrane and its substrates are distributed throughout various cellular compartments. To phosphorylate its substrates, Akt needs to be recruited to specific intracellular compartments. Thus, regulation of Akt cellular compartmentalization constitutes an important mechanism to specify Akt signaling. Here, we report the identification of ClipR-59 as an Akt interaction protein. We show that the interaction of ClipR-59 with Akt is mediated by the CAP-Gly domain of ClipR-59 and kinase domain of Akt and is regulated by Akt phosphorylation. We demonstrate that ClipR-59 regulates the Akt membrane association through its interaction with Akt and membrane localization and, by modulating Akt cellular compartmentalization, differentially modulates phosphorylation of Akt substrates in adipocytes. Finally, we provide evidence that one of the Akt substrates whose phosphorylation is upregulated by ClipR-59 is AS160, a negative regulator of adipocyte glucose transport. Accordingly, ectopic expression of ClipR-59 enhances, whereas knockdown of ClipR-59 suppresses, adipocyte glucose transport. We suggest that ClipR-59 functions as a scaffold protein that interacts with phospho-Akt and recruits active Akt on the membrane and may play an important role in adipocyte glucose transport.

Minimal plus-end tracking unit of the cytoplasmic linker protein CLIP-170

Cytoplasmic linker protein 170 (CLIP-170) is the prototype microtubule (MT) plus-end tracking protein (+TIP) and is involved in regulating MT dynamics. A comprehensive understanding of the process by which CLIP-170 tracks MT plus ends would provide insight into its function. However, the precise molecular mechanism of CLIP-170 +TIP behavior is unknown, and many potential models have been presented. Here, by separating the two CLIP-170 CAP-Gly domains and their adjacent serine-rich regions into fragments of varied size, we have characterized the minimal plus-end tracking unit of CLIP-170 in vivo. Each CLIP-170 fragment was also characterized for its tubulin polymerization activity in vitro. We found that the two CAP-Gly domains have different activities, whereas CAP-Gly-1 appears incompetent to mediate either +TIP behavior or MT nucleation, a CLIP-170 fragment consisting of the second CAP-Gly domain and its adjacent serine-rich region can both track MT plus ends in vivo and induce tubulin polymerization in vitro. These observations complement recent work on CLIP-170 fragments, demonstrate that CAP-Gly motifs do not require dimerization for +TIP and polymerization-promoting activities, and provide insight into CLIP-170 function and mechanism.

Capturing protein tails by CAP-Gly domains

Cytoskeleton-associated protein-glycine-rich (CAP-Gly) domains are protein-interaction modules implicated in important cellular processes and in hereditary human diseases. A prominent function of CAP-Gly domains is to bind to C-terminal EEY/F-COO(-) sequence motifs present in alpha-tubulin and in some microtubule-associated protein tails; however, CAP-Gly domains also interact with other structural elements including end-binding homology domains, zinc-finger motifs and proline-rich sequences. Recent findings unravelled the link between tubulin tyrosination and CAP-Gly-protein recruitment to microtubules. They further provided a molecular basis for understanding the role of CAP-Gly domains in controlling dynamic cellular processes including the tracking and regulation of microtubule ends. It is becoming increasingly clear that CAP-Gly domains are also involved in coordinating complex and diverse aspects of cell architecture and signalling.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

Centrosomal CAP350 protein stabilises microtubules associated with the Golgi complex

A comprehensive model of how the centrosome organises the microtubule network in animal cells has not yet been elucidated. Here we show that the centrosomal large CAP-Gly protein CAP350 is not only present at the centrosome, but is also present as numerous dots in the pericentrosomal area. Using in vitro and in vivo expression of partial constructs, we demonstrated that CAP350 binds microtubules through an N-terminal basic region rather than through its CAP-Gly domain. CAP-Gly-containing domains of CAP350 are targeted not only to the centrosome but also to a Golgi-like network. Interestingly, full-length GFP-tagged CAP350 bound preferentially to microtubules in the pericentrosomal area. These results indicate that the large CAP350 protein has a dual binding ability. Overexpression of CAP350 promoted an increase in the stability of the whole microtubule network, as judged by a significant decrease in the number of EB1 comets and by an enhanced microtubule resistance to Nocodazole treatment. In support of this, CAP350 depletion decreased microtubule stability. Moreover, both depletion and overexpression of CAP350 induced specific fragmentation of the Golgi complex while maintaining a juxtanuclear localisation. We propose that CAP350 specifically stabilises Golgi-associated microtubules and in this way participates in the maintenance of a continuous pericentrosomal Golgi ribbon.

