Targeting of SCG10 to the area of the Golgi complex is mediated by its NH2-terminal region

The Stathmin-2 membrane-targeting domain is required for axon protection and regulated degradation by DLK signaling

Axon integrity is essential for functional connectivity in the nervous system. The degeneration of stressed or damaged axons is a common and sometimes initiating event in neurodegenerative disorders. Stathmin-2 (Stmn2) is an axon maintenance factor that is depleted in amyotrophic lateral sclerosis, and replenishment of Stmn2 can restore neurite outgrowth in diseased neurons. However, mechanisms responsible for Stmn2-mediated axon maintenance in injured neurons are not known. We used primary sensory neurons to interrogate the role of Stmn2 in the degeneration of severed axons. We discover that membrane association of Stmn2 is critical for its axon-protective activity. Structure-function studies revealed that axonal enrichment of Stmn2 is driven by palmitoylation as well as tubulin interaction. Using live imaging, we discover that another Stmn, Stmn3, comigrates with Stmn2-containing vesicles. We also demonstrate that Stmn3 undergoes regulated degradation through dual leucine zipper kinase (DLK)-c-Jun N-terminal kinase signaling. The Stmn2 membrane-targeting domain is both necessary and sufficient for localization to a specific vesicle population and confers sensitivity to DLK-dependent degradation. Our findings reveal a broader role for DLK in tuning the local abundance of palmitoylated Stmns in axon segments. Moreover, palmitoylation is a critical component of Stmn-mediated axon protection, and defining the Stmn2-containing vesicle population will provide important clues toward mechanisms of axon maintenance.

A sticky situation: regulation and function of protein palmitoylation with a spotlight on the axon and axon initial segment

In neurons, the axon and axon initial segment (AIS) are critical structures for action potential initiation and propagation. Their formation and function rely on tight compartmentalisation, a process where specific proteins are trafficked to and retained at distinct subcellular locations. One mechanism which regulates protein trafficking and association with lipid membranes is the modification of protein cysteine residues with the 16-carbon palmitic acid, known as S-acylation or palmitoylation. Palmitoylation, akin to phosphorylation, is reversible, with palmitate cycling being mediated by substrate-specific enzymes. Palmitoylation is well-known to be highly prevalent among neuronal proteins and is well studied in the context of the synapse. Comparatively, how palmitoylation regulates trafficking and clustering of axonal and AIS proteins remains less understood. This review provides an overview of the current understanding of the biochemical regulation of palmitoylation, its involvement in various neurological diseases, and the most up-to-date perspective on axonal palmitoylation. Through a palmitoylation analysis of the AIS proteome, we also report that an overwhelming proportion of AIS proteins are likely palmitoylated. Overall, our review and analysis confirm a central role for palmitoylation in the formation and function of the axon and AIS and provide a resource for further exploration of palmitoylation-dependent protein targeting to and function at the AIS.

Ascending dorsal column sensory neurons respond to spinal cord injury and downregulate genes related to lipid metabolism

Regeneration failure after spinal cord injury (SCI) results in part from the lack of a pro-regenerative response in injured neurons, but the response to SCI has not been examined specifically in injured sensory neurons. Using RNA sequencing of dorsal root ganglion, we determined that thoracic SCI elicits a transcriptional response distinct from sciatic nerve injury (SNI). Both SNI and SCI induced upregulation of ATF3 and Jun, yet this response failed to promote growth in sensory neurons after SCI. RNA sequencing of purified sensory neurons one and three days after injury revealed that unlike SNI, the SCI response is not sustained. Both SCI and SNI elicited the expression of ATF3 target genes, with very little overlap between conditions. Pathway analysis of differentially expressed ATF3 target genes revealed that fatty acid biosynthesis and terpenoid backbone synthesis were downregulated after SCI but not SNI. Pharmacologic inhibition of fatty acid synthase, the enzyme generating palmitic acid, decreased axon growth and regeneration in vitro. These results support the notion that decreased expression of lipid metabolism-related genes after SCI, including fatty acid synthase, may restrict axon regenerative capacity after SCI.

RAS at the Golgi antagonizes malignant transformation through PTPRκ-mediated inhibition of ERK activation

RAS GTPases are frequently mutated in human cancer. H- and NRAS isoforms are distributed over both plasma-membrane and endomembranes, including the Golgi complex, but how this organizational context contributes to cellular transformation is unknown. Here we show that RAS at the Golgi is selectively activated by apoptogenic stimuli and antagonizes cell survival by suppressing ERK activity through the induction of PTPRκ, which targets CRAF for dephosphorylation. Consistently, in contrast to what occurs at the plasma-membrane, RAS at the Golgi cannot induce melanoma in zebrafish. Inactivation of PTPRκ, which occurs frequently in human melanoma, often coincident with TP53 inactivation, accelerates RAS-ERK pathway-driven melanomagenesis in zebrafish. Likewise, tp53 disruption in zebrafish facilitates oncogenesis driven by RAS from the Golgi complex. Thus, RAS oncogenic potential is strictly dependent on its sublocalization, with Golgi complex-located RAS antagonizing tumor development.

RAS isoforms are associated with the plasma membrane and endomembranes, but how their localization contributes to tumorigenesis is unclear. Here, the authors show that RAS signals from Golgi complex antagonize tumour formation by inducing apoptosis via ERK inhibition.

Palmitoylation enables MAPK-dependent proteostasis of axon survival factors

Axon degeneration is a prominent event in many neurodegenerative disorders. Axon injury stimulates an intrinsic self-destruction program that culminates in activation of the prodegeneration factor SARM1 and local dismantling of damaged axon segments. In healthy axons, SARM1 activity is restrained by constant delivery of the axon survival factor NMNAT2. Elevating NMNAT2 is neuroprotective, while loss of NMNAT2 evokes SARM1-dependent axon degeneration. As a gatekeeper of axon survival, NMNAT2 abundance is an important regulatory node in neuronal health, highlighting the need to understand the mechanisms behind NMNAT2 protein homeostasis. We demonstrate that pharmacological inhibition of the MAP3Ks dual leucine zipper kinase (DLK) and leucine zipper kinase (LZK) elevates NMNAT2 abundance and strongly protects axons from injury-induced degeneration. We discover that MAPK signaling selectively promotes degradation of palmitoylated NMNAT2, as well as palmitoylated SCG10. Conversely, nonpalmitoylated NMNAT2 is degraded by the Phr1/Skp1a/Fbxo45 ligase complex. Combined inactivation of both pathways leads to synergistic accumulation of NMNAT2 in axons and dramatically enhanced protection against pathological axon degeneration. Hence, the subcellular localization of distinct pools of NMNAT2 enables differential regulation of NMNAT2 abundance to control axon survival.

