mRNA association with the cytoskeletal framework likely represents a physiological binding event

Ataxin-2 associates with rough endoplasmic reticulum

Ataxin-2 is a novel protein, normally with a domain of 22 consecutive glutamine (Q) residues, which may expand beyond a threshold of (Q)(32), causing a neurodegenerative disease named Spinocerebellar ataxia type 2 (SCA2). To obtain clues about the functions of ataxin-2, we used fluorescence microscopy and centrifugation fractionation analyses. Immunocytochemical detection in non-neuronal and neuronal cells showed endogenous and transfected ataxin-2 distributed throughout the cytoplasm, with perinuclear preference and a granular appearance. Triple-labelling and confocal microscopy demonstrated co-localisation with the endoplasmic reticulum (ER) markers calreticulin, calnexin and CFP-ER. The pathogenic form of ataxin-2 with an expanded polyQ domain showed the same distribution pattern. Subcellular fractionation of mouse brain homogenates showed endogenous ataxin-2 associated with rough ER (rER) membranes, in a manner dependent on RNA, salt and phosphorylation. Our data are in agreement with recent findings that ataxin-2 directly interacts with poly(A)-binding protein (PABP), thus associating with polyribosomes under normal conditions and being recruited to stress granules under environmental stress. These data, in conjunction with the presence of Lsm domains within ataxin-2, suggest that ataxin-2 is involved in the processing of mRNA and/or the regulation of translation.

Translation termination factors function outside of translation: yeast eRF1 interacts with myosin light chain, Mlc1p, to effect cytokinesis

The translation termination factor eRF1 recognizes stop codons at the A site of the ribosome and induces peptidyl-tRNA hydrolysis at the peptidyl transferase centre. Recent data show that, besides translation, yeast eRF1 is also involved in cell cycle regulation. To clarify the mechanisms of non-translational functions of eRF1, we performed a genetic screen for its novel partner proteins. This screen revealed the gene for myosin light chain, Mlc1p, acting as a dosage suppressor of a temperature-sensitive mutation in the SUP45 gene encoding eRF1. eRF1 and Mlc1p are able to interact with each other and, similarly to depletion of Mlc1p, mutations in the SUP45 gene may affect cytokinesis. Immunofluorescent staining performed to determine localization of Mlc1p has shown that the sup45 mutation, which arrests cytokinesis, redistributed Mlc1p, causing its disappearance from the bud tip and the bud neck. The data obtained demonstrate that yeast eRF1 has an important non-translational function effecting cytokinesis via interaction with Mlc1p.

Subcellular fractionation and association with the cytoskeleton of messengers encoding myelin proteins

The targeting of polypeptides to restricted cytoplasmic domains by means of mRNA sorting is a widespread phenomena utilized by many cell types. In the central nervous system, in situ hybridization analysis has shown previously that the mRNAs encoding several myelin-specific proteins are specifically located within the myelinating processes of oligodendrocytes. Here, by means of biochemical and subcellular fractionation methods, we show that a myelin fraction is selectively enriched in those mRNAs. The four major myelin basic protein (MBP) mRNAs that arise by alternative splicing of exons II and VI of the MBP gene are concentrated in this subcellular fraction. Furthermore, an interaction of MBP and MOBP 81A mRNAs with the cytoskeleton was observed. This interaction might serve to mediate the anchoring of these messengers after translocation to the subcellular site of translation.

Synthesis of proto-oncogene proteins and cyclins depends on intact microfilaments

It is well established that microfilament disintegration by cytochalasin D (CD) as well as latrunculin (LAT)-A and LAT-B causes an inhibition of S phase entry of various nontransformed cell lines. Our experiments extended these observations to human embryonal diploid fibroblasts (Wi-38). To investigate the question whether this stop of DNA synthesis is due to a decline of the synthesis of proteins that are necessary for G1 progression and S phase entry, we examined the expression of two proto-oncogenes (c-fos, c-jun) and three cyclins (D1, E, A) after altering the microfilament system. Disintegration of microfilaments by CD, LAT-A, or LAT-B of asynchronously growing fibroblasts caused a strong dose-dependent and time-dependent inhibition of total protein synthesis. Expression of c-jun, cyclins D1, E, and A decreased by about the same percentage as total protein synthesis. The strong induction of total protein synthesis after reactivating serum-starved fibroblasts by adding fetal calf serum was suppressed, when CD or LAT-A were added to the culture medium during this reactivation process. While expression of cyclin E as well as cyclin A decreased by about the same percentage as total protein synthesis, cyclin D1 was more suppressed after microfilament disintegration. After reactivating growth-arrested Wi-38 fibroblasts, cultured in suspension for 12 h, by transferring them to a rigid substratum they could adhere to, total protein synthesis was strongly induced. Again alteration of microfilaments by CD suppressed that increase. The expression of cyclin D1 was slightly more suppressed than total protein synthesis after addition of CD during that reactivation process. Our results suggest that alteration of microfilaments causes a strong decline of total protein synthesis accompanied by a decrease of the expression of proteins that are required for G1 progression and S phase entry. The diminished presence of proteins that are important for cell cycle progression could explain the inhibition of DNA synthesis after microfilament disintegration by various drugs.

