Purification and assay of a 145-kDa protein (STOP145) with microtubule-stabilizing and motility behavior

Microtubule-Associated Proteins: Structuring the Cytoskeleton

Microtubule-associated proteins (MAPs) were initially discovered as proteins that bind to and stabilize microtubules. Today, an ever-growing number of MAPs reveals a more complex picture of these proteins as organizers of the microtubule cytoskeleton that have a large variety of functions. MAPs enable microtubules to participate in a plethora of cellular processes such as the assembly of mitotic and meiotic spindles, neuronal development, and the formation of the ciliary axoneme. Although some subgroups of MAPs have been exhaustively characterized, a strikingly large number of MAPs remain barely characterized other than their interactions with microtubules. We provide a comprehensive view on the currently known MAPs in mammals. We discuss their molecular mechanisms and functions, as well as their physiological role and links to pathologies.

MAP6 interacts with Tctex1 and Cav 2.2/N-type calcium channels to regulate calcium signalling in neurons

MAP6 proteins were first described as microtubule‐stabilizing agents, whose properties were thought to be essential for neuronal development and maintenance of complex neuronal networks. However, deletion of all MAP6 isoforms in MAP6 KO mice does not lead to dramatic morphological aberrations of the brain but rather to alterations in multiple neurotransmissions and severe behavioural impairments. A search for protein partners of MAP6 proteins identified Tctex1 – a dynein light chain with multiple non‐microtubule‐related functions. The involvement of Tctex1 in calcium signalling led to investigate it in MAP6 KO neurons. In this study, we show that functional Ca_v2.2/N‐type calcium channels are deficient in MAP6 KO neurons, due to improper location. We also show that MAP6 proteins interact directly with both Tctex1 and the C‐terminus of Ca_v2.2/N‐type calcium channels. A balance of these two interactions seems to be crucial for MAP6 to modulate calcium signalling in neurons.

Polarity of varicosity initiation in central neuron mechanosensation

Little is known about mechanical regulation of morphological and functional polarity of central neurons. In this study, we report that mechanical stress specifically induces varicosities in the axons but not the dendrites of central neurons by activating TRPV4, a Ca2+/Na+-permeable mechanosensitive channel. This process is unexpectedly rapid and reversible, consistent with the formation of axonal varicosities in vivo induced by mechanical impact in a mouse model of mild traumatic brain injury. In contrast, prolonged stimulation of glutamate receptors induces varicosities in dendrites but not in axons. We further show that axonal varicosities are induced by persistent Ca2+ increase, disassembled microtubules (MTs), and subsequently reversible disruption of axonal transport, and are regulated by stable tubulin-only polypeptide, an MT-associated protein. Finally, axonal varicosity initiation can trigger action potentials to antidromically propagate to the soma in retrograde signaling. Therefore, our study demonstrates a new feature of neuronal polarity: axons and dendrites preferentially respond to physical and chemical stresses, respectively.

Deficits in axon-associated proteins in prefrontal white matter in bipolar disorder but not schizophrenia

OBJECTIVES

Brain imaging studies have implicated white matter dysfunction in the pathophysiology of both bipolar disorder (BD) and schizophrenia (SCZ). However, the contribution of axons to white matter pathology in these disorders is not yet understood. Maintenance of neuronal function is dependent on the active transport of biological material, including synaptic proteins, along the axon. In this study, the expression of six proteins associated with axonal transport of synaptic cargoes was quantified in postmortem samples of prefrontal white matter in subjects with BD, those with SCZ, and matched controls, as a measure of axonal dysfunction in these disorders.

METHODS

Levels of the microtubule-associated proteins β-tubulin and microtubule-associated protein 6 (MAP6), the motor and accessory proteins kinesin-1 and disrupted-in-schizophrenia 1 (DISC1), and the synaptic cargoes synaptotagmin and synaptosomal-associated protein-25 (SNAP-25) were quantified in white matter adjacent to the dorsolateral prefrontal cortex in subjects with BD (n = 34), subjects with SCZ (n = 35), and non-psychiatric controls (n = 35) using immunoblotting and an enzyme-linked immunosorbent assay (ELISA).

RESULTS

Protein expression of β-tubulin, kinesin-1, DISC1, synaptotagmin, and SNAP-25 was significantly lower in subjects with BD compared to controls. Levels of axon-associated proteins were also lower in subjects with SCZ, but failed to reach statistical significance.

CONCLUSIONS

These data provide evidence for deficits in axon-associated proteins in prefrontal white matter in BD. Findings are suggestive of decreased axonal density or dysregulation of axonal function in this disorder.

Microtubule-associated protein 6 mediates neuronal connectivity through Semaphorin 3E-dependent signalling for axonal growth

Structural microtubule associated proteins (MAPs) stabilize microtubules, a property that was thought to be essential for development, maintenance and function of neuronal circuits. However, deletion of the structural MAPs in mice does not lead to major neurodevelopment defects. Here we demonstrate a role for MAP6 in brain wiring that is independent of microtubule binding. We find that MAP6 deletion disrupts brain connectivity and is associated with a lack of post-commissural fornix fibres. MAP6 contributes to fornix development by regulating axonal elongation induced by Semaphorin 3E. We show that MAP6 acts downstream of receptor activation through a mechanism that requires a proline-rich domain distinct from its microtubule-stabilizing domains. We also show that MAP6 directly binds to SH3 domain proteins known to be involved in neurite extension and semaphorin function. We conclude that MAP6 is critical to interface guidance molecules with intracellular signalling effectors during the development of cerebral axon tracts.

Loss of the structural microtubule-associated protein 6 (MAP6) leads to neuronal differentiation defects that are independent of MAP6's microtubule-binding properties. Here the authors establish a functional link between MAP6 and Semaphorin 3E signalling for proper formation of the fornix of the brain.

Human FAM154A (SAXO1) is a microtubule-stabilizing protein specific to cilia and related structures

Cilia and flagella are microtubule-based organelles present at the surface of most cells, ranging from protozoa to vertebrates, in which these structures are implicated in processes from morphogenesis to cell motility. In vertebrate neurons, microtubule-associated MAP6 proteins stabilize cold-resistant microtubules through their Mn and Mc modules, and play a role in synaptic plasticity. Although centrioles, cilia and flagella have cold-stable microtubules, MAP6 proteins have not been identified in these organelles, suggesting that additional proteins support this role in these structures. Here, we characterize human FAM154A (hereafter referred to as hSAXO1) as the first human member of a widely conserved family of MAP6-related proteins specific to centrioles and cilium microtubules. Our data demonstrate that hSAXO1 binds specifically to centriole and cilium microtubules. We identify, in vivo and in vitro, hSAXO1 Mn modules as responsible for microtubule binding and stabilization as well as being necessary for ciliary localization. Finally, overexpression and knockdown studies show that hSAXO1 modulates axoneme length. Taken together, our findings suggest a fine regulation of hSAXO1 localization and important roles in cilium biogenesis and function.

