Novel features of intermediate filament dynamics revealed by green fluorescent protein chimeras

Growth cones contain a dynamic population of neurofilament subunits

Neurofilaments (NFs) are classically considered to transport in a primarily anterograde direction along axons, and to undergo bulk degradation within the synapse or growth cone (GC). We compared overall NF protein distribution with that of newly expressed NF subunits within NB2a/d1 cells by transfection with a construct encoding green fluorescent protein (GFP) conjugated NF-M subunits. GCs lacked phosphorylated NF epitopes, and steady-state levels of non-phosphosphorylated NF subunits within GC were markedly reduced compared to those of neurite shaft as indicated by conventional immunofluorescence. However, GCs contained significant levels of GFP-tagged subunits in the form of punctate or short filamentous structures that in some cases exceeded that visualized along the shaft itself, suggesting that GCs contained a relatively higher concentration of newly synthesized subunits. GFP-tagged NF subunits within GCs co-localized with non-phosphorylated NF immunoreactivity. GFP-tagged subunits were observed within GC filopodia in which steady-state levels of NF subunits were too low to be detected by conventional immunofluorescence. Selective localization of fluorescein versus rhodamine fluorescene was observed within GCs following expression of NF-M conjugated to DsRed1-E5, which shifts from fluorescein to rhodamine fluorescence within hours after expression; axonal shafts contained a more even distribution of fluorescein and rhodamine fluorescence, further indicating that GCs contained relatively higher levels of the most-recently expressed subunits. GFP-tagged structures were rapidly extracted from GCs under conditions that preserved axonal structures. These short filamentous and punctate structures underwent rapid bi-directional movement within GCs. Movement of GFP-tagged structures within GCs ceased following application of nocodazole, cytochalasin B, and the kinase inhibitor olomoucine, indicating that their motility was dependent upon microtubules and actin and, moreover, was due to active transport rather than simple diffusion. Treatment with the protease inhibitor calpeptin increased overall NF subunits, but increased those within the GC to a greater extent than those along the shaft, indicating that subunits in the GC undergo more rapid turnover than do those within the shaft. Some GCs contained coiled aggregates of GFP-tagged NFs that appeared to be contiguous with axonal NFs. NFs extended from these aggregates into the advancing GC as axonal neurites elongated. These data are consistent with the presence of a population of dynamic NF subunits within GCs that is apparently capable of participating in regional filament formation during axonal elongation, and support the notion that NF polymerization and transport need not necessarily occur in a uniform proximal-distal manner.

Contrasting roles of endosomal pH and the cytoskeleton in infection of human glial cells by JC virus and simian virus 40

Infection of eukaryotic cells by pathogens requires the efficient use of host cell endocytic and cytoplasmic transport mechanisms. Understanding how these cellular functions are exploited by microorganisms allows us to better define the basic biology of pathogenesis while providing better insight into normal cellular functions. In this report we compare and contrast intracellular transport and trafficking of the human polyomavirus JC virus (JCV) with that of simian virus 40 (SV40). We have previously shown that infection of human glial cells by JCV requires clathrin-dependent endocytosis. In contrast, infection of cells by SV40 proceeds by caveola-dependent endocytosis. We now examine the roles of endosomal pH and the cellular cytoskeleton during infection of glial cells by both viruses. Our results demonstrate that JCV infection is sensitive to disruption of endosomal pH, whereas SV40 infection is pH independent. Infection by JCV is inhibited by treatment of glial cells with cytochalasin D, nocodazole, and acrylamide, whereas SV40 infection is affected only by nocodazole. These data point to critical differences between JCV and SV40 in terms of endocytosis and intracellular trafficking of their DNA genomes to the nucleus. These data also suggest a unique sequential involvement of cytoskeletal elements during infection of glial cells by JCV.

