Acceleration of bovine neurofilament L assembly by deprivation of acidic tail domain

Identification and characterization of novel filament-forming proteins in cyanobacteria

Filament-forming proteins in bacteria function in stabilization and localization of proteinaceous complexes and replicons; hence they are instrumental for myriad cellular processes such as cell division and growth. Here we present two novel filament-forming proteins in cyanobacteria. Surveying cyanobacterial genomes for coiled-coil-rich proteins (CCRPs) that are predicted as putative filament-forming proteins, we observed a higher proportion of CCRPs in filamentous cyanobacteria in comparison to unicellular cyanobacteria. Using our predictions, we identified nine protein families with putative intermediate filament (IF) properties. Polymerization assays revealed four proteins that formed polymers in vitro and three proteins that formed polymers in vivo. Fm7001 from Fischerella muscicola PCC 7414 polymerized in vitro and formed filaments in vivo in several organisms. Additionally, we identified a tetratricopeptide repeat protein - All4981 - in Anabaena sp. PCC 7120 that polymerized into filaments in vitro and in vivo. All4981 interacts with known cytoskeletal proteins and is indispensable for Anabaena viability. Although it did not form filaments in vitro, Syc2039 from Synechococcus elongatus PCC 7942 assembled into filaments in vivo and a Δsyc2039 mutant was characterized by an impaired cytokinesis. Our results expand the repertoire of known prokaryotic filament-forming CCRPs and demonstrate that cyanobacterial CCRPs are involved in cell morphology, motility, cytokinesis and colony integrity.

In vivo and in vitro phosphorylation at Ser-493 in the glutamate (E)-segment of neurofilament-H subunit by glycogen synthase kinase 3beta

Neurofilament (NF), a major neuronal intermediate filament, is composed of three subunits, NF-L, NF-M, and NF-H. All three subunits contain a well conserved glutamate (E)-rich region called "E-segment" in the N terminus of the tail region. Although the E-segments of NF-L and NF-M are phosphorylated by casein kinases, it has not been observed in NF-H. Using mass spectrometric analysis, we identified phosphorylation of the E-segment of NF-H, prepared from rat spinal cords, at Ser-493 and Ser-501 in the Ser-Pro sequences. The E-segment kinase was isolated from rat brain extract using column chromatography and identified as glycogen synthase kinase (GSK) 3beta. GSK3beta was shown to phosphorylate at Ser-493 in vitro by phosphopeptide mapping and site-directed mutagenesis, and in vivo in HEK293 cells using the phospho-Ser-493 antibody, but did not phosphorylate Ser-501. GSK3beta preferred Ser-493 to the KSP-repeated sequences for phosphorylation sites in the NF-H tail domain. Moreover, Ser-493 was a better phosphorylation site for GSK3beta than other proline-directed protein kinases, Cdk5/p35 and ERK. GSK3beta in the spinal cord extract was associated with NF cytoskeletons. Taken together, we concluded that Ser-493 in the E-segment of NF-H is phosphorylated by GSK3beta in rat spinal cords.

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.

Major phosphorylation site (Ser55) of neurofilament L by cyclic AMP-dependent protein kinase in rat primary neuronal culture

Ser55 of neurofilament L (NF-L) is reported to be partly phosphorylated in neurons and to be phosphorylated by cyclic AMP-dependent protein kinase (PKA). Bovine NF-L was phosphorylated by PKA in a low concentration of MgCl2 (0.3 mM) and digested by trypsin. Trypsin-digested fragments were assigned by MALDI/ TOF (matrix-assisted laser desorption and ionization/ time-of-flight) mass spectrometry. Phosphorylation sites were found at Ser41, Ser55, and Ser62 in the head region, with Ser55 considered the preferred site. A site-specific phosphorylation-dependent antibody against Ser55 rendered NF-L phosphorylated at Ser55 detectable in primary cultured rat neurons. One-hour treatment with 20 nM okadaic acid increased the phosphorylation level of Ser55, and co-treatment with 10 microM forskolin enhanced it. However, forskolin alone did not elevate the phosphorylation level. As a consequence, NF-L may be phosphorylated at Ser55 by PKA or by a PKA-like kinase in vivo; however, the phosphorylation level of Ser55 may be modulated by certain phosphatases sensitive to okadaic acid.