Structural basis for the association of MAP6 protein with microtubules and its regulation by calmodulin

Microtubules are highly dynamic αβ-tubulin polymers. In vitro and in living cells, microtubules are most often cold- and nocodazole-sensitive. When present, the MAP6/STOP family of proteins protects microtubules from cold- and nocodazole-induced depolymerization but the molecular and structure determinants by which these proteins stabilize microtubules remain under debate. We show here that a short protein fragment from MAP6-N, which encompasses its Mn1 and Mn2 modules (MAP6(90-177)), recapitulates the function of the full-length MAP6-N protein toward microtubules, i.e. its ability to stabilize microtubules in vitro and in cultured cells in ice-cold conditions or in the presence of nocodazole. We further show for the first time, using biochemical assays and NMR spectroscopy, that these effects result from the binding of MAP6(90-177) to microtubules with a 1:1 MAP6(90-177):tubulin heterodimer stoichiometry. NMR data demonstrate that the binding of MAP6(90-177) to microtubules involve its two Mn modules but that a single one is also able to interact with microtubules in a closely similar manner. This suggests that the Mn modules represent each a full microtubule binding domain and that MAP6 proteins may stabilize microtubules by bridging tubulin heterodimers from adjacent protofilaments or within a protofilament. Finally, we demonstrate that Ca(2+)-calmodulin competes with microtubules for MAP6(90-177) binding and that the binding mode of MAP6(90-177) to microtubules and Ca(2+)-calmodulin involves a common stretch of amino acid residues on the MAP6(90-177) side. This result accounts for the regulation of microtubule stability in cold condition by Ca(2+)-calmodulin.

Automated screening of microtubule growth dynamics identifies MARK2 as a regulator of leading edge microtubules downstream of Rac1 in migrating cells

Polarized microtubule (MT) growth in the leading edge is critical to directed cell migration, and is mediated by Rac1 GTPase. To find downstream targets of Rac1 that affect MT assembly dynamics, we performed an RNAi screen of 23 MT binding and regulatory factors and identified RNAi treatments that suppressed changes in MT dynamics induced by constitutively activated Rac1. By analyzing fluorescent EB3 dynamics with automated tracking, we found that RNAi treatments targeting p150^glued, APC2, spastin, EB1, Op18, or MARK2 blocked Rac1-mediated MT growth in lamellipodia. MARK2 was the only protein whose RNAi targeting additionally suppressed Rac1 effects on MT orientation in lamellipodia, and thus became the focus of further study. We show that GFP-MARK2 rescued effects of MARK2 depletion on MT growth lifetime and orientation, and GFP-MARK2 localized in lamellipodia in a Rac1-activity-dependent manner. In a wound-edge motility assay, MARK2-depleted cells failed to polarize their centrosomes or exhibit oriented MT growth in the leading edge, and displayed defects in directional cell migration. Thus, automated image analysis of MT assembly dynamics identified MARK2 as a target regulated downstream of Rac1 that promotes oriented MT growth in the leading edge to mediate directed cell migration.

Vinflunine: a new vision that may translate into antiangiogenic and antimetastatic activity

Microtubules and tubulin are major dynamic and structural cellular components that play a key role in several cell functions, including division, signalling and intracellular trafficking. Normal epithelial cells have a highly structured, rigid cytoskeletal network that is compatible with cell motility. Thus, tubulin and microtubules are compelling cellular targets for chemotherapy. In fact, among anticancer agents, those that target microtubules constitute one of the most effective classes of chemotherapeutics in cancer. The list of compounds that target either tubulin or microtubules is extensive and consists of chemically unique compounds that bind to the tubulin dimers and destabilize microtubules (Vinca alkaloids) and those that bind to the microtubule polymer and stabilize microtubules (taxanes). Tumour-induced angiogenesis, the formation of new capillaries from existing blood vessels, and epithelial-mesenchymal transition are two steps that are critical for both tumour growth and metastatic spread. Three possible mechanisms of action are described with vinflunine, the new-generation Vinca alkaloid to arrive in clinical practice are as follows: it acts against tubulin and microtubules, disrupts newly formed blood vessels and seems to be able to reduce the metastatic process as shown in preclinical studies. These findings support the hypothesis that vinflunine, by blocking microtubule functions that contribute to cell shape, polarization, migration and other processes, might be responsible not only for tumour-cytostatic but also for specific antiangiogenic or antiepithelial-mesenchymal transition effects.

Bmcc1s, a novel brain-isoform of Bmcc1, affects cell morphology by regulating MAP6/STOP functions

The BCH (BNIP2 and Cdc42GAP Homology) domain-containing protein Bmcc1/Prune2 is highly enriched in the brain and is involved in the regulation of cytoskeleton dynamics and cell survival. However, the molecular mechanisms accounting for these functions are poorly defined. Here, we have identified Bmcc1s, a novel isoform of Bmcc1 predominantly expressed in the mouse brain. In primary cultures of astrocytes and neurons, Bmcc1s localized on intermediate filaments and microtubules and interacted directly with MAP6/STOP, a microtubule-binding protein responsible for microtubule cold stability. Bmcc1s overexpression inhibited MAP6-induced microtubule cold stability by displacing MAP6 away from microtubules. It also resulted in the formation of membrane protrusions for which MAP6 was a necessary cofactor of Bmcc1s. This study identifies Bmcc1s as a new MAP6 interacting protein able to modulate MAP6-induced microtubule cold stability. Moreover, it illustrates a novel mechanism by which Bmcc1 regulates cell morphology.

A MAP6-related protein is present in protozoa and is involved in flagellum motility

In vertebrates the microtubule-associated proteins MAP6 and MAP6d1 stabilize cold-resistant microtubules. Cilia and flagella have cold-stable microtubules but MAP6 proteins have not been identified in these organelles. Here, we describe TbSAXO as the first MAP6-related protein to be identified in a protozoan, Trypanosoma brucei. Using a heterologous expression system, we show that TbSAXO is a microtubule stabilizing protein. Furthermore we identify the domains of the protein responsible for microtubule binding and stabilizing and show that they share homologies with the microtubule-stabilizing Mn domains of the MAP6 proteins. We demonstrate, in the flagellated parasite, that TbSAXO is an axonemal protein that plays a role in flagellum motility. Lastly we provide evidence that TbSAXO belongs to a group of MAP6-related proteins (SAXO proteins) present only in ciliated or flagellated organisms ranging from protozoa to mammals. We discuss the potential roles of the SAXO proteins in cilia and flagella function.