Roles of palmitoylation in axon growth, degeneration and regeneration

The protein-lipid modification palmitoylation plays important roles in neurons, but most attention has focused on roles of this modification in the regulation of mature pre- and post-synapses. However, exciting recent findings suggest that palmitoylation is also critical for both the growth and integrity of neuronal axons and plays previously unappreciated roles in conveying axonal anterograde and retrograde signals. Here we review these emerging roles for palmitoylation in the regulation of axons in health and disease. © 2017 Wiley Periodicals, Inc.

Proximity of SCG10 and prion protein in membrane rafts

The conversion of normal cellular prion protein (PrPC) into its pathogenic isoform (PrPSc) is an essential event in prion pathogenesis. In culture models, membrane rafts are suggested to play a critical role in PrPSc formation. To identify the candidate molecules capable of interacting with PrPC and facilitating PrPSc formation in membrane rafts, we applied a novel biochemical labeling method termed enzyme-mediated activation of radical sources. Enzyme-mediated activation of radical sources was applied to the Lubrol WX insoluble detergent-resistant membrane fractions from mouse neuroblastoma (N2a) cells in which the surface PrPC was labeled with HRP-conjugated anti-PrP antibody. Two-dimensional western blots of these preparations revealed biotinylated spots of approximately 20 kDa with an isoelectric point of 8.0-9.0. Liquid chromatography-tandem mass spectrometry analysis resulted in the identification of peptides containing SCG10, the neuron-specific microtubule regulator. Proximity of SCG10 and PrPC was confirmed using proximity ligation assay and co-immunoprecipitation assay. Transfection of persistently 22L prion-infected N2a cells with SCG10 small interfering RNA reduced SCG10 expression, but did not prevent PrPSc accumulation, indicating that SCG10 appears to be unrelated to PrPSc formation of 22L prion. Immunofluorescence and western blot analyses showed reduced levels of SCG10 in the hippocampus of prion-infected mice, suggesting a possible association between SCG10 levels and the prion neuropathogenesis. By applying a novel biochemical labeling method against detergent-resistant membrane fractions from mouse neuroblastoma cells, the neuron-specific microtubule-destabilization protein, SCG10 was identified as a novel candidate that is proximate to normal prion protein (PrP) in membrane rafts. SCG10 seemed unrelated to disease-related PrP formation under certain conditions, while there is a possible association between SCG10 levels and prion neuropathogenesis. Cover Image for this issue: doi: 10.1111/jnc.13310.

The myristoylated amino-terminus of an Arabidopsis calcium-dependent protein kinase mediates plasma membrane localization

Calcium-dependent protein kinases (CDPK) are a major group of calcium-stimulated kinases found in plants and some protists. Many CDPKs are membrane-associated, presumably because of lipid modifications at their amino termini. We investigated the subcellular location and myristoylation of AtCPK5, a member of the Arabidopsis CDPK family. Most AtCPK5 was associated with the plasma membrane as demonstrated by two-phase fractionation of plant microsomes and by in vivo detection of AtCPK5-GFP fusion proteins. AtCPK5 was a substrate for plant N-myristoyltransferase and myristoylation was prevented by converting the glycine at the proposed site of myristate attachment to alanine (G2A). In transgenic plants, a G2A mutation completely abolished AtCPK5 membrane association, indicating that myristoylation was essential for membrane binding. The first sixteen amino acids of AtCPK5 were sufficient to direct plasma membrane localization. In addition, differentially phosphorylated forms of AtCPK5 were detected both in planta and after expression of AtCPK5 in a cell-free plant extract. Our results demonstrate that AtCPK5 is myristoylated at its amino terminus and that myristoylation is required for membrane binding.

Subcellular Golgi localization of stathmin family proteins is promoted by a specific set of DHHC palmitoyl transferases

Palmitoylation of the neuronal microtubule regulatory stathmin phosphoproteins is crucial for their Golgi and vesicle localization and trafficking along neurites. It is promoted by a specific set of palmitoyl transferases, which suggests that palmitoylation can be viewed as a crucial regulatory process for neuronal stathmin functions.

 Protein palmitoylation is a reversible lipid modification that plays critical roles in protein sorting and targeting to specific cellular compartments. The neuronal microtubule-regulatory phosphoproteins of the stathmin family (SCG10/stathmin 2, SCLIP/stathmin 3, and RB3/stathmin 4) are peripheral proteins that fulfill specific and complementary roles in the formation and maturation of the nervous system. All neuronal stathmins are localized at the Golgi complex and at vesicles along axons and dendrites. Their membrane anchoring results from palmitoylation of two close cysteine residues present within their homologous N-terminal targeting domains. By preventing palmitoylation with 2-bromopalmitate or disrupting the integrity of the Golgi with brefeldin A, we were able to show that palmitoylation of stathmins 2 and 3 likely occurs at the Golgi and is crucial for their specific subcellular localization and trafficking. In addition, this membrane binding is promoted by a specific set of palmitoyl transferases that localize with stathmins 2 and 3 at the Golgi, directly interact with them, and enhance their membrane association. The subcellular membrane–associated microtubule-regulatory activity of stathmins might then be fine-tuned by extracellular stimuli controlling their reversible palmitoylation, which can be viewed as a crucial regulatory process for specific and local functions of stathmins in neurons.