A Cellular Networking Model Involving Interactions Among Glycosyl-Phosphatidylinositol (GPI)-Anchored Plasma Membrane Arabinogalactan Proteins (AGPs), Microtubules and F-actin in Tobacco BY-2 Cells

Arabinogalactan-proteins (AGPs) are perhaps the most abundantly expressed set of proteins at the plant cell surface and play probable roles in cellular architecture and signaling. Although considerable progress has been made to understand the role of AGPs in plant growth and development, their exact functional roles and the molecular mechanisms underlying their interactions with either intra- or extra-cellular molecules are unknown. These unknown interactions were addressed in a recent research article in Plant Physiology. This study reported molecular interactions between AGPs and the cytoskeleton [microtubules, (MTs) and F-actin] in tobacco BY-2 cells. Here in this addendum, a summary of this recent publication and additional perspectives are presented. As reported, perturbation studies were conducted in tobacco BY-2 cells to analyze the effects of an AGP inhibitor (beta-Yariv reagent) on the organization of microtubules [labeled by GFP-MBD (green fluorescent protein-microtubule binding domain)] and F-actin (labeled by rhodamine-phalloidin) and conversely to analyze the effects of a microtubule inhibitor (amiprophosmethyl) and an F-actin inhibitor (cytochalasin-D) on the localization of GPI-anchored GFP-LeAGP-1. These studies implicate a role for GPI-anchored LeAGP-1 in mediating a cell wall-plasma membrane-cytoskeleton connection.

Tau interacts with Golgi membranes and mediates their association with microtubules

Tau, a microtubule-associated protein enriched in the axon, is known to stabilize and promote the formation of microtubules during axonal outgrowth. Several studies have reported that tau was associated with membranes. In the present study, we further characterized the interaction of tau with membranous elements by examining its distribution in subfractions enriched in either Golgi or endoplasmic reticulum membranes isolated from rat brain. A subfraction enriched with markers of the medial Golgi compartment, MG160 and mannosidase II, presented a high tau content indicating that tau was associated with these membranes. Electron microscope morphometry confirmed the enrichment of this subfraction with Golgi membranes. Double-immunogold labeling experiments conducted on this subfraction showed the direct association of tau with vesicles labeled with either an antibody directed against MG160 or TGN38. The association of tau with the Golgi membranes was further confirmed by immunoisolating Golgi membranes with an anti-tau antibody. Immunogold labeling confirmed the presence of tau on the Golgi membranes in neurons in vivo. Overexpression of human tau in primary hippocampal neurons induced the formation of large Golgi vesicles that were found in close vicinity to tau-containing microtubules. This suggested that tau could serve as a link between Golgi membranes and microtubules. Such role for tau was demonstrated in an in vitro reconstitution assay. Finally, our results showed that some tau isoforms present in the Golgi subfraction were phosphorylated at the sites recognized by the phosphorylation-dependent antibodies PHF-1 and AT-8.

The CLIP-170 orthologue Bik1p and positioning the mitotic spindle in yeast

Bik1p is the yeast Saccharomyces cerevisiae representative of the CLIP-170 family of microtubule plus-end tracking proteins. Bik1p shares a number of similarities with its mammalian counterpart CLIP-170, including an important role in dynein function. However, Bik1p and CLIP-170 differ in several significant ways, including the mechanisms utilized to track microtubule plus ends. In addition to presenting functional comparisons between Bik1p and CLIP-170, we provide sequence analyses that reveal previously unrecognized similarities between Bik1p and its animal counterparts. We examine in detail what is known about the functions of Bik1p and consider the various roles that Bik1p plays in positioning the yeast mitotic spindle. This chapter also highlights several recent findings, including the contribution of Bik1p to the yeast mating pathway.

CLIPs and CLASPs and cellular dynamics

The dynamic properties of microtubules are regulated by plus-end tracking proteins (+TIPs), which associate with the distal ends of microtubules. Among the +TIPs are cytoplasmic linker proteins (CLIPs), which promote microtubule growth and regulate dynein-dynactin localization, and CLIP-associating proteins (CLASPs), which stabilize specific subsets of microtubules on reception of signalling cues. CLIPs and CLASPs interact and cooperate to direct the microtubule network, thereby regulating cellular asymmetry.