Cold exposure reveals two populations of microtubules in pulmonary endothelia

Microtubules are composed of α-tubulin and β-tubulin dimers. Microtubules yield tubulin dimers when exposed to cold, which reassemble spontaneously to form microtubule fibers at 37°C. However, mammalian neurons, glial cells, and fibroblasts have cold-stable microtubules. While studying the microtubule toxicity mechanisms of the exotoxin Y from Pseudomonas aeruginosa in pulmonary microvascular endothelial cells, we observed that some endothelial microtubules were very difficult to disassemble in the cold. As a consequence, we designed studies to test the hypothesis that microvascular endothelium has a population of cold-stable microtubules. Pulmonary microvascular endothelial cells and HeLa cells (control) were grown under regular cell culture conditions, followed by exposure to an ice-cold water bath and a microtubule extraction protocol. Polymerized microtubules were detected by immunofluorescence confocal microscopy and Western blot analyses. After cold exposure, immunofluorescence revealed that the majority of HeLa cell microtubules disassembled, whereas a smaller population of endothelial cell microtubules disassembled. Immunoblot analyses showed that microvascular endothelial cells express the microtubule cold-stabilizing protein N-STOP (neuronal stable tubule-only polypeptides), and that N-STOP binds to endothelial microtubules after cold exposure, but not if microtubules are disassembled with nocodazole before cold exposure. Hence, pulmonary endothelia have a population of cold-stable microtubules.

Submembraneous microtubule cytoskeleton: regulation of microtubule assembly by heterotrimeric Gproteins

Heterotrimeric Gproteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Gproteins also interact with microtubules and participate in microtubule-dependent centrosome/chromosome movement during cell division, as well as neuronal differentiation. In recent years, significant progress has been made in our understanding of the biochemical/functional interactions between Gprotein subunits (alpha and betagamma) and microtubules, and the molecular details emerging from these studies suggest that alpha and betagamma subunits of Gproteins interact with tubulin/microtubules to regulate the assembly/dynamics of microtubules, providing a novel mechanism for hormone- or neurotransmitter-induced rapid remodeling of cytoskeleton, regulation of the mitotic spindle for centrosome/chromosome movements in cell division, and neuronal differentiation in which structural plasticity mediated by microtubules is important for appropriate synaptic connections and signal transmission.

G protein βγ subunits interact with αβ- and γ-tubulin and play a role in microtubule assembly in PC12 cells

The betagamma subunit of G proteins (Gbetagamma) is known to transfer signals from cell surface receptors to intracellular effector molecules. Recent results suggest that Gbetagamma also interacts with microtubules and is involved in the regulation of the mitotic spindle. In the current study, the anti-microtubular drug nocodazole was employed to investigate the mechanism by which Gbetagamma interacts with tubulin and its possible implications in microtubule assembly in cultured PC12 cells. Nocodazole-induced depolymerization of microtubules drastically inhibited the interaction between Gbetagamma and tubulin. Gbetagamma was preferentially bound to microtubules and treatment with nocodazole suggested that the dissociation of Gbetagamma from microtubules is an early step in the depolymerization process. When microtubules were allowed to recover after removal of nocodazole, the tubulin-Gbetagamma interaction was restored. Unlike Gbetagamma, however, the interaction between tubulin and the alpha subunit of the Gs protein (Gsalpha) was not inhibited by nocodazole, indicating that the inhibition of tubulin-Gbetagamma interactions during microtubule depolymerization is selective. We found that Gbetagamma also interacts with gamma-tubulin, colocalizes with gamma-tubulin in centrosomes, and co-sediments in centrosomal fractions. The interaction between Gbetagamma and gamma-tubulin was unaffected by nocodazole, suggesting that the Gbetagamma-gamma-tubulin interaction is not dependent on assembled microtubules. Taken together, our results suggest that Gbetagamma may play an important and definitive role in microtubule assembly and/or stability. We propose that betagamma-microtubule interaction is an important step for G protein-mediated cell activation. These results may also provide new insights into the mechanism of action of anti-microtubule drugs.

A cAMP-activated pathway, including PKA and PI3K, regulates neuronal differentiation

Neuronal differentiation is a complex process in which many different signalling pathways may be involved. An increase in the intracellular levels of cyclic AMP (cAMP) has been shown to induce neuronal differentiation and also to cooperate with NGF to induce PC12 neurite outgrowth in a Ras-dependent manner. However, the neuritogenic activities associated with cAMP are still not well understood. The purpose of this study was to investigate the potential neuritogenic activities mediated by cAMP. For this purpose, we used the human neuroblastoma cell line SH-SY5Y. These neuroblastoma cells respond to cAMP by forming neurite-like extensions. We tried to identify some essential pathways involved in the cAMP-induced neurite elongation of these cells. Our results indicated that PKA is transiently activated in this elongation model. When we blocked PKA activity, elongation did not take place. Similarly, PI3K also plays an essential role because when we blocked this kinase activity, there was no neurite elongation. Indeed, over-expression of the p110-catalytic subunit or an activating form of the p85-regulatory subunit (p65) is able to induce some degree of neurite extension. Moreover, our results showed that when elongation is initiated, PI3K is still essential for maintenance of the neuronal morphology, whereas PKA or MAPK (ERKs or p38) activation does not appear to be necessary during this process.

Stable Tubule Only Polypeptides (STOP) Proteins Co-Aggregate with Spheroid Neurofilaments in Amyotrophic Lateral Sclerosis

A major cytopathological hallmark of amyotrophic lateral sclerosis (ALS) is the presence of axonal spheroids containing abnormally accumulated neurofilaments. The mechanism of their formation, their contribution to the disease, and the possibility of other co-aggregated components are still enigmatic. Here we analyze the composition of such lesions with special reference to stable tubule only polypeptide (STOP), a protein responsible for microtubule cold stabilization. In normal human brain and spinal cord, the distribution of STOP proteins is uniform between the cytoplasm and neurites of neurons. However, all the neurofilament-rich spheroids present in the tissues of affected patients are intensely labeled with 3 different anti-STOP antibodies. Moreover, when neurofilaments and microtubules are isolated from spinal cord and brain, STOP proteins are systematically co-purified with neurofilaments. By SDS-PAGE analysis, no alteration of the migration profile of STOP proteins is observed in pathological samples. Other microtubular proteins, like tubulin or kinesin, are inconstantly present in spheroids, suggesting that a microtubule destabilizing process may be involved in the pathogenesis of ALS. These results indicate that the selective co-aggregation of neurofilament and STOP proteins represent a new cytopathological marker for spheroids.