Calmyrin1 binds to SCG10 protein (stathmin2) to modulate neurite outgrowth

Calmyrin1 (CaMy1) is an EF-hand Ca(2+)-binding protein expressed in several cell types, including brain neurons. Using a yeast two-hybrid screen of a human fetal brain cDNA library, we identified SCG10 protein (stathmin2) as a CaMy1 partner. SCG10 is a microtubule-destabilizing factor involved in neuronal growth during brain development. We found increased mRNA and protein levels of CaMy1 during neuronal development, which paralleled the changes in SCG10 levels. In developing primary rat hippocampal neurons in culture, CaMy1 and SCG10 colocalized in cell soma, neurites, and growth cones. Pull-down, coimmunoprecipitation, and proximity ligation assays demonstrated that the interaction between CaMy1 and SCG10 is direct and Ca(2+)-dependent in vivo and requires the C-terminal domain of CaMy1 (residues 99-192) and the N-terminal domain of SCG10 (residues 1-35). CaMy1 did not interact with stathmin1, a protein that is homologous with SCG10 but lacks the N-terminal domain characteristic of SCG10. CaMy1 interfered with SCG10 inhibitory activity in a microtubule polymerization assay. Moreover, CaMy1 overexpression inhibited SCG10-mediated neurite outgrowth in nerve growth factor (NGF)-stimulated PC12 cells. This CaMy1 activity did not occur when an N-terminally truncated SCG10 mutant unable to interact with CaMy1 was expressed. Altogether, these data suggest that CaMy1 via SCG10 couples Ca(2+) signals with the dynamics of microtubules during neuronal outgrowth in the developing brain. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

SCG10 expression on activation of hepatic stellate cells promotes cell motility through interference with microtubules

During liver fibrogenesis, quiescent hepatic stellate cells switch their phenotype toward a myofibroblastic-like pattern with a gain in motility. Here, we show that SCG10 (superior cervical ganglia 10) mRNA expression, a microtubule-destabilizing protein that favors cell growth and motility in neurons, both increases and correlates with the stage of fibrosis in patients with chronic hepatitis C. We also show the de novo expression of SCG10 mRNA in two rat models of liver fibrosis. We demonstrate that activated hepatic stellate cells appear to be the major cellular sources of SCG10 in the liver. Tracking of the SCG10 pathway in hepatic stellate cells shows that SCG10 initially accumulates in the perinuclear Golgi area then migrates in small vesicle-like structures along individual microtubules. Moreover, SCG10 vesicles cluster at the distal ends of microtubules in areas where tubules are spread and decompacted, suggesting their preferential association with destabilized and dynamic microtubules. Inhibition of SCG10 expression by gene-specific short interfering RNA in primary rat hepatic stellate cells is associated with a significant reduction in microtubule-dependent cellular functions, such as proliferation and migration. In conclusion, the de novo expression of SCG10 by hepatic stellate cells may play a major role in cellular mechanisms associated with HSC activation, namely cell motility and division, through interference with microtubules. SCG10 may represent a potential molecular target for anti-fibrosis therapies.

KBP interacts with SCG10, linking Goldberg-Shprintzen syndrome to microtubule dynamics and neuronal differentiation

Goldberg–Shprintzen syndrome (GOSHS) is a rare clinical disorder characterized by central and enteric nervous system defects. This syndrome is caused by inactivating mutations in the Kinesin Binding Protein (KBP) gene, which encodes a protein of which the precise function is largely unclear. We show that KBP expression is up-regulated during neuronal development in mouse cortical neurons. Moreover, KBP-depleted PC12 cells were defective in nerve growth factor-induced differentiation and neurite outgrowth, suggesting that KBP is required for cell differentiation and neurite development. To identify KBP interacting proteins, we performed a yeast two-hybrid screen and found that KBP binds almost exclusively to microtubule associated or related proteins, specifically SCG10 and several kinesins. We confirmed these results by validating KBP interaction with one of these proteins: SCG10, a microtubule destabilizing protein. Zebrafish studies further demonstrated an epistatic interaction between KBP and SCG10 in vivo . To investigate the possibility of direct interaction between KBP and microtubules, we undertook co-localization and in vitro binding assays, but found no evidence of direct binding. Thus, our data indicate that KBP is involved in neuronal differentiation and that the central and enteric nervous system defects seen in GOSHS are likely caused by microtubule-related defects.

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.

Drosophila stathmins bind tubulin heterodimers with high and variable stoichiometries

In vertebrates, stathmins form a family of proteins possessing two tubulin binding repeats (TBRs), which each binds one soluble tubulin heterodimer. The stathmins thus sequester two tubulins in a phosphorylation-dependent manner, providing a link between signal transduction and microtubule dynamics. In Drosophila, we show here that a single stathmin gene (stai) encodes a family of D-stathmin proteins. Two of the D-stathmins are maternally deposited and then restricted to germ cells, and the other two are detected in the nervous system during embryo development. Like in vertebrates, the nervous system-enriched stathmins contain an N-terminal domain involved in subcellular targeting. All the D-stathmins possess a domain containing three or four predicted TBRs, and we demonstrate here, using complementary biochemical and biophysical methods, that all four predicted TBR domains actually bind tubulin. D-stathmins can indeed bind up to four tubulins, the resulting complex being directly visualized by electron microscopy. Phylogenetic analysis shows that the presence of regulated multiple tubulin sites is a conserved characteristic of stathmins in invertebrates and allows us to predict key residues in stathmin for the binding of tubulin. Altogether, our results reveal that the single Drosophila stathmin gene codes for a stathmin family similar to the multigene vertebrate one, but with particular tubulin binding properties.

Regulation of neurite outgrowth by interactions between the scaffolding protein, JNK-associated leucine zipper protein, and neuronal growth-associated protein superior cervical ganglia clone 10

JLP (JNK-associated leucine zipper protein) is a novel scaffolding protein involved in JNK signaling. Although it is known that JLP is highly expressed in brain, the biological function of JLP in neuronal systems remains unknown. Here, we report a novel interaction between JLP and SCG10 (superior cervical ganglia clone 10), which is a microtubule-destabilizing factor that is essential for neurite outgrowth. Inhibition of endogenous JLP expression using small interference RNA methodology strongly enhanced nerve growth factor (NGF)-induced neurite outgrowth in PC12 cells. Our results show that JLP negatively regulates NGF-induced neurite outgrowth by decreasing the level of phosphorylated SCG10. Furthermore, inhibition of JNK phosphorylation by a small molecule inhibitor, SP600125, resulted in inhibition of SCG10 phosphorylation and inhibition of neurite growth. Taken together, our results suggest that JLP negatively regulates NGF-induced neurite outgrowth through a sequestering mechanism that results in an attenuation of NGF-induced SCG10 phosphorylation.