Quantification of cytoskeletal deformation in living cells based on hierarchical feature vector matching

The cytoskeleton is a dynamic scaffold in living cells even in the absence of externally imposed forces. In this study on cytoskeletal deformation, the applicability of hierarchical feature vector matching (HFVM), a new matching method, currently applied in space research and three-dimensional surface reconstruction, was investigated. Stably transfected CHO-K1 cells expressing green fluorescent protein (GFP) coupled to vimentin were used to visualize spontaneous movement of the vimentin cytoskeleton of individual cells using a confocal laser scanning system. We showed that, with proper parameter and configuration settings, HFVM could recognize and trace 60-70% of all image points in artificially translated, rotated, or deformed images. If only points belonging to the cytoskeleton were selected for matching purposes, the percentage of matched points increased to 98%. This high percentage of recognition also could be reached in a time series of images, in which a certain degree of bleaching of the fluorescence over the recording time of 30 min was inevitable. In these images, HFVM allowed the detection as well as the quantification of spontaneous cytoskeletal movements of up to 10% of the cell width. Therefore, HFVM appears to be a reliable method of quantifying dynamic cytoskeletal behavior in living cells.

Polycystin-1 interacts with intermediate filaments

Polycystin-1, the protein defective in a majority of patients with autosomal dominant polycystic kidney disease, is a ubiquitously expressed multi-span transmembrane protein of unknown function. Subcellular localization studies found this protein to be a component of various cell junctional complexes and to be associated with the cytoskeleton, but the specificity and nature of such associations are not known. To identify proteins that interact with the polycystin-1 C-tail (P1CT), this segment was used as bait in a yeast two-hybrid screening of a kidney epithelial cell library. The intermediate filament (IF) protein vimentin was identified as a strong polycystin-1-interacting partner. Cytokeratins K8 and K18 and desmin were also found to interact with P1CT. These interactions were mediated by coiled-coil motifs in polycystin-1 and IF proteins. Vimentin, cytokeratins K8 and K18, and desmin also bound directly to P1CT in GST pull-down and in in vitro filament assembly assays. Two observations confirmed these interactions in vivo: (i) a cell membrane-anchored form of recombinant P1CT decorated the IF network and was found to associate with the cytoskeleton in detergent-solubilized cells and (ii) endogenous polycystin-1 distributed with IF at desmosomal junctions. Polycystin-1 may utilize this association for structural, storage, or signaling functions.

Ypel1: a novel nuclear protein that induces an epithelial-like morphology in fibroblasts

BACKGROUND

Embryonic development depends on the regulation of cell morphology and behaviour to carry out morphogenesis. One example of this is the development of the face, which is constructed from independent blocks of tissue that must grow in a coordinated way and then fuse to form a continuous tissue.

RESULTS

We have isolated a novel gene of unknown function from mouse and quail embryos in a search for genes involved in craniofacial development. Sequence analysis of this gene, known as Ypel1, demonstrates a striking level of amino acid conservation between vertebrates and invertebrates but no significant homology with any other characterized genes. Ypel1 is expressed in the ventral half of early embryos including the branchial arches from which the face derives. Ypel1 localizes to the nucleus, and transfection into fibroblasts induces an epithelial-like transition, which is accompanied by alterations to the cytoskeleton and cell adhesion machinery. In addition, human YPEL1 localizes to chromosome 22q11.2, a region associated with a number of syndromes involving malformation of the craniofacial complex.

CONCLUSION

These data suggest a role in regulation of cellular morphology and behaviour that is important for development of the craniofacial complex.

Insights into the dynamic properties of keratin intermediate filaments in living epithelial cells

The properties of keratin intermediate filaments (IFs) have been studied after transfection with green fluorescent protein (GFP)-tagged K18 and/or K8 (type I/II IF proteins). GFP-K8 and -K18 become incorporated into tonofibrils, which are comprised of bundles of keratin IFs. These tonofibrils exhibit a remarkably wide range of motile and dynamic activities. Fluorescence recovery after photobleaching (FRAP) analyses show that they recover their fluorescence slowly with a recovery t(1/2) of approximately 100 min. The movements of bleach zones during recovery show that closely spaced tonofibrils (<1 microm apart) often move at different rates and in different directions. Individual tonofibrils frequently change their shapes, and in some cases these changes appear as propagated waveforms along their long axes. In addition, short fibrils, termed keratin squiggles, are seen at the cell periphery where they move mainly towards the cell center. The motile properties of keratin IFs are also compared with those of type III IFs (vimentin) in PtK2 cells. Intriguingly, the dynamic properties of keratin tonofibrils and squiggles are dramatically different from those of vimentin fibrils and squiggles within the same cytoplasmic regions. This suggests that there are different factors regulating the dynamic properties of different types of IFs within the same cytoplasmic regions.