Identification of novel bifunctional calmodulin-binding and microtubule-stabilizing motifs in STOP proteins

Although microtubules are intrinsically labile tubulin assemblies, many cell types contain stable polymers, resisting depolymerizing conditions such as exposure to the cold or the drug nocodazole. This microtubule stabilization is largely due to polymer association with STOP proteins. There are several STOP variants, some with capacity to induce microtubule resistance to both the cold and nocodazole, others with microtubule cold stabilizing activity only. These microtubule-stabilizing effects of STOP proteins are inhibited by calmodulin and we now demonstrate that they are determined by two distinct kinds of repeated modular sequences (Mn and Mc), both containing a calmodulin-binding peptide, but displaying different microtubule stabilizing activities. Mn modules induce microtubule resistance to both the cold and nocodazole when expressed in cells. Mc modules, which correspond to the STOP central repeats, have microtubule cold stabilizing activity only. Mouse neuronal STOPs, which induce both cold and drug resistance in cellular microtubules, contain three Mn modules and four Mc modules. Compared with neuronal STOPs, the non-neuronal F-STOP lacks multiple Mn modules and this corresponds with an inability to induce nocodazole resistance. STOP modules represent novel bifunctional calmodulin-binding and microtubule-stabilizing sequences that may be essential for the generation of the different patterns of microtubule stabilization observed in cells.

Detyrosinated (Glu) microtubules are stabilized by an ATP-sensitive plus-end cap

Many cell types contain a subset of long-lived, ‘stable’ microtubules that differ from dynamic microtubules in that they are enriched in post-translationally detyrosinated tubulin (Glu-tubulin). Elevated Glu tubulin does not stabilize the microtubules and the mechanism for the stability of Glu microtubules is not known. We used detergent-extracted cell models to investigate the nature of Glu microtubule stability. In these cell models, Glu microtubules did not incorporate exogenously added tubulin subunits on their distal ends, while >70% of the bulk microtubules did. Ca(2+)-generated fragments of Glu microtubules incorporated tubulin, showing that Glu microtubule ends are capped. Consistent with this, Glu microtubules in cell models were resistant to dilution-induced breakdown. Known microtubule end-associated proteins (EB1, APC, p150(Glued) and vinculin focal adhesions) were not localized on Glu microtubule ends. ATP, but not nonhydrolyzable analogues, induced depolymerization of Glu microtubules in cell models. Timelapse and photobleaching studies showed that ATP triggered subunit loss from the plus end. ATP breakdown of Glu microtubules was inhibited by AMP-PNP and vanadate, but not by kinase or other inhibitors. Additional experiments showed that conventional kinesin or kif3 were not involved in Glu microtubule capping. We conclude that Glu microtubules are stabilized by a plus-end cap that includes an ATPase with properties similar to kinesins.

Doublecortin is a microtubule-associated protein and is expressed widely by migrating neurons

Doublecortin (DCX) is required for normal migration of neurons into the cerebral cortex, since mutations in the human gene cause a disruption of cortical neuronal migration. To date, little is known about the distribution of DCX protein or its function. Here, we demonstrate that DCX is expressed in migrating neurons throughout the central and peripheral nervous system during embryonic and postnatal development. DCX protein localization overlaps with microtubules in cultured primary cortical neurons, and this overlapping expression is disrupted by microtubule depolymerization. DCX coassembles with brain microtubules, and recombinant DCX stimulates the polymerization of purified tubulin. Finally, overexpression of DCX in heterologous cells leads to a dramatic microtubule phenotype that is resistant to depolymerization. Therefore, DCX likely directs neuronal migration by regulating the organization and stability of microtubules.

G protein alpha subunits activate tubulin GTPase and modulate microtubule polymerization dynamics

G proteins serve many functions involving the transfer of signals from cell surface receptors to intracellular effector molecules. Considerable evidence suggests that there is an interaction between G proteins and the cytoskeleton. In this report, G protein alpha subunits Gi1alpha, Gsalpha, and Goalpha are shown to activate the GTPase activity of tubulin, inhibit microtubule assembly, and accelerate microtubule dynamics. Gialpha inhibited polymerization of tubulin-GTP into microtubules by 80-90% in the absence of exogenous GTP. Addition of exogenous GTP, but not guanylylimidodiphosphate, which is resistant to hydrolysis, overcame the inhibition. Analysis of the dynamics of individual microtubules by video microscopy demonstrated that Gi1alpha increases the catastrophe frequency, the frequency of transition from growth to shortening. Thus, Galpha may play a role in modulating microtubule dynamic instability, providing a mechanism for the modification of the cytoskeleton by extracellular signals.

Posttranslational arginylation of soluble rat brain proteins after whole body hyperthermia

We have previously reported the posttranslational addition of [14C]-arginine in the N-terminus of several soluble rat brain proteins. One of these proteins was identified as the microtubule-associated protein, the stable tubule only polypeptide (STOP). However, despite the fact that the biological significance of arginylation is not completely understood, some evidence associates it with proteolysis via the ubiquitin pathway. Since this degradative via is exacerbated as a response to stress, we studied in vitro the posttranslational [14C]-arginylation of cytosolic brain proteins of rats subjected to hyperthermia in vivo. Immediately after subjecting the animals to hyperthermia, a minor reduction (16%) in the acceptor capacity of [14C]-arginine into proteins was observed in comparison with animals maintained at 28 degrees C. However, in the animals allowed to recover for 3 h, an increase (46%) in the arginylation was observed concomitantly with a significant accumulation of the heat shock protein (70 kDa; hsp 70) when compared to the control animals. These findings suggest that the posttranslational arginylation of proteins participate in the heat shock response. The STOP protein of the soluble brain fraction of control animals, which in Western blot appears as a doublet band (125 and 130 kDa, respectively), is seen, after the hyperthermic treatment, as a single band of 125 kDa. The amount of 125 kDa protein, as well as the in vitro incorporation of [14C]-arginine, increases after hyperthermia in comparison with control animals. Following hyperthermic treatment, we observed a decrease in the amount of in vivo [35S]-methionine-labeled brain proteins. We speculate that, as observed for STOP protein, the increase in the degradation of protein that occurs in hyperthermia, would produce an increase in the amount of arginine acceptor proteins.

STOP Proteins

Microtubules assembled from pure tubulin in vitro are labile, rapidly depolymerized upon exposure to the cold. In contrast, in a number of cell types, cytoplasmic microtubules are stable, resistant to prolonged cold exposure. During the past years, the molecular basis of this microtubule stabilization in cells has been elucidated. Cold stability is due to polymer association with different variants of a calmodulin-regulated protein, STOP protein. The dynamic and hence the physiological consequences of STOP association with microtubules vary in different tissues. In neurons, STOP seems almost permanently associated with microtubules. STOP is apparently a major determinant of microtubule turnover in such cells and is required for normal neuronal differentiation. In cycling cells, only minor amounts of STOP are associated with interphase microtubules and STOP does not measurably affects microtubule dynamics. However, STOP is associated with mitotic microtubules in the spindle. Recent results indicate that such an association could be vital for meiosis and for the long-term fidelity of the mitotic process.