Characterization of spatial and temporal expression pattern of SCG10 during zebrafish development

SCG10 (Superior Cervical Ganglia 10, STMN2) is a member of the stathmin family of proteins. Stathmins regulate microtubule dynamics by inhibiting polymerization and promoting their depolymerization. SCG10 is believed to be a neuronal-specific stathmin that is enriched in the growth cones of developing neurons and plays a role in regulating neurite outgrowth. In all species examined so far, SCG10 is expressed in both the CNS and PNS. We have cloned two zebrafish SCG10 homologues and have determined the temporal and spatial expression pattern of both of these genes by RT-PCR and in situ hybridization. RT-PCR shows that both transcripts are expressed maternally and zygotically through at least 5 days. In situ hybridization analysis reveals that both SCG10 orthologues have dynamic, spatial expression patterns that are nearly identical to each other. Initially, these orthologues are expressed in discrete areas of the forebrain, midbrain, and hindbrain, as well as in the anterior and posterior lateral line ganglia and transiently in the spinal cord Rohon-Beard neurons. From 48hpf onwards, the level of expression of both genes increases and becomes mainly restricted to the anterior CNS (the forebrain region, retina, optic tectum, and hindbrain), and to the cranial ganglia. From 72 to 96hpf, SCG10 genes are also expressed in the developing neurons in the gut and in the surrounding intestinal mesenchyme. Our results provide a starting point for future studies that will investigate the in vivo function of SCG10 orthologues in zebrafish neural development.

Rnd1 regulates axon extension by enhancing the microtubule destabilizing activity of SCG10

The GTPase Rnd1 affects actin dynamics antagonistically to Rho and has been implicated in the regulation of neurite outgrowth, dendrite development, and axon guidance. Here we show that Rnd1 interacts with the microtubule regulator SCG10. This interaction requires a central domain of SCG10 comprising about 40 amino acids located within the N-terminal-half of a putative alpha-helical domain and is independent of phosphorylation at the four identified phosphorylation sites that regulate SCG10 activity. Rnd1 enhances the microtubule destabilizing activity of SCG10 and both proteins colocalize in neurons. Knockdown of Rnd1 or SCG10 by RNAi suppressed axon extension, indicating a critical role for both proteins during neuronal differentiation. Overexpression of Rnd1 in neurons induces the formation of multiple axons. The effect of Rnd1 on axon extension depends on SCG10. These results indicate that SCG10 acts as an effector downstream of Rnd1 to regulate axon extensions by modulating microtubule organization.

The trans-Golgi proteins SCLIP and SCG10 interact with chromogranin A to regulate neuroendocrine secretion

Secretion of proteins and peptides from eukaryotic cells takes place by both constitutive and regulated pathways. Regulated secretion may involve interplay of proteins that are currently unknown. Recent studies suggest an important role of chromogranin A (CHGA) in the regulated secretory pathway in neuroendocrine cells, but the mechanism by which CHGA enters the regulated pathway, or even triggers the formation of the pathway, remains unclear. In this study, we used a transcriptome/proteome-wide approach, to discover binding partners for CHGA, by employing a phage display cDNA library method. Several proteins within or adjacent to the secretory pathway were initially detected as binding partners of recombinant human CHGA. We then focused on the trans-Golgi protein SCLIP (STMN3) and its stathmin paralog SCG10 (STMN2) for functional study. Co-immunoprecipitation experiments confirmed the interaction of each of these two proteins with CHGA in vitro. SCLIP and SCG10 were colocalized to the Golgi apparatus of chromaffin cells in vivo and shared localization with CHGA as it transited the Golgi. Downregulation of either SCLIP or SCG10 by synthetic siRNAs virtually abolished chromaffin cell secretion of a transfected CHGA-EAP chimera (expressing CHGA fused to an enzymatic reporter, and trafficked to the regulated pathway). SCLIP siRNA also decreased the level of secretion of endogenous CHGA and SCG2, as well as transfected human growth hormone, while SCG10 siRNA decreased the level of regulated secretion of endogenous CHGB. Moreover, a dominant negative mutant of SCG10 (Cys 22,Cys 24-->Ala 22,Ala 24) significantly blocked secretion of the transfected CHGA-EAP chimera. A decrease in the buoyant density of chromaffin granules was observed after downregulation of SCG10 by siRNA, suggesting participation of these stathmins in granule formation or maturation. We conclude that SCLIP and SCG10 interact with CHGA, share partial colocalization in the Golgi apparatus, and may be necessary for typical transmitter storage and release from chromaffin cells.

Stathmin family protein SCG10 differentially regulates the plus and minus end dynamics of microtubules at steady state in vitro: implications for its role in neurite outgrowth

SCG10 (superior cervical ganglia neural-specific 10 protein) is a neuron specific member of the stathmin family of microtubule regulatory proteins that like stathmin can bind to soluble tubulin and depolymerize microtubules. The direct actions of SCG10 on microtubules themselves and on their dynamics have not been investigated previously. Here, we analyzed the effects of SCG10 on the dynamic instability behavior of microtubules in vitro, both at steady state and early during microtubule polymerization. In contrast to stathmin, whose major action on dynamics is to destabilize microtubules by increasing the switching frequency from growth to shortening (the catastrophe frequency) at microtubule ends, SCG10 stabilized the plus ends both at steady state and early during polymerization by increasing the rate and extent of growth. For example, early during polymerization at high initial tubulin concentrations (20 microM), a low molar ratio of SCG10 to tubulin of 1:30 increased the growth rate by approximately 50%. In contrast to its effects at plus ends, SCG10 destabilized minus ends by increasing the shortening rate, the length shortened during shortening events, and the catastrophe frequency. Consistent with its ability to modulate microtubule dynamics at steady state, SCG10 bound to purified microtubules along their lengths. The dual activity of SCG10 at opposite microtubule ends may be important for its role in regulating growth cone microtubule dynamics. SCG10's ability to promote plus end growth may facilitate microtubule extension into filopodia, and its ability to destabilize minus ends could provide soluble tubulin for net plus end elongation.

SCLIP, a Microtubule-destabilizing Factor, Interacts with RasGRF1 and Inhibits Its Ability to Promote Rac Activation and Neurite Outgrowth

RasGRF1 is a neuron-specific guanine nucleotide exchange factor for the small GTPases Ras and Rac. It is implicated in the regulation of memory formation and in the development of tolerance to drug abuse, although the mechanisms have been elucidated only in part. Here we report the isolation, by the yeast two-hybrid screen, of the microtubule-destabilizing factor SCLIP (SCG10-like protein) as a novel RasGRF1-interacting protein. This interaction requires the region spanning the Dbl-homology domain of RasGRF1, endowed with catalytic activity on Rac. In search for a possible function we found by biochemical means that SCLIP influences the signaling properties of RasGRF1, greatly reducing its ability to activate the Rac/p38 MAPK pathway, while the Ras/Erk one remains unaffected. Moreover, a potential role is suggested by transfection studies in neuronal PC12 cells in which RasGRF1 induces neurite outgrowth, and coexpression of SCLIP counteracts this effect, causing a dramatic decrease in the percentage of cells bearing neurites, which also appear significantly shortened. This study unveils a physical and functional interaction between RasGRF1 and SCLIP. We suggest that this novel interplay may have possible implications in mechanisms that regulate neuronal morphology and structural plasticity.