Efficient mammalian protein synthesis requires an intact F-actin system

The mammalian protein synthesizing system is highly organized in vivo, and its substrate, tRNA, is channeled throughout the translation process. However, the cellular components responsible for this organization are not known. To examine this question a series of studies was carried out using intact and permeabilized Chinese hamster ovary cells. We show that cold shock dramatically reduces the protein synthetic capacity of these cells by as much as 95%. The loss of activity can be reversed by a short recovery period under conditions that allow energy metabolism to occur; transcription and translation during the recovery period are not needed. While individual components of the translation apparatus are not inactivated by the cold shock, the supramolecular organization of the system appears to be altered and F-actin levels are found to decrease. Resumption of protein synthesis during the recovery period coincides closely with the restoration of F-actin to normal levels. Moreover, disruption of actin filaments, but not microtubules, also leads to a major reduction in translation. These data support the conclusion that the cellular microfilament network plays an important role in the structure and function of the translation system and that perturbations of this network can have profound effects on protein synthesis.

Modification of cytoskeleton and prosome networks in relation to protein synthesis in influenza A virus-infected LLC-MK2 cells

Modifications of the cytoskeleton and protein synthesis were investigated in LLC-MK2 cells during infection by FPV/Ulster 73, an avian strain of influenza A virus. During infection, the cytoskeleton and the prosome networks undergo a dramatic reorganization, which seems to be at least temporally differentiated for each cytoskeletal system, i.e. microfilaments (MFs), microtubules (MTs), intermediate filaments (IFs). In order to evaluate the role of the three different cytoskeletal networks during FPV/Ulster infection, studies were carried out on cellular and virus-specific protein synthesis and viral production, using drugs which selectively affect individual cytoskeletal systems. Our data show that the perturbation of the IF system, but not that of the MFs or MTs, seems to have a strong inhibitory effect on virus production and cellular and viral protein synthesis. Furthermore, the dynamics of IFs and prosomes were investigated during viral infection and, at no time, dissociation of the prosome and IF networks was observed. Taken together, these results strongly support the idea that the interactions between the protein synthesis machinery, the cytoskeleton, and the prosomes are all affected by viral infection in a partially coordinated manner.

Growth of brain microvessel endothelial cells on collagen gels: applications to the study of blood-brain barrier physiology and CNS inflammation

Brain microvessel endothelial cells (BMEC) exhibit the tendency to migrate through 3.0-vm pore semipermeable inserts and establish monolayers on both apical and basal filter surfaces. This can potentially lead to complications in accurately assessing a wide variety of physiologic parameters uniquely associated with these cells. To avoid this problem, we have explored growing BMEC on Transwell filters coated with hydrated collagen gels. BMEC seeded on such gels grow as a monolayer until confluency, but do not invade the subendothelial collagen matrix or the underlying support filter. Furthermore, BMEC grown in this manner exhibit biochemical, morphologic, and electrophysiologic properties reflective of the endothelial cells that comprise the blood-brain barrier in vivo. Although the collagen gel acts as an impenetrable barrier to BMEC, and thus ensures the growth of only a single layer of cells, it nevertheless can be infiltrated by monocytes that have been stimulated by a chemotaxin to undergo diapedesis. Thus, growing BMEC on collagen gel-coated Transwells has broad applications for the in vitro study of both blood-brain barrier physiology as well as the mechanisms underlying central nervous system inflammation.

Actin depolymerization is not involved in phototransduction

The rhabdomeral microvilli in the eyes of invertebrates have a cytoskeletal core (Saibil, 1982; Blest et al., 1982a, b, 1983) of actin (de Couet et al., 1984; Arikawa et al., 1990; Williams, 1991; Caiman & Chamberlain, 1992) which might be involved in phototransduction. Tsukita et al. (1988) suggested that light stimulation triggers the breakdown of the microvillar actin filament complex and that this may play a role in phototransduction. To test their suggestion, we pressure-injected phalloidin into Limulus ventral photoreceptors to prevent actin depolymerization (Cooper, J.A., 1987) and tested to see if this blocked phototransduction. We have previously shown that injected rho-damine-phalloidin brightly labels the microvillar actin filaments in living Limulus ventral photoreceptors for several hours (Feng et al., 1994). In the experiments reported here, phalloidin unconjugated with fluorophore was used, and the final concentration in the cell after many small injections from a pipette containing 10 mg/ml phalloidin was estimated to be 1 mg/ml (1.2 mM). This concentration is 100 times higher than that used in an extracted cellular preparation to stabilize actin (Biegel & Pachter, 1992). Fig. 1 shows that the responses to steps of light at different intensities are normal 1 h after phalloidin injection. Similar results were obtained in two other cells.