RNA splicing of bacterial genes in eukaryotes

The presence of intervening sequences or introns in eukaryotic genes has been known for more than 20 years, and the mechanisms underlying RNA splicing have been studied in depth both genetically and biochemically. In recent years, however, an increasing number of bacterial genes have been introduced into higher eukaryotes as important tools for genetic studies. Their gene products are frequently used as an indirect measure for cell type-specific promoter activity, as, for example, in the case of chloramphenicol acetyl transferase (CAT assay) or beta-galactosidase. Here we show that RNA splicing of two prokaryotic genes encoding site-specific DNA recombinases occurs in eukaryotic cells. In one case, splicing is only observed after treatment of cells with the cytokine alpha interferon. We further demonstrate that mutating an intragenic donor splice site in a bacterial gene apparently activates a second, alternative splicing pathway. In conjunction with previous reports, our findings should also be regarded as a warning that splicing of bacterial genes in higher eukaryotes is a more common phenomenon than presently recognized, which may be difficult to overcome and may cause problems in the interpretation of experimental results.

STOP proteins are responsible for the high degree of microtubule stabilization observed in neuronal cells

Neuronal differentiation and function require extensive stabilization of the microtubule cytoskeleton. Neurons contain a large proportion of microtubules that resist the cold and depolymerizing drugs and exhibit slow subunit turnover. The origin of this stabilization is unclear. Here we have examined the role of STOP, a calmodulin-regulated protein previously isolated from cold-stable brain microtubules. We find that neuronal cells express increasing levels of STOP and of STOP variants during differentiation. These STOP proteins are associated with a large proportion of microtubules in neuronal cells, and are concentrated on cold-stable, drug-resistant, and long-lived polymers. STOP inhibition abolishes microtubule cold and drug stability in established neurites and impairs neurite formation. Thus, STOP proteins are responsible for microtubule stabilization in neurons, and are apparently required for normal neurite formation.

Nonneuronal isoforms of STOP protein are responsible for microtubule cold stability in mammalian fibroblasts

A number of cycling mammalian cells, such as NIH 3T3, contain abundant subsets of cold-stable microtubules. The origin of such microtubule stabilization in nonneuronal cells is unknown. We have previously described a neuronal protein, stable tubule-only polypeptide (STOP), that binds to microtubules and induces cold stability. We find that NIH 3T3 fibroblasts contain a major 42-kDa isoform of STOP (fibroblastic STOP, F-STOP). F-STOP contains the central repeats characteristic of brain STOP but shows extensive deletions of N- and C-terminal protein domains that are present in brain STOP. These deletions arise from differences in STOP RNA splicing. Despite such deletions, F-STOP has full microtubule stabilizing activity. F-STOP accumulates on cold-stable microtubules of interphase arrays and is present on stable microtubules within the mitotic spindle of NIH 3T3 cells. STOP inhibition by microinjection of affinity-purified STOP central repeat antibodies into NIH 3T3 cells abolishes both interphase and spindle microtubule cold stability. Similar results were obtained with Rat2 cells. These results show that STOP proteins have nonneuronal isoforms that are responsible for the microtubule cold stability observed in mammalian fibroblasts.

Genomic structure and chromosomal mapping of the mouse STOP gene (Mtap6)

The microtubule associated protein STOP (Stable Tubule Only Polypeptide) is a calmodulin-regulated protein able to induce a high degree of microtubule stability. STOP is abundant in neurons which contain large subpopulations of stable microtubules. Genomic clones spanning 67 kb and encompassing the mouse STOP gene (Mtap6) have been isolated and characterized. These clones derive from a single gene mapping to the E2-F1 region of mouse chromosome 7. The gene is composed of 4 exons that exhibit conventional vertebrate splicing sequences. Transcription of the gene initiate at multiple sites in a 85 nucleotide region located 530 bases upstream the translation initiation codon. Accordingly, the 5' flanking region of the gene lacks a TATA box or an initiator element at usual position. The protein encoded by the mouse STOP gene (Mtap6) is composed of 906 amino acids and presents a 91% identities with the rat brain STOP.

Evidence for Opposing Effects of Calmodulin on Cortical Microtubules

Microtubule integrity within the cortical array was visualized in detergent-lysed carrot (Daucus carota L.) protoplasts that were exposed to various exogenous levels of Ca2+ and calmodulin (CaM). CaM appears to help stabilize cortical microtubules against the destabilizing action of Ca2+/CaM complexes at low Ca2+ concentrations, but not at higher Ca2+ concentrations. The hypothesis that CaM interacts with microtubules at two different sites, determined by the concentration of Ca2+, is supported by the effects of the CaM antagonists N-(6-aminohexyl)-1-naphthalene-sulfonamide and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfanamide (20 [mu]M) and by affinity chromatography. Two classes of proteins were identified that interact with tubulin and bind to CaM. One class required Ca2+ for CaM binding, whereas the second class bound only when Ca2+ concentrations were low (<320 nM). Thus, CaM's ability to have two opposing effects upon microtubules may be regulated by the concentration of intracellular Ca2+ and its differential interactions with microtubule-associated proteins. Experimental manipulation of intracellular Ca2+ concentrations, as monitored by Indo-1, revealed that the effect of Ca2+ is specific to the cortical microtubules and does not affect actin microfilaments in these cells.

Cloning, expression, and properties of the microtubule-stabilizing protein STOP

Nerve cells contain abundant subpopulations of cold-stable microtubules. We have previously isolated a calmodulin-regulated brain protein, STOP (stable tubule-only polypeptide), which reconstitutes microtubule cold stability when added to cold-labile microtubules in vitro. We have now cloned cDNA encoding STOP. We find that STOP is a 100.5-kDa protein with no homology to known proteins. The primary structure of STOP includes two distinct domains of repeated motifs. The central region of STOP contains 5 tandem repeats of 46 amino acids, 4 with 98% homology to the consensus sequence. The STOP C terminus contains 28 imperfect repeats of an 11-amino acid motif. STOP also contains a putative SH3-binding motif close to its N terminus. In vitro translated STOP binds to both microtubules and Ca2+-calmodulin. When STOP cDNA is expressed in cells that lack cold-stable microtubules, STOP associates with microtubules at 37 degrees C, and stabilizes microtubule networks, inducing cold stability, nocodazole resistance, and tubulin detyrosination on microtubules in transfected cells. We conclude that STOP must play an important role in the generation of microtubule cold stability and in the control of microtubule dynamics in brain.

Identification and partial purification of calmodulin-binding microtubule-associated proteins from Neurospora crassa

We have purified microtubule-associated proteins from Neurospora crassa on the basis of heat stability and affinity to calmodulin. Two proteins of molecular masses 170 kDa and 190 kDa have been partially purified. A third protein of 145 kDa was purified almost to homogeneity, and we present evidence that this protein is a specific substrate for a Ca2+/calmodulin-dependent protein kinase. The purified 170-, 190-, and 145-kDa proteins induce the assembly of microtubules from purified porcine brain tubulin. We demonstrate that all three proteins are microtubule-associated proteins on the basis of an in vitro microtubule-binding assay.