The "SCG10-LIke Protein" SCLIP is a novel regulator of axonal branching in hippocampal neurons, unlike SCG10

Stathmin family phosphoproteins participate in the control of microtubule dynamics and have been proposed to be involved in the control of neuronal differentiation. We examined the biological properties and functions of SCLIP, the stathmin family protein most similar to SCG10, a widely studied and recognized neuronal differentiation marker. SCLIP, like SCG10, is present from the earliest stages of hippocampal neuron differentiation in culture at vesicle-like structures following dynamic microtubules. Its inhibition by RNA interference resulted in increased axonal branching, revealing a novel biological role for SCLIP, distinct from SCG10 whose down-regulation in the same conditions promoted growth cone expansion. The enhanced axonal branching resulted from the formation of collateral lamellar protrusions, with cytoskeleton reorganization typical of normal branching. In addition to revealing a novel function for SCLIP in axonal morphogenesis, our results demonstrate for the first time that stathmin family proteins fulfill different and complementary roles during neuronal differentiation.

STOP-like Protein 21 Is a Novel Member of the STOP Family, Revealing a Golgi Localization of STOP Proteins

Neuronal microtubules are stabilized by two calmodulin-regulated microtubule-associated proteins, E-STOP and N-STOP, which when suppressed in mice induce severe synaptic and behavioral deficits. Here we show that mature neurons also contain a 21-kDa STOP-like protein, SL21, which shares calmodulin-binding and microtubule-stabilizing homology domains with STOP proteins. Accordingly, in different biochemical or cellular assays, SL21 has calmodulin binding and microtubule stabilizing activity. However, in cultured hippocampal neurons, SL21 antibodies principally stain the somatic Golgi and punctate Golgi material in neurites. In cycling cells, transfected SL21 decorates microtubules when expressed at high levels but is otherwise principally visible at the Golgi. The Golgi targeting of SL21 depends on the presence of cysteine residues located within the SL21 N-terminal domain, suggesting that Golgi targeting may require SL21 palmitoylation. Accordingly we find that SL21 is palmitoylated in vivo. N-STOP and E-STOP, which contain the Golgi targeting sequences present in SL21, also display distinct Golgi staining when expressed at low level in cycling cells. Thus neuronal proteins of the STOP family have the capacity to associate with Golgi material, which could be important for STOP synaptic functions.

Taxol and tau overexpression induced calpain-dependent degradation of the microtubule-destabilizing protein SCG10

Microtubule-stabilizing and -destabilizing proteins play a crucial role in regulating the dynamic instability of microtubules during neuronal development and synaptic transmission. The microtubule-destabilizing protein SCG10 is a neuron-specific protein implicated in neurite outgrowth. The SCG10 protein is significantly reduced in mature neurons, suggesting that its expression is developmentally regulated. In contrast, the microtubule-stabilizing protein tau is expressed in mature neurons and its function is essential for the maintenance of neuronal polarity and neuronal survival. Thus, the establishment and maintenance of neuronal polarity may down-regulate the protein level/function of SCG10. In this report, we show that treatment of PC12 cells and neuroblastoma cells with the microtubule-stabilizing drug Taxol induced a rapid degradation of the SCG10 protein. Consistently, overexpression of tau protein in neuroblastoma cells also induced a reduction in SCG10 protein levels. Calpain inhibitor MDL-28170, but not caspase inhibitors, blocked a significant decrease in SCG10 protein levels. Collectively, these results indicate that tau overexpression and Taxol treatment induced a calpain-dependent degradation of the microtubule-destabilizing protein SCG10. The results provide evidence for the existence of an intracellular mechanism involved in the regulation of SCG10 upon microtubule stabilization.

Expression of stathmin family genes in the murine uterus during early pregnancy

Stathmin, a cytosolic phosphoprotein that regulates microtubule dynamics during cell-cycle progression, is abundantly expressed at embryo implantation sites in rats. Here, we characterized the expression of stathmin and its family genes in the murine uterus during the peri-implantation period. Stathmin protein was expressed in the glandular and luminal epithelium, blood vessels, and stromal cells on day 3 of pregnancy. On the day of implantation (day 5), stathmin was mainly localized in blood vessels in the endometrium. On day 7, intense stathmin expression was limited to capillary vessels and secondary decidual cells. Stathmin expression was higher at implantation sites than at uterine segments between implantation sites and increased during oil-induced decidualization. Although the artificially-induced deciduoma weights and number of implantation sites were similar between stathmin-knockout (KO) and wild-type (WT) mice, the stathmin-KO mice had fewer newborn pups (reduced by 30%). The expression of alkaline phosphatase, desmin, and cyclin D3 was attenuated in decidual zones of stathmin-KO mice. Messenger RNA level of the stathmin family gene, SCG10, was high at the time of decidualization in WT and stathmin-KO mice. In contrast, the others of stathmin family members, SCLIP and RB3 were highly expressed in stathmin-KO mice compared to WT mice. These results suggest that stathmin and stathmin family genes are expressed in the murine endometrium with enhanced expression in the implantation or the decidualization process.

The control of microtubule stability in vitro and in transfected cells by MAP1B and SCG10

In neurons, the regulation of microtubules plays an important role for neurite outgrowth, axonal elongation, and growth cone steering. SCG10 family proteins are the only known neuronal proteins that have a strong destabilizing effect, are highly enriched in growth cones and are thought to play an important role during axonal elongation. MAP1B, a microtubule-stabilizing protein, is found in growth cones as well, therefore it was important to test their effect on microtubules in the presence of both proteins. We used recombinant proteins in microtubule assembly assays and in transfected COS-7 cells to analyze their combined effects in vitro and in living cells, respectively. Individually, both proteins showed their expected activities in microtubule stabilization and destruction respectively. In MAP1B/SCG10 double-transfected cells, MAP1B could not protect microtubules from SCG10-induced disassembly in most cells, in particular not in cells that contained high levels of SCG10. This suggests that SCG10 is more potent to destabilize microtubules than MAP1B to rescue them. In microtubule assembly assays, MAP1B promoted microtubule formation at a ratio of 1 MAP1B per 70 tubulin dimers while a ratio of 1 SCG10 per two tubulin dimers was needed to destroy microtubules. In addition to its known binding to tubulin dimers, SCG10 binds also to purified microtubules in growth cones of dorsal root ganglion neurons in culture. In conclusion, neuronal microtubules are regulated by antagonistic effects of MAP1B and SCG10 and a fine tuning of the balance of these proteins may be critical for the regulation of microtubule dynamics in growth cones.