The predominant form in which neurofilament subunits undergo axonal transport varies during axonal initiation, elongation, and maturation

The forms in which neurofilament (NF) subunits undergo axonal transport is controversial. Recent studies from have provided real-time visualization of the slow axonal transport of NF subunits by transfecting neuronal cultures with constructs encoding green fluorescent protein (GFP)-conjugated NF-M subunits. In our studies in differentiated NB2a/d1 cells, the majority NF subunits underwent transport in the form of punctate NF precursors, while studies in cultured neurons have demonstrated transport of NF subunits in predominantly filamentous form. Although different constructs were used in these studies, transfection of the same cultured neurons with our construct yielded the filamentous pattern observed by others, while transfection of our cultures with their construct generated punctate structures, confirming that the observed differences did not reflect variances in assembly-competence among the constructs. Manipulation of intracellular kinase, phosphatase, and protease activities shifted the predominant form of GFP-conjugated subunits between punctate and filamentous, confirming, as shown previously for vimentin, that punctate structures represent precursors for intermediate filament formation. Since these prior studies were conducted at markedly differing neuronal differentiation states, we tested the alternate hypothesis that these differing results reflected developmental alterations in NF dynamics that accompany various stages of neuritogenesis. We conducted time-course analyses of transfected NB2a/d1 cells, including monitoring of transfected cells over several days, as well as transfecting cells at varying intervals prior to and following induction of differentiation and axonal neurite outgrowth. GFP-conjugated subunits were predominantly filamentous during the period of most robust axonal outgrowth and NF accumulation, and presented a mixed profile of punctate and filamentous forms prior to neuritogenesis and following the developmental slowing of neurite outgrowth. These analyses demonstrate that NF subunits are capable of undergoing axonal transport in multiple forms, and that the predominant form in which NF subunits undergo axonal transport varies in accord with the rate of axonal elongation and accumulation of NFs within developing axons.

Spatiotemporal analysis of flow-induced intermediate filament displacement in living endothelial cells

The distribution of hemodynamic shear stress throughout the arterial tree is transduced by the endothelium into local cellular responses that regulate vasoactivity, vessel wall remodeling, and atherogenesis. Although the exact mechanisms of mechanotransduction remain unknown, the endothelial cytoskeleton has been implicated in transmitting extracellular force to cytoplasmic sites of signal generation via connections to the lumenal, intercellular, and basal surfaces. Direct observation of intermediate filament (IF) displacement in cells expressing green fluorescent protein-vimentin has suggested that cytoskeletal mechanics are rapidly altered by the onset of fluid shear stress. Here, restored images from time-lapse optical sectioning fluorescence microscopy were analyzed as a four-dimensional intensity distribution function that represented IF positions. A displacement index, related to the product moment correlation coefficient as a function of time and subcellular spatial location, demonstrated patterns of IF displacement within endothelial cells in a confluent monolayer. Flow onset induced a significant increase in IF displacement above the nucleus compared with that measured near the coverslip surface, and displacement downstream from the nucleus was larger than in upstream areas. Furthermore, coordinated displacement of IF near the edges of adjacent cells suggested the existence of mechanical continuity between cells. Thus, quantitative analysis of the spatiotemporal patterns of flow-induced IF displacement suggests redistribution of intracellular force in response to alterations in hemodynamic shear stress acting at the lumenal surface.

Properties of the nonhelical end domains of vimentin suggest a role in maintaining intermediate filament network structure

To investigate the functional role of the nonhelical domains of the intermediate filament (IF) protein vimentin, we carried out transient transfection of constructs encoding fusion proteins of these domains with enhanced green fluorescent protein (EGFP). Expression of these fusion proteins did not have any effect on the endogenous IF networks of transfected cells. However, the head domain-EGFP fusion protein localized almost exclusively to the nucleus. This localization could be disrupted in a reversible fashion by chilling cells. Furthermore, the head domain was capable of targeting to the nucleus a strictly cytoplasmic protein, pyruvate kinase. Thus, the vimentin head domain contains information that specifically directs proteins into the nucleus. In contrast, the nonhelical tail domain of vimentin, when expressed as a fusion protein with EGFP, was retained in the cytoplasm. Cytoplasmic retention of tail domain-containing fusion proteins appeared to be dependent on the integrity of the microtubule network. Our results are consistent with a proposal that the nonhelical end domains of vimentin are involved in maintaining an extended IF network by exerting oppositely directed forces along the filaments. The head domains exert a nuclear-directed force while the tail domains extend the IF network toward the cell periphery via a microtubule-dependent mechanism.