Translation and the cytoskeleton: a mechanism for targeted protein synthesis

This review describes the critical evidence that in eukaryotic cells polyribosomes, mRNAs and components of the protein synthetic machinery are associated with the cytoskeleton. The role of microtubules, intermediate filaments and microfilaments are discussed; at present most evidence suggests that polyribosomes interact with the actin filaments. The use of non-ionic detergent/deoxycholate treatment in the isolation of cytoskeletal-bound polysomes is described and the conclusion reached that at low salt concentrations this leads to mixed preparations of polysomes derived from both the cytoskeleton and the endoplasmic reticulum. At present the best approach for isolation of cytoskeletal-bound polysomes appears to involve extraction with salt concentrations greater than 130 mM after an initial non-ionic detergent treatment. Such polysomes appear to be enriched in certain mRNAs and thus it is suggested that they are involved in translation of a unique set of proteins. The evidence for mRNA localisation is presented and the role of the cytoskeleton in transport and localisation of RNA discussed. Recent data on the role of the 3' untranslated region in the targeting of mRNAs both to particular regions of the cell and for translation on cytoskeletal-bound polysomes is described. The hypothesis is developed that the association of polysomes with the cytoskeleton is the basis of a mechanism for the targeting of mRNAs and the compartmentalization of protein synthesis.

Mechanisms of myelin basic protein and proteolipid protein targeting in oligodendrocytes (review)

The segregation of proteins to specific cellular membranes is recognized as a common phenomenon. In oligodendrocytes of the central nervous system, localization of certain proteins to select regions of the plasma membrane gives rise to the myelin membrane. Whilst the fundamental structure and composition of myelin is well understood, less is known of the mechanisms by which the constituent proteins are specifically recruited to those regions of plasma membrane that are forming myelin. The two principal proteins of myelin, the myelin basic protein and proteolipid protein, differ greatly in character and sites of synthesis. The message for myelin basic protein is selectively translocated to the ends of the cell processes, where it is translated on free ribosomes and is incorporated directly into the membrane. Proteolipid protein synthesized at the rough endoplasmic reticulum, processed through the Golgi apparatus, and presumably transported via vesicles to the myelin membrane. This review examines the mechanisms by which these two proteins are targeted to the myelin membrane.

Spatial organization of the synthesis of cytoskeletal proteins

The cytoskeleton of most cells is complex and spatially diverse. The mRNAs for some cytoskeletal proteins are localized, suggesting that synthesis of these proteins may occur at sites appropriate for function or assembly. mRNA concentrations were first observed for several oocyte and embryonic mRNAs. Some insight has been gained into the mechanisms that help to position these mRNAs. More surprising to some, many cytoskeletal mRNAs are also localized. Among them are mRNAs for actin, tubulin, intermediate filaments, and a variety of associated proteins. Different mRNAs in the same cell can be located in different places; the same mRNA can be located in different places; the same mRNA can be located differently at different times of development. For example, we observed vimentin mRNA in developing chicken muscle cultures by fluorescent in situ hybridization. We found that vimentin mRNA takes on a variety of positions during myogenesis, ending up located with its cognate protein at costameres. This last pattern is significant because it is too finely structured to have a function in the soluble phase and probably reflects cotranslational assembly of this particular protein. Analogies can be made between oocyte or embryonic positions (animal/vegetal poles, oocyte cortex, and interior) and somatic cell positions (anterior/posterior and cell cortex/cell center). These analogies may point to conserved mechanisms for moving and retaining mRNA. Localization of cytoskeletal synthesis, through the mRNA or by other means, may prove as important for assembling and maintaining differentiated cytoskeletal structures and somatic cells as mRNA location is for organizing the embryo. Mechanisms that permit mRNA localization are likely to be conserved.

Beta-actin mRNA-binding proteins associated with the cytoskeletal framework

Association of mRNA with the cytoskeletal framework (CSK) is thought to play a strategic role in the placement of mRNA in the cytoplasm. However, the molecular determinants underlying mRNA/CSK association are completely unknown. To begin addressing this issue, we have employed a binding assay to identify proteins of the CSK compartment of NIH 3T3 cells that bind in-vitro-transcribed 32P-labelled beta-actin mRNA with high affinity. Three proteins, of approximate molecular masses 27, 50 and 97 kDa, were observed to exhibit strong binding. Binding to these proteins took place at physiological salt concentration and withstood washing in 0.5 M salt. Furthermore, binding was unaffected by heparin but was inhibited by unlabelled beta-actin mRNA. Treatment of isolated CSKs with the microfilament-severing agent DNase I abolished all beta-actin mRNA-binding activities, thus suggesting a possible association of beta-actin mRNA with the microfilament network in situ. Removal of the 3' untranslated region (UTR) significantly reduced beta-actin mRNA binding to all three CSK proteins but removal of the 5' UTR mainly affected binding to the 97-kDa species and that to a lesser extent. beta-Tubulin mRNA bound to the same three CSK proteins as did beta-actin mRNA, but with considerably less avidity. In contrast, vimentin mRNA strongly recognized these CSK proteins, and further bound to a group of smaller proteins (< 29 kDa). As beta-actin mRNA, beta-tubulin mRNA and vimentin mRNA have been observed to occupy separate cytoplasmic locales, the proteins detected here may be operative both in binding mRNAs to the CSK in situ, as well as in localizing mRNA in the cytoplasm.