Casein kinase II is responsible for phosphorylation of NF-L at Ser-473

Ser-473 is solely phosphorylated in vivo in the tail region of neurofilament L (NF-L). With peptides including the native phosphorylation site, it was not possible to locate responsible kinases. We therefore adopted full-length dephosphorylated NF-L as the substrate, and employed MALDI/TOF (matrix-assisted laser desorption and ionization/time of flight) mass spectrometry and a site-specific phosphorylation-dependent antibody recognizing Ser-473 phosphorylation. The antibody showed that casein kinase I (CK I) as well as casein kinase II (CK II) phosphorylated Ser-473 in vitro, while neither GSK-3beta nor calcium/calmodulin-dependent protein kinase II did so. However, the mass spectra of the tail fragments of the phosphorylated NF-L indicated that CK II was the kinase mediating Ser-473 phosphorylation in vitro as opposed to CK I, because CK I phosphorylated another site as well as Ser-473 in vitro. The antibody also demonstrated that NF-L phosphorylated at Ser-473 was abundant in the neuronal perikarya of the rat cortex, indicating that phosphorylation of Ser-473 may take place there. This result may support the suggestion that CK II is the kinase responsible for Ser-473 phosphorylation. Despite many reports showing that CK I mediates phosphorylation of neurofilaments, CK II may phosphorylate NF-L in vivo.

Pulse exposure of cultured rat neurons to aluminum-maltol affected the axonal transport system

Although chronic aluminum neurotoxicity has been well established, the mechanism of the toxicity has not been elucidated yet. In order to simplify the study of the aluminum neurotoxicity, we employed the pulse exposure of cultured rat cortical neurons to 250 microM aluminum-maltol for 1 h at the early stage (6 h after plating), which resulted in abnormal distribution of neurofilament L (NFL) and fast axonal transported proteins, whereas the axonal transport of tubulin, actin, and clathrin were not impaired. Otherwise, the pulse exposure of neurons at the late stage (4 days after plating) to the same concentration of aluminum-maltol did not affect the cell morphology and the distribution of NFL. The pulse exposure of cultured neurons to aluminum-maltol at the early stage might affect the axonal transport system of NFL and fast axonal transported proteins.

Abnormal distribution of neurofilament L in neurons with Alzheimer's disease

Abnormality of cytoskeletal proteins is closely related to the pathology of Alzheimer's disease. As neurofilament proteins are major cytoskeletal components of neurons, abnormality of neurofilaments is proposed in brain with Alzheimer's disease. Free-floating sections of the hippocampus with Alzheimer's disease were studied immunohistochemically, using a polyclonal antibody specifically bound to the tail region of neurofilament L (NF-L). In brains with early onset type of Alzheimer's disease, many neurons and dystrophic neurites were labeled by the antibody, while these observations were not seen in either brains with late onset type or control brains. Double immunohistochemical staining of NF-L and tau protein demonstrated that abnormal deposition of NF-L was not always accompanied with that of tau protein, indicating that the abnormal deposition of NF-L might not occur in parallel with that of tau protein. These observations suggest the involvement of neurofilament proteins on the pathology of Alzheimer's disease in a different way than tau protein.