Cdc42Hs and Rac1 GTPases induce the collapse of the vimentin intermediate filament network

In this study we show that expression of active Cdc42Hs and Rac1 GTPases, two Rho family members, leads to the reorganization of the vimentin intermediate filament (IF) network, showing a perinuclear collapse. Cdc42Hs displays a stronger effect than Rac1 as 90% versus 75% of GTPase-expressing cells show vimentin collapse. Similar vimentin IF modifications were observed when endogenous Cdc42Hs was activated by bradykinin treatment, endogenous Rac1 by platelet-derived growth factor/epidermal growth factor, or both endogenous proteins upon expression of active RhoG. This reorganization of the vimentin IF network is not associated with any significant increase in soluble vimentin. Using effector loop mutants of Cdc42Hs and Rac1, we show that the vimentin collapse is mostly independent of CRIB (Cdc42Hs or Rac-interacting binding)-mediated pathways such as JNK or PAK activation but is associated with actin reorganization. This does not result from F-actin depolymerization, because cytochalasin D treatment or Scar-WA expression have merely no effect on vimentin organization. Finally, we show that genistein treatment of Cdc42 and Rac1-expressing cells strongly reduces vimentin collapse, whereas staurosporin, wortmannin, LY-294002, R(p)-cAMP, or RII, the regulatory subunit of protein kinase A, remain ineffective. Moreover, we detected an increase in cellular tyrosine phosphorylation content after Cdc42Hs and Rac1 expression without modification of the vimentin phosphorylation status. These data indicate that Cdc42Hs and Rac1 GTPases control vimentin IF organization involving tyrosine phosphorylation events.

Plasticin, a type III neuronal intermediate filament protein, assembles as an obligate heteropolymer: implications for axonal flexibility

The assembly characteristics of the neuronal intermediate filament protein plasticin were studied in SW13 cells in the presence and absence of a cytoplasmic filament network. Full-length plasticin cannot polymerize into homopolymers in filament-less SW13c1.2Vim(-) cells but efficiently coassembles with vimentin in SW13c1.1Vim(-) cells. By cotransfecting plasticin and vimentin in SW13c1.1Vim(-) cells, we show that plasticin assembly requires vimentin in noncatalytic amounts. Differing effects on assembly were seen with point mutations of plasticin monomers that were analogous to the keratin mutations that cause epidermolysis bullosa simplex (EBS). In particular, plasticin monomers with point mutations analogous to those in EBS do not uniformly inhibit neurofilament (NF) network formation. A point mutation in the helix termination sequence resulted in complete filament aggregation when coexpressed with vimentin but showed limited coassembly with low- and medium-molecular-weight NF proteins (NF-L and NF-M, respectively). In transfected SW13c1.1Vim(+) cells, a point mutation in the first heptad of the alpha-helical coil region formed equal amounts of filaments, aggregates, and a mixture of filaments and aggregates. Furthermore, coexpression of this point mutation with NF-L and NF-M was associated with a shift toward increased numbers of aggregates. These results suggest that there are important structural differences in assembly properties between homologous fish and mammalian intermediate filament proteins. These structural differences may contribute to the distinctive growth characteristics of the teleost visual pathway.

Glutamate slows axonal transport of neurofilaments in transfected neurons

Neurofilaments are transported through axons by slow axonal transport. Abnormal accumulations of neurofilaments are seen in several neurodegenerative diseases, and this suggests that neurofilament transport is defective. Excitotoxic mechanisms involving glutamate are believed to be part of the pathogenic process in some neurodegenerative diseases, but there is currently little evidence to link glutamate with neurofilament transport. We have used a novel technique involving transfection of the green fluorescent protein-tagged neurofilament middle chain to measure neurofilament transport in cultured neurons. Treatment of the cells with glutamate induces a slowing of neurofilament transport. Phosphorylation of the side-arm domains of neurofilaments has been associated with a slowing of neurofilament transport, and we show that glutamate causes increased phosphorylation of these domains in cell bodies. We also show that glutamate activates members of the mitogen-activated protein kinase family, and that these kinases will phosphorylate neurofilament side-arm domains. These results provide a molecular framework to link glutamate excitotoxicity with neurofilament accumulation seen in some neurodegenerative diseases.