Intrinsic microtubule stability in interphase cells

Interphase microtubule arrays are dynamic in intact cells under normal conditions and for this reason they are currently assumed to be composed of polymers that are intrinsically labile, with dynamics that correspond to the behavior of microtubules assembled in vitro from purified tubulin preparations. Here, we propose that this apparent lability is due to the activity of regulatory effectors that modify otherwise stable polymers in the living cell. We demonstrate that there is an intrinsic stability in the microtubule network in a variety of fibroblast and epithelial cells. In the absence of regulatory factors, fibroblast cell interphase microtubules are for the most part resistant to cold temperature exposure, to dilution-induced disassembly and to nocodazole-induced disassembly. In epithelial cells, microtubules are cold-labile, but otherwise similar in behavior to polymers observed in fibroblast cells. Factors that regulate stability of microtubules appear to include Ca2+ and the p34cdc2 protein kinase. Indeed, this kinase induced complete destabilization of microtubules when applied to lysed cells, while a variety of other protein kinases were ineffective. This suggests that p34cdc2, or a kinase of similar specificity, may phosphorylate and inactivate microtubule-associated proteins, thereby conferring lability to otherwise length-wise stabilized microtubules.

Tau confers drug stability but not cold stability to microtubules in living cells

We previously defined two classes of microtubule polymer in the axons of cultured sympathetic neurons that differ in their sensitivity to nocodazole by roughly 35-fold (Baas and Black (1990) J. Cell Biol. 111, 495–509). Here we demonstrate that virtually all of the microtubule polymer in these axons, including the drug-labile polymer, is stable to cold. What factors account for the unique stability properties of axonal microtubules? In the present study, we have focused on the role of tau, a microtubule-associated protein that is highly enriched in the axon, in determining the stability of microtubules to nocodazole and/or cold in living cells. We used a baculovirus vector to express very high levels of tau in insect ovarian Sf9 cells. The cells respond by extending processes that contain dense bundles of microtubules (Knops et al. (1991) J. Cell Biol. 114, 725–734). Cells induced to express tau were treated with either cold or 2 micrograms/ml nocodazole for times ranging from 5 minutes to 6 hours. The results with each treatment were very different from one another. Virtually all of the polymer was depolymerized within the first 30 minutes in cold, while little or no microtubule depolymerization was detected even after 6 hours in nocodazole. Based on these results, we conclude that tau is almost certainly a factor in conferring drug stability to axonal microtubules, but that factors other than or in addition to tau are required to confer cold stability.

Calcium Levels Affect the Ability to Immunolocalize Calmodulin to Cortical Microtubules

Calcium affects the stability of cortical microtubules (MTs) in lysed protoplasts. This calmodulin (CaM)-mediated interaction may provide a mechanism that serves to integrate cellular behavior with MT function. To test the hypothesis that CaM associates with these MTs, monoclonal antibodies were produced against CaM, and one (designated mAb1D10) was selected for its suitability as an immunocytochemical reagent. It is shown that CaM associates with the cortical MTs of cultured carrot (Daucus carota L.) and tobacco (Nicotiana tabacum L.) cells. Inasmuch as CaM interacts with calcium and affects the behavior of these MTs, we hypothesized that calcium would alter this association. To test this, protoplasts containing taxol-stabilized MTs were lysed in the presence of various concentrations of calcium and examined for the association of CaM with cortical MTs. At 1 [mu]M calcium, many protoplasts did not have CaM in association with the cortical MTs, whereas at 3.6 [mu]M calcium, this association was completely abolished. Control experiments were performed to eliminate alternate explanations including differential antibody binding in the presence of calcium and/or taxol, detergent-induced redistribution of antigen, and epitope masking. The results are discussed in terms of a model in which CaM associates with MTs via two types of interactions, one that occurs in the presence of calcium and another that occurs only in its absence.

Induction of tumor-cell lysis by bi-specific antibody recognizing ganglioside GD2 and T-cell antigen CD3

Human tumor cells expressing ganglioside GD2 were lysed by various effector populations targeted with an anti-CD3-anti-GD2 bi-specific antibody (BAb CD3 x GD2). This antibody-heteroconjugate was prepared by chemically cross-linking the OKT-3 monoclonal antibody (MAb) reactive with CD3 antigen on T lymphocytes with the ganglioside MAb ME 361, which binds preferentially to the tumor-associated ganglioside GD2. The specificity of target-cell lysis by the cytotoxic T cells (CTL) was mediated by the specificity of the targeting antibody: GD2-negative cells were not lysed in the presence of the CD3 x GD2 BAb. A dose-dependent response was observed in a range of 10 to 10,000 ng/ml. In contrast, 2 other BAbs recognizing the tumor-associated antigens EGF-R and TKB-2 had greater potency to mediate tumor-cell lysis than the GD2 x CD3 BAb. Peripheral-blood cells (PBL) stimulated with OKT-3 MAb or with irradiated tumor cells in a mixed lymphocyte culture (MLTC) could be induced to lyse GD2-positive tumor cells in the presence of CD3 x GD2 BAb. The tumor-cell lysis could be mediated by autologous or allogeneic effector cells. NK cells had no influence on the BAb-induced cytotoxicity.

Mutations in yeast calmodulin cause defects in spindle pole body functions and nuclear integrity

Yeast calmodulin (CaM) is required for the progression of nuclear division (Ohya, Y. and Y. Anraku. 1989. Curr. Genet. 15:113-120), although the precise mechanism and physiological role of CaM in this process are unclear. In this paper we have characterized the phenotype caused by a temperature-sensitive lethal mutation (cmdl-101) in the yeast CaM. The cmdl-101 mutation expresses a carboxyl-terminal half of the yeast CaM (Met72-Cys147) under the control of an inducible GAL1 promoter. Incubation of the cmdl-101 cells at a nonpermissive temperature causes a severe defect in chromosome segregation. The rate of chromosome loss in the cmdl-101 mutant is higher than wild-type cell even at permissive temperature. The primary visible defect observed by immunofluorescence and electron microscopic analyses is that the organization of spindle microtubules is abnormal in the cmdl-101 cells grown at nonpermissive temperature. Majority of budded cells arrested at the high temperature contain only one spindle pole body (SPB), which forms monopolar spindle, whereas the budded cells of the same strain incubated at permissive temperature all contain two SPBs. Using the freeze-substituted fixation method, we found that the integrity of the nuclear morphology of the cmdl-101 mutant cell is significantly disturbed. The nucleus in wild-type cells is round with smooth contours of nuclear envelope. However, the nuclear envelope in the mutant cells appears to be very flexible and forms irregular projections and invaginations that are never seen in wild-type cells. The deformation of the nuclear becomes much more severe as the incubation at nonpermissive temperature continues. The single SPB frequently localizes on the projections or the invaginations of the nuclear envelope. These observations suggest that CaM is required for the functions of SPB and spindle, and the integrity of nucleus.