Filensin and phakinin form a novel type of beaded intermediate filaments and coassemble de novo in cultured cells

The fiber cells of the eye lens possess a unique cytoskeletal system known as the "beaded-chain filaments" (BFs). BFs consist of filensin and phakinin, two recently characterized intermediate filament (IF) proteins. To examine the organization and the assembly of these heteropolymeric IFs, we have performed a series of in vitro polymerization studies and transfection experiments. Filaments assembled from purified filensin and phakinin exhibit the characteristic 19-21-nm periodicity seen in many types of IFs upon low angle rotary shadowing. However, quantitative mass-per-length (MPL) measurements indicate that filensin/phakinin filaments comprise two distinct and dissociable components: a core filament and a peripheral filament moiety. Consistent with a nonuniform organization, visualization of unfixed and unstained specimens by scanning transmission electron microscopy (STEM) reveals the the existence of a central filament which is decorated by regularly spaced 12-15-nm-diam beads. Our data suggest that the filamentous core is composed of phakinin, which exhibits a tendency to self-assemble into filament bundles, whereas the beads contain filensin/phakinin hetero-oligomers. Filensin and phakinin copolymerize and form filamentous structures when expressed transiently in cultured cells. Experiments in IF-free SW13 cells reveal that coassembly of the lens-specific proteins in vivo does not require a preexisting IF system. In epithelial MCF-7 cells de novo forming filaments appear to grow from distinct foci and organize as thick, fibrous laminae which line the plasma membrane and the nuclear envelope. However, filament assembly in CHO and SV40-transformed lens-epithelial cells (both of which are fibroblast-like) yields radial networks which codistribute with the endogenous vimentin IFs. These observations document that the filaments formed by lens-specific IF proteins are structurally distinct from ordinary cytoplasmic IFs. Furthermore, the results suggest that the spatial arrangement of filensin/phakinin filaments in vivo is subject to regulation by host-specific factors. These factors may involve cytoskeletal networks (e.g., vimentin IFs) and/or specific sites associated with the cellular membranes.

Contributions of the structural domains of filensin in polymer formation and filament distribution

Filensin and phakinin constitute the subunits of a heteropolymeric, lens-specific intermediate filament (IF) system known as the beaded-chain filaments (BFs). Since the rod of filensin is four heptads shorter than the rods of all other IF proteins, we decided to examine the specific contribution of this protein in filament assembly. For these purposes, we constructed chimeric proteins in which regions of filensin were exchanged with the equivalent ones of vimentin, a self-polymerizing IF protein. Our in vitro studies show that the filensin rod domain does not allow homopolymeric filament elongation. However, the filensin rod is necessary for co-polymerization of filensin with phakinin and seems to counteract the inherent tendency of the latter protein to homopolymerize into large, laterally associated filament bundles. Apart from the rod domain, the presence of an authentic or substituted tail domain in filensin is also essential for co-assembly with the naturally tail-less phakinin and formation of extended filaments in vitro. Finally, transfection experiments in CHO and MCF-7 cells show that the rod domain of filensin plays an important role in de novo filament formation and distribution. The same type of analysis further suggests that the end-domains of filensin interact with cell-specific, assembly-modulating factors.

Accumulation of amyloid beta-protein precursor (APP) in Purkinje cells and increase of amino-terminal fragments of APP in cerebrum and cerebellum of aged rat brain

In aged rat brain, amyloid beta-protein precursor (APP) is accumulated in dendrites and cell bodies of Purkinje cells as full-length or truncated APP, because dendrites and cell bodies are positively stained by antibodies against both the amino- and carboxy-termini of APP. Western blot analysis of homogenates of brains of aged and young rats showed no apparent differences except for an increase in amino-terminal fragments in cerebrum and cerebellum of aged rat. These results indicate that the expression, transport or metabolism of APP in specific regions of brains may be affected by the aging process.

Making heads and tails of intermediate filament assembly, dynamics and networks

Thus far, intermediate filaments (IFs) have been the least understood of the three cytoskeletal filament systems with regard to their structure, assembly, network formation, and dynamics. This picture is now slowly but definitely changing, as recent in vivo and in vitro experiments, including generation of transgenic animals, have yielded important new data shedding light on the following areas: the molecular architecture of IFs; the role of the highly variable end domains during IF assembly and network formation; the factors that govern whether IF proteins are involved in de novo filament formation or are incorporated into a pre-existing IF network; and the effects of post-translational modifications, such as phosphorylation and glycosylation of IF polypeptides, on filament assembly, dynamics and turnover.