SCG10, a microtubule destabilizing factor, stimulates the neurite outgrowth by modulating microtubule dynamics in rat hippocampal primary cultured neurons

Microtubule dynamics, one of the key elements in neurite outgrowth, is regulated by various regulatory factors to determine the behavior of the neuronal growth cone and to form the specialized neuronal shape. SCG10 is a neuron-specific stathmin protein with a potent microtubule destabilizing factor and is enriched in the growth cones of the developing neurons. We investigated the functional role of SCG10 in neurite outgrowth using rat hippocampal primary cultured neurons. Genetic manipulation of SCG10 using a short-interfering RNA duplex markedly decreased the SCG10 expression level and significantly suppressed neurite outgrowth. This result was confirmed by immunodepletion experiments. On the other hand, the protein transduction of SCG10 using a polyarginine tag stimulated neurite outgrowth. Such manipulation of the SCG10 expression level affected microtubule morphology within the growth cones. A decrease in the SCG10 level converted the morphology to a more stable state, while an increase converted the morphology to a more dynamic state. However, an excess of SCG10 induced neurite retraction due to an excess of microtubule disassembly. These results suggest that SCG10 serves as an important regulatory factor of growth cone motility by enhancing microtubule dynamics, possibly through increasing the catastrophe frequency.

Site-specific phosphorylation of SCG10 in neuronal plasticity: role of Ser73 phosphorylation by N-methyl D-aspartic acid receptor activation in rat hippocampus

Accumulated evidence suggests that actin and microtubule regulating proteins contribute to neuronal structural dynamics, which subsequently affect neuronal plasticity. SCG10 is a neuronal-specific stathmin protein with microtubule destabilizing activity that is affected by multiple phosphorylation, at least in vitro. SCG10 has four major phosphorylation sites: Ser50 and Ser97 targeted by protein kinase A (PKA), and Ser62 and Ser73 targeted by mitogen-activated protein kinase (MAPK). To explore the potential roles of site-specific phosphorylation in physiological models, we developed phosphorylation site-specific antibodies and examined the SCG10 status in primary cultured hippocampal neurons and tissues. Although SCG10 is concentrated in growth cones and the Golgi apparatus in primary cultured neurons, the phosphorylated form was also detected in both regions, suggesting that MT dynamics within the growth cone may be regulated by protein phosphorylation. In the adult hippocampus, an intense stimulus such as kainate treatment induced a rapid phosphorylation of Ser73 within 15 min that was sustained for at least 60 min. This response was mediated through the N-methyl D-aspartic acid (NMDA) receptor and was ablated by the antagonist MK-801. The MAPK enzyme Erk2 was simultaneously activated along a similar time course to SCG10, suggesting that Erk2 may directly phosphorylate Ser73. These results demonstrate that changes in the phosphorylation status of SCG10 in vivo, dependent upon neural activity and/or plasticity, could affect the microtubule dynamics in neuronal dendrites.

Improved sciatic nerve regeneration by local thyroid hormone treatment in adult rat is accompanied by increased expression of SCG10

Thyroid hormone plays an important role in regulating the development and regeneration of the nervous system. Our previous work showed that local administration of triiodothyronine (T3) at the level of transected rat sciatic nerve increased the number and diameter of regenerated axons, but the mechanism underlying the improved regeneration is still unclear. Here, we have investigated the effect of T3 on the expression of SCG10, a regulator of microtubule dynamics in growth cones. After transection of adult rat sciatic nerves, silicone tubes were implanted and filled with T3 or phosphate-buffered solution. At various time points following surgery, the expression of SCG10 protein and mRNA was analyzed. Semi-quantitative Western blot analysis revealed that sciatic nerve transection induced a more than 20-fold upregulation of SCG10 protein in proximal nerve segments at 1 day post-lesion, while at this time point, SCG10 mRNA in dorsal root ganglion neurons was not increased yet. The increase in SCG10 protein and mRNA could be observed over 30 days. Local T3 treatment significantly enhanced the increase in SCG10 protein levels about two-fold in the different segments of transected nerve during the regeneration period. Also SCG10 mRNA levels in lumbar ganglia were enhanced. Immunohistochemical analysis showed that T3 treatment not only increased the number of SCG10 positive axons but also the intensity of their staining. These results suggest that SCG10 is involved in the regulation of regeneration. The stimulating effect of T3 on SCG10 expression could provide a mechanism by which T3 enhances peripheral nerve regeneration.

Two separate motifs cooperate to target stathmin-related proteins to the Golgi complex

The appropriate targeting of membrane-associated proteins involves a diversity of motifs including post-translational modifications and specific protein sequences. Phosphoproteins of the stathmin family are important regulators of microtubule dynamics, in particular in the developing and mature nervous system. Whereas stathmin is cytosolic, SCG10, SCLIP and the splice variants RB3/RB3′/RB3″ are associated with Golgi and vesicular membranes, through their palmitoylated N-terminal A domains. In order to identify essential motifs involved in this specific targeting, we examined the subcellular distribution of various subdomains derived from domain A of SCG10 fused with GFP. We show that the Golgi localization of SCG10 results from the cooperation of two motifs: a membrane-anchoring palmitoylation motif and a newly identified Golgi-specifying sequence. The latter displayed no targeting activity by itself, but retained a Golgi-specifying activity when associated with another membrane-anchoring palmitoylation motif derived from the protein GAP-43. We further identified critical residues for the specific Golgi targeting of domain A. Altogether, our results give new insight into the regulation of the subcellular localization of stathmin family proteins, an important feature of their physiological functions in differentiating and mature neural cells. More generally we provide new information on essential mechanisms of functional protein subcellular targeting.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Palmitoylation of intracellular signaling proteins: regulation and function

Protein S-palmitoylation is the thioester linkage of long-chain fatty acids to cysteine residues in proteins. Addition of palmitate to proteins facilitates their membrane interactions and trafficking, and it modulates protein-protein interactions and enzyme activity. The reversibility of palmitoylation makes it an attractive mechanism for regulating protein activity, and this feature has generated intensive investigation of this modification. The regulation of palmitoylation occurs through the actions of protein acyltransferases and protein acylthioesterases. Identification of the protein acyltransferases Erf2/Erf4 and Akr1 in yeast has provided new insight into the palmitoylation reaction. These molecules work in concert with thioesterases, such as acyl-protein thioesterase 1, to regulate the palmitoylation status of numerous signaling molecules, ultimately influencing their function. This review discusses the function and regulation of protein palmitoylation, focusing on intracellular proteins that participate in cell signaling or protein trafficking.