Polyreactive autoantibodies to negatively charged epitopes following Trypanosoma cruzi infection

During the course of many human autoimmune diseases, antibodies which recognize negatively charged epitopes on self antigens are detected. Trypanosoma cruzi, an intracellular protozoan parasite capable of infecting a wide variety of vertebrates, is the cause of Chagas disease in humans. Infection with the parasite frequently results in autoimmune and inflammatory pathology. We report here on an affinity-purified population of antibodies that bind to a broad class of antigens that contain runs of acidic amino acids, including tubulin. Although these antibodies can be isolated from both uninfected and T. cruzi chronically infected C3H/He mice, the antibodies from the normal mice (the natural autoantibodies) bind to tubulin poorly at physiological pH, whereas the antibodies isolated from the infected animals bind well at physiological pH. We propose that similar processes may occur in humans following other infections accounting for the detection of antibodies to negatively charged epitopes in a variety of autoimmune diseases.

The plus ends of stable microtubules are the exclusive nucleating structures for microtubules in the axon

Microtubules (MTs) in the axon have a uniform polarity orientation that is recapitulated during recovery from episodes of MT depolymerization (Heidemann, S. R., M. A. Hamborg, S. J. Thomas, B. Song, S. Lindley, and D. Chu. 1984. J. Cell Biol. 99:1289-1295). This tight regulation of their organization indicates that axonal MTs are spatially regulated by discrete nucleating structures comparable in function to the centrosome. Several authors have proposed that an especially stable class of MTs in the axon may serve as these nucleating structures. In a previous report (Baas, P. W., and M. M. Black. 1990. J. Cell Biol. 111:495-509), we determined that the axons of cultured sympathetic neurons contain two classes of MT polymer, stable and labile, that differ in their sensitivity to nocodazole by roughly 35-fold. The stable and labile polymer represent long-lived and recently assembled polymer, respectively. We also determined that these two classes of polymer can be visually distinguished at the immunoelectron microscopic level based on their content of tyrosinated alpha-tubulin: the labile polymer stains densely, while the stable polymer does not stain. In the present study, we have taken advantage of these observations to directly identify MT nucleating structures in the axon. Neuron cultures were treated with nocodazole for 6 h to completely depolymerize the labile polymer in the axon, and substantially shorten the stable polymer. The cultures were then rinsed free of the drug, permitted to reassemble polymer for various periods of time, and prepared for immunoelectron microscopic localization of tyrosinated alpha-tubulin. Serial reconstruction of consecutive thin sections was undertaken to determine the spatial relationship between the stable MTs and the newly assembled polymer. All of the new polymer assembled in direct continuity with the plus ends of stable MTs, indicating that these ends are assembly competent, and hence capable of acting as nucleating structures. Our results further indicate that no self-assembly of MTs occurs in the axon, nor do any MT nucleating structures exist in the axon other than the plus ends of stable MTs. Thus the plus ends of stable MTs are the exclusive nucleating structures for MTs in the axon.

Distribution of acetylated alpha-tubulin in brain. In situ localization and biochemical characterization

We studied the solubility properties of brain acetylated alpha-tubulin, as well as the localization of this tubulin in brain tissue. Endogenous unpolymerized tubulin and cytoskeletal tubulin were fractionated after brain Triton-solubilization. Using the immunoblotting technique, we found that acetylated alpha-tubulin was recovered in the cytoskeletal fraction, and that most (92%) of the acetylated microtubules of this fraction were depolymerized by cold/Ca2+ treatment. In another set of experiments, axonal and soma-dendritic preparations were found to have equivalent amounts of acetylated alpha-tubulin. By immunogold electron microscopy, we established that acetylated microtubules are widely distributed in dendrites of the central nervous system.

Assembly of chick brain MAP2-tubulin microtubule protein. Analysis of tubulin subunit flux rates by immunofluorescence microscopy

A filter-based immunofluorescence-microscopy method for obtaining microtubule lengths has been developed and evaluated. Kinetic constants and mean lengths obtained show close agreement with those obtained by complementary methods applied to chick brain MAP2-tubulin microtubule protein in NaCl-supplemented buffer. The filter-based method has been used to estimate tubulin subunit flux (Jon) resulting from isothermal dilution of microtubule populations to various free tubulin concentrations, (c). This experimental Jon(c) plot is significantly different from that predicted by a variety of theoretical models, but is consistent with a 'lateral cap' model of dynamic instability [Bayley, Schilstra & Martin (1990) J. Cell. Sci. 95, 33-48] adapted to accommodate the observed vectorial GTP hydrolysis.

Microtubule-associated proteins-dependent colchicine stability of acetylated cold-labile brain microtubules from the Atlantic cod, Gadus morhua

Assembly of brain microtubule proteins isolated from the Atlantic cod, Gadus morhua, was found to be much less sensitive to colchicine than assembly of bovine brain microtubules, which was completely inhibited by low colchicine concentrations (10 microM). The degree of disassembly by colchicine was also less for cod microtubules. The lack of colchicine effect was not caused by a lower affinity of colchicine to cod tubulin, as colchicine bound to cod tubulin with a dissociation constant, Kd, and a binding ratio close to that of bovine tubulin. Cod brain tubulin was highly acetylated and mainly detyrosinated, as opposed to bovine tubulin. When cod tubulin, purified by means of phosphocellulose chromatography, was assembled by addition of DMSO in the absence of microtubule-associated proteins (MAPs), the microtubules became sensitive to low concentrations of colchicine. They were, however, slightly more stable to disassembly, indicating that posttranslational modifications induce a somewhat increased stability to colchicine. The stability was mainly MAPs dependent, as it increased markedly in the presence of MAPs. The stability was not caused by an extremely large amount of cod MAPs, since there were slightly less MAPs in cod than in bovine microtubules. When "hybrid" microtubules were assembled from cod tubulin and bovine MAPs, these microtubules became less sensitive to colchicine. This was not a general effect of MAPs, since bovine MAPs did not induce a colchicine stability of microtubules assembled from bovine tubulin. We can therefore conclude that MAPs can induce colchicine stability of colchicine labile acetylated tubulin.