The 47-kD lens-specific protein phakinin is a tailless intermediate filament protein and an assembly partner of filensin

In previous studies we have characterized a lens-specific intermediate filament (IF) protein, termed filensin. Filensin does not self-assemble into regular IFs but is known to associate with another 47-kD lens-specific protein which has been suggested to represent its assembly partner. To address this possibility, we cloned and sequenced the cDNA coding for the bovine 47-kD protein which we have termed phakinin (from the greek phi alpha kappa omicron sigma = phakos = lens). The predicted sequence comprises 406 amino acids and shows significant similarity (31.3% identity over 358 residues) to type I cytokeratins. Phakinin possesses a 95-residue, non-helical domain (head) and a 311 amino acid long alpha-helical domain punctuated with heptad repeats (rod). Similar to cytokeratin 19, phakinin lacks a COOH-terminal tail domain and it therefore represents the second known example of a naturally tailless IF protein. Confocal microscopy on frozen lens sections reveals that phakinin colocalizes with filensin and is distributed along the periphery of the lens fiber cells. Quantitative immunoblotting with whole lens fiber cell preparations and fractions of washed lens membranes suggest that the natural stoichiometry of phakinin to filensin is approximately 3:1. Under in vitro conditions, phakinin self-assembles into metastable filamentous structures which tend to aggregate into thick bundles. However, mixing of phakinin and filensin at an optimal ratio of 3:1 yields stable 10-nm filaments which have a smooth surface and are ultrastructurally indistinguishable from "mainstream" IFs. Immunolabeling with specific antibodies shows that these filaments represent phakinin/filensin heteropolymers. Despite its homology to the cytokeratins, phakinin does not coassemble with acidic (type I), or basic (type II) cytokeratins. From these data we conclude that filensin and phakinin are obligate heteropolymers which constitute a new membrane-associated, lens-specific filament system related to, but distinct from the known classes of IFs.

In vitro assembly properties of vimentin mutagenized at the beta-site tail motif

The intermediate filament (IF) proteins vimentin, desmin and peripherin share a 9-residue sequence motif (beta-site) located near the end of their COOH-terminal tail domain. Peptide inhibition experiments have previously suggested that the beta-site is involved in interactions that limit the lateral growth of IFs and prevent inappropriate filament-filament associations. To investigate this question further, we have constructed and expressed, in Escherichia coli, hamster vimentin bearing different mutations in the beta-site. We show here that a single exchange of glycine 450 with a valine residue, or an internal deletion of amino acids 444–452, strongly interferes with the normal assembly of IFs under in vitro conditions. These mutants polymerize into irregular fibrils that have a strong tendency to anastomose and laterally aggregate under isotonic conditions. In contrast, a non-conservative substitution of arginine 448 for glutamic acid does not significantly interfere with filament structure and yields subunits that polymerize into long, smooth filaments that show a slight aberration in thickness. All mutant proteins are soluble in low salt and form oligomers similar to the ones formed by wild-type vimentin. On the basis of these findings and on related observations, we propose that the tail domain of type III IF proteins contains important structural elements involved in lateral protofilament-protofilament interactions.

Assembly of intermediate filaments

The assembly of intermediate filaments is a fundamental property of the central rod domain of the individual subunit proteins. This rod domain, with its high propensity for alpha-helix formation, is the common and identifying feature of this family of proteins. Assembly occurs in vitro in the absence of other proteins or exogenous sources of energy; in vivo, it appears as if other factors, as yet poorly understood, modulate the assembly of intermediate filaments. Parallel, in-register dimers form via coiled-coil interactions of the rod domain. Tetramers may form from staggered arrays of parallel or antiparallel arrangements of dimers. Higher-order polymerization, which occurs spontaneously if the ionic strength of a mixture of dimers and tetramers is raised, proceeds rapidly through poorly described intermediates to the final 10 nm filament. This process is dependent on and modulated by the non-alpha-helical end domains, as well as those amino acids present at the very beginning and end of the rod domain. The interactions governing tetramer formation are most probably the same ones that are responsible for the lateral and longitudinal associations within intermediate filaments.