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

We recently have identified a new cytoplasmic linker protein (CLIP), CLIPR-59, which is involved in the regulation of early endosome/trans-Golgi network dynamics. In contrast with CLIP-170, CLIPR-59 is not localized to microtubules at steady state but is associated with the trans-Golgi network and the plasma membrane. Here we show that the last 30 amino acids (C30) are sufficient for membrane targeting and that two cysteines in the C30 domain are palmitoylated. We demonstrate that CLIPR-59 is associated with lipid rafts via its C-terminal palmitoylated domain. In vitro experiments suggest that CLIPR-59 and its microtubule-binding domain alone have a better affinity for unpolymerized tubulin or small oligomers than for microtubules. In contrast with the CLIP-170 microtubule-binding domain, the CLIPR-59 microtubule-binding domain diminishes microtubule regrowth after nocodazole washout in vivo, showing that this domain can prevent microtubule polymerization. In contrast with the role of linker between membranes and microtubules that was proposed for CLIP function, CLIPR-59 thus may have an "anti-CLIP" function by preventing microtubule-raft interactions.

Stathmin and its phosphoprotein family: general properties, biochemical and functional interaction with tubulin

Stathmin, also referred to as Op18, is a ubiquitous cytosolic phosphoprotein, proposed to be a small regulatory protein and a relay integrating diverse intracellular signaling pathways involved in the control of cell proliferation, differentiation and activities. It interacts with several putative downstream target and/or partner proteins. One major action of stathmin is to interfere with microtubule dynamics, by inhibiting the formation of microtubules and/or favoring their depolymerization. Stathmin (S) interacts directly with soluble tubulin (T), which results in the formation of a T2S complex which sequesters free tubulin and therefore impedes microtubule formation. However, it has been also proposed that stathmin's action on microtubules might result from the direct promotion of catastrophes, which is still controversial. Phosphorylation of stathmin regulates its biological actions: it reduces its affinity for tubulin and hence its action on microtubule dynamics, which allows for example progression of cells through mitosis. Stathmin is also the generic element of a protein family including the neural proteins SCG10, SCLIP and RB3/RB3'/RB3". Interestingly, the stathmin-like domains of these proteins also possess a tubulin binding activity in vitro. In vivo, the transient expression of neural phosphoproteins of the stathmin family leads to their localization at Golgi membranes and, as previously described for stathmin and SCG10, to the depolymerization of interphasic microtubules. Altogether, the same mechanism for microtubule destabilization, that implies tubulin sequestration, is a common feature likely involved in the specific biological roles of each member of the stathmin family.

A synergistic relationship between three regions of stathmin family proteins is required for the formation of a stable complex with tubulin

Stathmin is a ubiquitous 17 kDa cytosolic phosphoprotein proposed to play a general role in the integration and relay of intracellular signalling pathways. It is believed to regulate microtubule dynamics by sequestering tubulin in a complex made of two tubulin heterodimers per stathmin molecule (T2S complex). The other proteins of the stathmin family can also bind two tubulin heterodimers through their SLD (stathmin-like domain), but the different tubulin:SLD complexes display varying stabilities. In this study, we analysed the relative influence of three regions of SLDs on the interaction with tubulin and the mechanistic processes that lead to its sequestration. Tubulin-binding properties of fragments and chimaeras of stathmin and RB3(SLD) were studied in vitro by tubulin polymerization, size-exclusion chromatography and surface plasmon resonance assays. Our results show that the N-terminal region of SLDs favours the binding of the first tubulin heterodimer and that the second C-terminal tubulinbinding site confers the specific stability of a given tubulin:SLD complex. Our results highlight the molecular processes by which tubulin co-operatively interacts with the SLDs. This knowledge may contribute to drug development aimed at disturbing microtubules that could be used for the treatment of cancer.

Modulation of the Stathmin-like Microtubule Destabilizing Activity of RB3, a Neuron-specific Member of the SCG10 Family, by Its N-terminal Domain

RB3 is a neuron-specific homologue of the SCG10/stathmin family proteins, possessing a unique N-terminal membrane-associated domain and the stathmin-like domain at the C terminus, which promotes microtubule (MT) catastrophe and/or tubulin sequestering. We examined herein the contribution of the N-terminal subdomain of RB3 to the regulation of MT dynamics. To begin with, we determined the effects of full-length (RB3-f) and short truncated (RB3-s) forms of RB3 on the polymerization of MT in vitro. RB3-s had a deletion of amino acids 1-75 from the N terminus, leaving the so-called stathmin-like domain, consisting of residues 76-217. Although both RB3-f and RB3-s exhibited MT-depolymerizing activity, RB3-f was less effective. The binding affinity for tubulin was also lower in RB3-f. Direct observation of the dynamics of individual MTs using dark field microscopy revealed that RB3-s slowed MT elongation velocity, increased catastrophes, and reduced rescues. This effect is almost identical to that by stathmin/oncoprotein 18. On the other hand, the MT elongation rate increased at lower concentrations of RB3-f. In addition, RB3-f, indicated higher rescue frequency than control as well as the catastrophe in a dose-dependent manner. The functionality of RB3-f indicated that full-length RB3 has not only stathmin-like MT destabilizing activity but also MT-associated protein-like MT stabilizing activity. Possibly, the balance of these activities is altered in a concentration-dependent manner in vitro. This interesting regulatory role of the unique N-terminal domain of RB3 in MT dynamics would contribute to the physiological regulation of neuronal morphogenesis.