Biochemical and immunochemical analysis of rat brain dynamin interaction with microtubules and organelles in vivo and in vitro

We have purified a 100-kD rat brain protein that has microtubule cross-linking activity in vitro, and have determined that it is dynamin, a putative microtubule-associated motility protein. We find that dynamin appears to be specific to neuronal tissue where it is present in both soluble and particulate tissue fractions. In the cytosol it is abundant, representing as much as 1.5% of the total extractable protein. Dynamin appears to be in particulate material due to association with a distinct subcellular membrane fraction. Surprisingly, by immunofluorescence analysis of PC12 cells we find that dynamin is distributed uniformly throughout the cytoplasm with no apparent microtubule association in either interphase, mitotic, or taxol-treated cells. Upon nerve growth factor (NGF) induction of PC12 cell differentiation into neurons, dynamin levels increase approximately twofold. In the cell body, the distribution of dynamin again remains clearly distinct from that of tubulin, and in axons, where microtubules are numerous and ordered into bundles, dynamin staining is sparse and punctate. On the other hand, in the most distal domain of growth cones, where there are relatively few microtubules, dynamin is particularly abundant. The dynamin staining of neurites is abolished by extraction of the cells with detergent under conditions that preserve microtubules, suggesting that dynamin in neurites is associated with membranes. We conclude that dynamin is a neuronal protein that is specifically associated with as yet unidentified vesicles. It is possible, but unproven, that it may link vesicles to microtubules for transport in differentiated axons.

Presence of a new microtubule cold-stabilizing factor in bull sperm dynein preparations

Brain tubulin polymerized with dynein isolated from bull spermatozoa forms cold-stable microtubules, in contrast with microtubules made of brain tubulin polymerized by brain microtubule-associated proteins (MAPs). The level of cold-stable microtubules depends on the concentration of dynein used. Addition of dynein to cold-unstable microtubules renders these microtubules stable to cold. Although ATP and a non-hydrolysable ATP analogue increase the formation of microtubules made of tubulin and dynein, these nucleotides have no effect on dynein cold-stabilizing properties. The data suggests that a new factor, not involving the dynein ATPase active site and present in bull sperm dynein preparations, confers cold-stability to microtubules.

An antigen located in the kinetochore region in metaphase and on polar microtubule ends in the midbody region in anaphase, characterised using a monoclonal antibody

We describe a new component of the kinetochore region of Chinese hamster ovary cells, which was characterised using a monoclonal antibody (mAb). This antigen was localised on the kinetochore regions of purified metaphase chromosomes, but in anaphase it was instead located on the polar microtubules in the midbody region, where they terminate in the stembody. It was not detectable in prophase or interphase cells by immunofluorescence, but was present in the interphase nucleus as shown by immunoblotting after SDS-polyacrylamide gel electrophoresis. The mAb recognised two polypeptides of Mr 140,000 and 155,000. The localisation of this antigen in metaphase on the kinetochore region, where the plus ends of the kinetochore microtubules are temporarily stabilised when they attach, and later in the stembody and midbody where the plus ends of the polar microtubules are stabilised in anaphase and telophase, suggests that it could play a role in stabilising the plus ends of microtubules and thus in the control of microtubule dynamics during mitosis.

Polypeptides from the myxomycete Physarum polycephalum interacting in vitro with microtubules

Microtubule-interacting proteins have been studied in the lower eukaryote Physarum polycephalum. We show for the first time 1) the presence in Physarum amoebal crude extracts of at least six polypeptides that bind specifically to amoebal microtubules, 2) the binding between these proteins and mammalian microtubules, 3) the heat stability of two of these polypeptides (125 and 235 kDa), 4) the functional properties of a fraction containing a heat-soluble 125 kDa polypeptide, and 5) the phosphorylation of the 125 kDa polypeptide during two distinct periods of the cell cycle in Physarum synchronous plasmodia, first at late S/early G2 phase and second at late G2/prophase.

Microtubule-associated proteins from Antarctic fishes

Microtubules and presumptive microtubule-associated proteins (MAPs) were isolated from the brain tissues of four Antarctic fishes (Notothenia gibberifrons, N. coriiceps neglecta, Chaenocephalus aceratus, and a Chionodraco sp.) by means of a taxol-dependent, microtubule-affinity procedure (cf. Vallee: Journal of Cell Biology 92:435-442, 1982). MAPs from these fishes were similar to each other in electrophoretic pattern. Prominent in each preparation were proteins in the molecular weight ranges 410,000-430,000, 220,000-280,000, 140,000-155,000, 85,000-95,000, 40,000-45,000, and 32,000-34,000. The surfaces of MAP-rich microtubules were decorated by numerous filamentous projections. Exposure to elevated ionic strength released the MAPs from the microtubules and also removed the filamentous projections. Addition of fish MAPs to subcritical concentrations of fish tubulins at 0-5 degrees C induced the assembly of microtubules. Both the rate and the extent of this assembly increased with increasing concentrations of the MAPs. Sedimentation revealed that approximately six proteins, with apparent molecular weights between 60,000 and 300,000, became incorporated into the microtubule polymer. Bovine MAPs promoted microtubule formation by fish tubulin at 2-5 degrees C, and proteins corresponding to MAPs 1 and 2 co-sedimented with the polymer. MAPs from C. aceratus also enhanced the polymerization of bovine tubulin at 33 degrees C, but the microtubules depolymerized at 0 degrees C. We conclude that MAPs are part of the microtubules of Antarctic fishes, that these proteins promote microtubule assembly in much the same way as mammalian MAPs, and that they do not possess special capacities to promote microtubule assembly at low temperatures or to prevent cold-induced microtubule depolymerization.

Polymerization of Antarctic fish tubulins at low temperatures: energetic aspects

Tubulins were purified from the brain tissues of three Antarctic fishes, Notothenia gibberifrons, Notothenia coriiceps neglecta, and Chaenocephalus aceratus, by ion-exchange chromatography and one cycle of temperature-dependent microtubule assembly and disassembly in vitro, and the functional properties of the protein were examined. The preparations contained the alpha- and beta-tubulins and were free of microtubule-associated proteins. At temperatures between 0 and 24 degrees C, the purified tubulins polymerized readily and reversibly to yield both microtubules and microtubule polymorphs (e.g., "hooked" microtubules and protofilament sheets). Critical concentrations for polymerization of the tubulins ranged from 0.87 mg/mL at 0 degrees C to 0.02 mg/mL at 18 degrees C. The van't Hoff plot of the apparent equilibrium constant for microtubule elongation at temperatures between 0 and 18 degrees C was linear and gave a standard enthalpy change (delta H degree) of +26.9 kcal/mol and a standard entropy change (delta S degree) of +123 eu. At 10 degrees C, tubulin from N. gibberifrons polymerized efficiently at high ionic strength; the critical concentration increased monotonically from 0.041 to 0.34 mg/mL as the concentration of NaCl added to the assembly buffer was increased from 0 to 0.4 M. Together, the results indicate that the polymerization of tubulins from the Antarctic fishes is entropically driven and suggest that an increased reliance on hydrophobic interactions underlies the energetics of microtubule formation at low temperatures. Thus, evolutionary modification to increase the proportion of hydrophobic interactions (relative to other bond types) at sites of interdimer contact may be one adaptive mechanism that enables the tubulins of cold-living poikilotherms to polymerize efficiently at low temperatures.