Misdirected vimentin messenger RNA alters cell morphology and motility

Localized messenger RNAs were first observed as embryonic determinants that altered development when mislocalized. In recent years localized mRNAs have been found for several cytoskeletal proteins, including actin, vimentin and several microtubule associated proteins. We sought to determine whether redirecting mRNA for a cytoskeletal protein to an inappropriate address would alter cellular phenotypes. To do so we generated vimentin mRNAs with a myc epitope tag and the (beta)-actin 3′ untranslated region (3′ UTR) as a localization signal. When misdirected vimentin mRNAs are expressed in either fibroblasts or SW13 cells, cells develop numerous, extremely long processes; these cells also move more slowly to enter a wound of the monolayer. In situ hybridization revealed that the misdirected mRNA was often localized in the processes, in contrast to endogenous vimentin mRNA. The processes usually contained actin distal to the transgenic vimentin and microtubules proximal to it. SW13 cells lacking vimentin produced fewer and shorter processes, suggesting a dominant negative effect that involves recruitment of endogenous vimentin. Control experiments that transfected in constructs expressing tagged, correctly localized vimentin, or (beta)-galactosidase that localized through the (beta)-actin 3′ UTR, indicate that neither the shape nor the motility changes are solely due to the level of vimentin expression in the cell. This is direct evidence that the site of expression for at least one cytoskeletal mRNA alters the phenotype of the cell in which it is expressed. Messenger RNA localization is proving to be as essential for the normal maintenance of somatic cell phenotypes as embryonic determinants are for embryogenesis.

Detection of cytokeratin dynamics by time-lapse fluorescence microscopy in living cells

To monitor the desmosome-anchored cytokeratin network in living cells fusion protein HK13-EGFP consisting of human cytokeratin 13 and the enhanced green fluorescent protein was stably expressed in vulvar carcinoma-derived A-431 cells. It is shown for A-431 subclone AK13-1 that HK13-EGFP emits strong fluorescence in fixed and living cells, being part of an extended cytoplasmic intermediate filament network that is indistinguishable from that of parent A-431 cells. Biochemical, immunological and ultrastructural analyses demonstrate that HK13-EGFP behaves identically to the endogenous cytokeratin 13 and is therefore a reliable in vivo tag for this polypeptide and the structures formed by it. Time-lapse fluorescence microscopy reveals that the cytokeratin 13-containing network is in constant motion, resulting in continuous restructuring occurring in single and migratory cells, as well as in desmosome-anchored cells. Two major types of movement are distinguished: (i) oscillations of mostly long filaments, and (ii) an inward-directed flow of fluorescence originating as diffuse material at the cell periphery and moving in the form of dots and thin filaments toward the deeper cytoplasm where it coalesces with other filaments and filament bundles. Both movements are energy dependent and can be inhibited by nocodazole, but not by cytochalasin D. Finally, disassembly and reformation of cytokeratin filament networks are documented in dividing cells revealing distinct and rapidly occurring stages of cytokeratin organisation and distribution.

Dynamics of gap junctions observed in living cells with connexin43-GFP chimeric protein

To study the aggregation of cell-to-cell channels into gap junctions at individual cell-cell contacts, we transfected cells with an expression vector for a chimeric protein composed of the cell-to-cell channel protein connexin43 and a green fluorescent protein. The chimeric channel protein was visualized in the fluorescence microscope and was found to form gap junctions at the cell-cell contacts just like wild-type connexin43. Cells expressing the chimeric protein had functional cell-to-cell channels. Using timelapse videomicroscopy on live cells we observed individual gap junctions over long periods and recorded the time course of aggregation of the chimeric channel protein into gap junctions at newly formed cell-cell contacts. We found that individual small gap junctions were very dynamic, moving about or becoming assembled and disassembled in the course of minutes. Larger gap junctions were more stable than small punctate ones. In control condition, stable new gap junctions were not formed during observation times of 30 min or longer. But at elevated levels of cyclic adenosine monophosphate, the chimeric channel protein began aggregating at new junctions 5-10 minutes after cell-cell contact and continued to concentrate there for at least one hour. Also already established junctions grew in size. The fluorescent chimeric channel protein will be an excellent tool to investigate the regulation of trafficking of connexin from and to the membrane and the mechanism of connexin channel aggregation into gap junctions.