Role of the microtubule destabilizing proteins SCG10 and stathmin in neuronal growth

The related proteins SCG10 and stathmin are highly expressed in the developing nervous system. Recently it was discovered that they are potent microtubule destabilizing factors. While stathmin is expressed in a variety of cell types and shows a cytosolic distribution, SCG10 is neuron-specific and membrane-associated. It contains an N-terminal targeting sequence that mediates its transport to the growing tips of axons and dendrites. SCG10 accumulates in the central domain of the growth cone, a region that also contains highly dynamic microtubules. These dynamic microtubules are known to be important for growth cone advance and responses to guidance cues. Because overexpression of SCG10 strongly enhances neurite outgrowth, SCG10 appears to be an important factor for the dynamic assembly and disassembly of growth cone microtubules during axonal elongation. Phosphorylation negatively regulates the microtubule destabilizing activity of SCG10 and stathmin, suggesting that these proteins may link extracellular signals to the rearrangement of the neuronal cytoskeleton. A role for these proteins in axonal elongation is also supported by their growth-associated expression pattern in nervous system development as well as during neuronal regeneration.

Identification of upregulated SCG10 mRNA expression associated with late-phase long-term potentiation in the rat hippocampal Schaffer-CA1 pathway in vivo

The maintenance of long-term potentiation (LTP) depends on alteration of gene transcription. By screening a subtracted cDNA library that is enriched in upregulated transcripts in rat hippocampus 3 hr after Schaffer-CA1 LTP induction in vivo, we identified a neural growth-associated protein SCG10 (superior cervical ganglia clone 10) gene. The semiquantitative reverse transcription-PCR and Northern blot experiments confirmed that SCG10 mRNA levels were elevated in tetanized rat hippocampi compared with those of sham controls that received only low-frequency stimulation. Both 1 and 2 kb forms of SCG10 mRNAs contributed to the increased expression. Using a riboprobe with a sequence specific to the 3'-untranslated region of rat SCG10 mRNA, in situ hybridization further revealed a significant increase of the SCG10 mRNA 2 kb form in the ipsilateral CA3 and CA1 regions of LTP animals. In addition, we systemically injected the competitive NMDA receptor antagonist d,l-3[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid (CPP) to determine whether the alteration of SCG10 expression depends on NMDA receptor activation or tetanus alone. Administration of CPP 1 hr before tetanus completely blocked LTP induction and the increase of SCG10 mRNA levels. Thus, these results suggest that the transcription of SCG10 in vivo is regulated by long-lasting synaptic activity and may contribute to the maintenance of long-term synaptic plasticity via a presynaptic remodeling mechanism.

Deciphering the cellular functions of the Op18/Stathmin family of microtubule-regulators by plasma membrane-targeted localization

The Op18/stathmin family of microtubule regulators includes the ubiquitous cytosolic Op18/stathmin (Op18) and the neuronal, primarily Golgi-associated proteins SCG10 and RB3, which all form ternary complexes with two head-to-tail-aligned tubulin heterodimers. To understand the physiological significance of previously observed differences in ternary complex stability, we have fused each of the heterodimer-binding regions of these three proteins with the CD2 cell surface protein to generate confined plasma membrane localization of the resulting CD2 chimeras. Herein, we show that, in contrast to constitutively active CD2-Op18-tetraA, both the CD2-SCG10 and CD2-RB3 chimeras sequestered tubulin at the plasma membrane, which results in >35% reduction of cytosolic tubulin heterodimer levels and consequent delayed formation of mitotic spindles. However, all three CD2 chimeras, including the tubulin sequestration-incompetent CD2-Op18-tetraA, destabilize interphase microtubules. Given that microtubules are in extensive contact with the plasma membrane during interphase, but not during mitosis, these findings indicate that Op18-like proteins have the potential to destabilize microtubules by both sequestration and direct interaction with microtubules. However, the differences in tubulin binding observed in cells also indicate conceptual differences between the functions of low-abundance neural family members, which will accumulate tubulin at specific cellular compartments, and the abundant cytosolic Op18 protein, which will not.

Expression of stathmin family genes in human tissues: non-neural-restricted expression for SCLIP

The stathmin family consists of phosphoproteins highly conserved in vertebrates and thought to be implicated in the development and functional regulation of various organs, most notably the nervous system. This family includes stathmin, SCG10, SCLIP, and RB3, phosphoproteins that are related by structural and functional homologies. They all sequester tubulin and interfere with microtubule dynamics, a property due to their shared stathmin-like domain. Little is known about the expression of the stathmin gene family in humans. Herein, we describe for the first time, for a collection of human tissues, the expression of each member of this family, using real-time quantitative RT-PCR. We found that stathmin is ubiquitously expressed, whereas SCG10 and RB3 are neural enriched, expression patterns similar to those reported for other mammals. Surprisingly, SCLIP, whose expression is thought to be neural-specific, exhibits a broader tissue distribution. Analyses of the SCLIP gene (approved symbol STMN3) show that it contains several NRSE-like elements that display low or no affinity for the cognate binding protein NRSF. The substantial expression of SCLIP in most tissues points out a novel function for this protein outside the nervous system and raises the possibility that its coexpression with stathmin could provide some degree of functional redundancy.

Subcellular localization of herpes simplex virus type 1 UL51 protein and role of palmitoylation in Golgi apparatus targeting

The herpes simplex virus type 1 (HSV-1) UL51 gene products are virion-associated phosphoproteins with apparent molecular masses of 27, 29, and 30 kDa in HSV-1-infected cells. In this study, we have investigated the intracellular localization and distribution of UL51 protein both in infected cells and in transfected cells expressing only UL51. We found that this protein colocalized closely with Golgi marker proteins such as the Golgi-58K protein and GM130 in transfected cells expressing only UL51. However, in infected cells, the UL51 protein localized to the juxtanuclear region but only partially colocalized with the Golgi maker proteins. Mutant protein analysis revealed that the N-terminal 15 amino acid residues of the UL51 protein sufficed for this Golgi localization property. The UL51 protein redistributed on addition of brefeldin A. This was prevented by pretreatment with 2-deoxyglucose and sodium azide, which results in ATP depletion, but not by pretreatment with NaF and AlCl(3), which activates heterotrimeric G proteins. Moreover, we found that palmitoylation of the UL51 protein through the N-terminal cysteine at position 9 was necessary for its Golgi localization. Protease digestion analysis suggested that the UL51 protein localized on the cytoplasmic face of the membrane in UL51-transfected cells, while in infected cells it localized mainly to the inside of cytoplasmic vesicles and/or the viral envelope. Transmission immunoelectron microscopy revealed an association of UL51 protein-specific labeling with cytoplasmic virions and also with some membranous structure. We infer from these observations that internalization of UL51 protein into the cytoplasmic vesicle and/or virion may occur in association with viral envelopment in HSV-infected cells.