Fluorescence tracing of intracellular proteins

Developments in fluorescence microscopy and the availability of fluorescently labeled antibodies and probes for localization of molecules and organelles have made the microscope an indispensable tool with which one can map specific molecules to subcellular loci allowing deep insight into cell and organelle biology. Furthermore, confocal microscopy permits analysis of the three dimensional architecture of cells that could not be accomplished by conventional light microscopy. The goal of fluorescence protein tracing by microscopy is to visualize cellular constituents and general cytoarchitecture as close to native organization as possible. To achieve this, and to preserve cellular structure in the best possible manner, the specimen is usually fixed chemically. Here I review several standard fixation, permeabilization and labeling schemes followed by examples of several standard imaging techniques.

Kinesin-mediated transport of neurofilament protein oligomers in growing axons

We examined cytoskeleton-associated forms of NF proteins during axonal neuritogenesis in cultured dorsal root ganglion (DRG) neurons and NB2a/d1 neuroblastoma. In addition to filamentous immunoreactivity, we observed punctate NF immunoreactivity throughout perikarya and neurites. Immuno-electron microscopy revealed this punctate immunoreactivity to consist of non-membrane-bound 75 nm round/ovoid structures consisting of amorphous, fibrous material. Endogenous and microinjected NF subunits incorporated into dots prior to their accumulation within filaments. A transfected GFP-conjugated NF-M incorporated into dots and translocated at a rate consistent with slow axonal transport in real-time video analyses. Some dots converted into a filamentous form or exuded filamentous material during transport. Dots contained conventional kinesin immunoreactivity, associated with microtubules, and their transport into axons was blocked by anti-kinesin antibodies and nocodazole. These oligomeric structures apparently represent one form in which NF subunits are transported in growing axons and may utilize kinesin as a transport motor.

Specific accumulation of Rho-associated kinase at the cleavage furrow during cytokinesis: cleavage furrow-specific phosphorylation of intermediate filaments

The small GTPase Rho and one of its targets, Rho-associated kinase (Rho-kinase), are implicated in a wide spectrum of cellular functions, including cytoskeletal rearrangements, transcriptional activation and smooth muscle contraction. Since Rho also plays an essential role in cytokinesis, Rho-kinase may possibly mediate some biological aspects of cytokinesis. Here, using a series of monoclonal antibodies that can specifically recognize distinct phosphorylated sites on glial fibrillary acidic protein (GFAP) and vimentin, phosphorylation sites by Rho-kinase in vitro were revealed to be identical to in vivo phosphorylation sites on these intermediate filament (IF) proteins at the cleavage furrow in dividing cells. We then found, by preparing two types of anti-Rho-kinase antibodies, that Rho-kinase accumulated highly and circumferentially at the cleavage furrow in various cell lines. This subcellular distribution during cytokinesis was very similar to that of ezrin/radixin/moesin (ERM) proteins and Ser19-phosphorylated myosin light chain. These results raise the possibility that Rho-kinase might be involved in the formation of the contractile ring by modulating these F-actin-binding proteins during cytokinesis and in the phosphorylation and regulation of IF proteins at the cleavage furrow.

Detyrosination of tubulin regulates the interaction of intermediate filaments with microtubules in vivo via a kinesin-dependent mechanism

Posttranslationally modified forms of tubulin accumulate in the subset of stabilized microtubules (MTs) in cells but are not themselves involved in generating MT stability. We showed previously that stabilized, detyrosinated (Glu) MTs function to localize vimentin intermediate filaments (IFs) in fibroblasts. To determine whether tubulin detyrosination or MT stability is the critical element in the preferential association of IFs with Glu MTs, we microinjected nonpolymerizable Glu tubulin into cells. If detyrosination is critical, then soluble Glu tubulin should be a competitive inhibitor of the IF-MT interaction. Before microinjection, Glu tubulin was rendered nonpolymerizable and nontyrosinatable by treatment with iodoacetamide (IAA). Microinjected IAA-Glu tubulin disrupted the interaction of IFs with MTs, as assayed by the collapse of IFs to a perinuclear location, and had no detectable effect on the array of Glu or tyrosinated MTs in cells. Conversely, neither IAA-tyrosinated tubulin nor untreated Glu tubulin, which assembled into MTs, caused collapse of IFs when microinjected. The epitope on Glu tubulin responsible for interfering with the Glu MT-IF interaction was mapped by microinjecting tubulin fragments of alpha-tubulin. The 14-kDa C-terminal fragment of Glu tubulin (alpha-C Glu) induced IF collapse, whereas the 36-kDa N-terminal fragment of alpha-tubulin did not alter the IF array. The epitope required more than the detyrosination site at the C terminus, because a short peptide (a 7-mer) mimicking the C terminus of Glu tubulin did not disrupt the IF distribution. We previously showed that kinesin may mediate the interaction of Glu MTs and IFs. In this study we found that kinesin binding to MTs in vitro was inhibited by the same reagents (i.e., IAA-Glu tubulin and alpha-C Glu) that disrupted the IF-Glu MT interaction in vivo. These results demonstrate for the first time that tubulin detyrosination functions as a signal for the recruitment of IFs to MTs via a mechanism that is likely to involve kinesin.

RhoA-binding kinase alpha translocation is facilitated by the collapse of the vimentin intermediate filament network

The regulation of morphological changes in eukaryotic cells is a complex process involving major components of the cytoskeleton including actin microfilaments, microtubules, and intermediate filaments (IFs). The putative effector of RhoA, RhoA-binding kinase alpha (ROKalpha), is a serine/threonine kinase that has been implicated in the reorganization of actin filaments and in myosin contractility. Here, we show that ROKalpha also directly affects the structural integrity of IFs. Overexpression of active ROKalpha, like that of RhoA, caused the collapse of filamentous vimentin, a type III IF. A RhoA-binding-deficient, kinase-inactive ROKalpha inhibited the collapse of vimentin IFs induced by RhoA in HeLa cells. In vitro, ROKalpha bound and phosphorylated vimentin at its head-rod domain, thereby inhibiting the assembly of vimentin. ROKalpha colocalized predominantly with the filamentous vimentin network, which remained intact in serum-starved cells. Treatment of cells with vinblastine, a microtubule-disrupting agent, also resulted in filamentous vimentin collapse and concomitant ROKalpha translocation to the cell periphery. ROKalpha translocation did not occur when the vimentin network remained intact in vinblastine-treated cells at 4 degreesC or in the presence of the dominant-negative RhoAN19 mutant. Transient translocation of ROKalpha was also observed in cells subjected to heat shock, which caused the disassembly of the vimentin network. Thus, the translocation of ROKalpha to the cell periphery upon overexpression of RhoAV14 or growth factor treatment is associated with disassembly of vimentin IFs. These results indicate that Rho effectors known to act on microfilaments may be involved in regulating the assembly of IFs. Vimentin when phosphorylated also exhibits reduced affinity for the inactive ROKalpha. The translocation of ROKalpha from IFs to the cell periphery upon action by activated RhoA and ROKalpha suggests that ROKalpha may initiate its own cascade of activation.

Motile properties of vimentin intermediate filament networks in living cells

The motile properties of intermediate filament (IF) networks have been studied in living cells expressing vimentin tagged with green fluorescent protein (GFP-vimentin). In interphase and mitotic cells, GFP-vimentin is incorporated into the endogenous IF network, and accurately reports the behavior of IF. Time-lapse observations of interphase arrays of vimentin fibrils demonstrate that they are constantly changing their configurations in the absence of alterations in cell shape. Intersecting points of vimentin fibrils, or foci, frequently move towards or away from each other, indicating that the fibrils can lengthen or shorten. Fluorescence recovery after photobleaching shows that bleach zones across fibrils rapidly recover their fluorescence. During this recovery, bleached zones frequently move, indicating translocation of fibrils. Intriguingly, neighboring fibrils within a cell can exhibit different rates and directions of movement, and they often appear to extend or elongate into the peripheral regions of the cytoplasm. In these same regions, short filamentous structures are also seen actively translocating. All of these motile properties require energy, and the majority appear to be mediated by interactions of IF with microtubules and microfilaments.