Control of actin filament length by phosphorylation of fragmin-actin complex

Early signaling pathways mediating dormant cyst formation in terrestrial unicellular eukaryote Colpoda

Dormant (resting) cyst formation (encystment) in unicellular eukaryotes is the process of a large-scale digestion of vegetative cell structures and reconstruction into the dormant form, which is performed by cell signaling pathways accompanied by up- or down-regulation of protein expression, and by posttranslational modification such as phosphorylation. In this review, the author describes the morphogenetic events during encystment of Colpoda and the early molecular events in the Ca2+/calmodulin-triggered signaling pathways for encystment, based mainly on our research results of the past 10 years; especially, the author discusses the role of c-AMP dependently phosphorylated proteins (ribosomal P0 protein, ribosomal S5 protein, Rieske iron-sulfur protein, actin and histone H4) and encystment-dependently upregulated (EF-1α-HSP60, actin-related protein) and downregulated proteins (ATP synthase β-chain). In addition, the roles of AMPK, a key molecule in the signaling pathways leading to Colpoda encystment, and differentially expressed genes and proteins during encystment of other ciliates are discussed.

Analysis of Water-Soluble Proteins by Two-Dimensional Electrophoresis in the Encystment Process of Colpoda cucullus Nag-1 and Cytoskeletal Dynamics

Assays of protein contained in water-soluble fraction of encysting cells Colpoda cucullus Nag-1 by two-dimensional electrophoresis (2-D PAGE) and mass spectrometry (MS) revealed that the amount of β-tubulin abruptly increased in 2.5–10 h after encystment induction. Judging from the results that total α-tubulin content did not decrease much until 12 h after encystment induction, the result indicates that disassembly of microtubules may occur soon after encystment is induced. Therefore, we tried to visualize dynamics of microtubules. Immunofluorescence microscopy using anti-α-tubulin antibody indicated that disassembly of axonemal microtubules of cilia became within 1.5 h after encystment induction, and resorbed in 3 days. Although the cytoplasmic microtubules failed to be visualized clearly, encystmentdependent globulation of cells was promoted by taxol, an inhibitor of disassembly of microtubules. It is possible that a temporary formation of cytoplasmic microtubules may be involved in cell globulation. The phosphorylation level of actin (43 kDa) became slightly elevated just after encystment induction. Lepidosomes, the sticky small globes surrounding encysting cells, were vividly stained with Acti-stain 555 phalloidin, suggesting that 43-kDa actin or its homologues may be contained in lepidosomes.

Age-Onset Phosphorylation of a Minor Actin Variant Promotes Intestinal Barrier Dysfunction

Age-associated decay of intercellular interactions impairs the cells' capacity to tightly associate within tissues and form a functional barrier. This barrier dysfunction compromises organ physiology and contributes to systemic failure. The actin cytoskeleton represents a key determinant in maintaining tissue architecture. Yet, it is unclear how age disrupts the actin cytoskeleton and how this, in turn, promotes mortality. Here, we show that an uncharacterized phosphorylation of a low-abundant actin variant, ACT-5, compromises integrity of the C. elegans intestinal barrier and accelerates pathogenesis. Age-related loss of the heat-shock transcription factor, HSF-1, disrupts the JUN kinase and protein phosphatase I equilibrium which increases ACT-5 phosphorylation within its troponin binding site. Phosphorylated ACT-5 accelerates decay of the intestinal subapical terminal web and impairs its interactions with cell junctions. This compromises barrier integrity, promotes pathogenesis, and drives mortality. Thus, we provide the molecular mechanism by which age-associated loss of specialized actin networks impacts tissue integrity.

Tyrosine nitration as a key event of signal transduction that regulates functional state of the cell

This review is dedicated to the role of nitration of proteins by tyrosine residues in physiological and pathological conditions. First of all, we analyze the biochemical evidence of peroxynitrite formation and reactions that lead to its formation, types of posttranslational modifications (PTMs) induced by reactive nitrogen species, as well as three biological pathways of tyrosine nitration. Then, we describe two possible mechanisms of protein nitration that are involved in intracellular signal transduction, as well as its interconnection with phosphorylation/dephosphorylation of tyrosine. Next part of the review is dedicated to the role of proteins nitration in different pathological conditions. In this section, special attention is devoted to the role of nitration in changes of functional properties of actin-protein that undergoes PTMs both in normal and pathological conditions. Overall, this review is devoted to the main features of protein nitration by tyrosine residue and the role of this process in intracellular signal transduction in basal and pathological conditions.

Post-translational modification-regulated leukocyte adhesion and migration

Leukocytes undergo frequent phenotypic changes and rapidly infiltrate peripheral and lymphoid tissues in order to carry out immune responses. The recruitment of circulating leukocytes into inflamed tissues depends on integrin-mediated tethering and rolling of these cells on the vascular endothelium, followed by transmigration into the tissues. This dynamic process of migration requires the coordination of large numbers of cytosolic and transmembrane proteins whose functional activities are typically regulated by post-translational modifications (PTMs). Our recent studies have shown that the lysine methyltransferase, Ezh2, critically regulates integrin signalling and governs the adhesion dynamics of leukocytes via direct methylation of talin, a key molecule that controls these processes by linking integrins to the actin cytoskeleton. In this review, we will discuss the various modes of leukocyte migration and examine how PTMs of cytoskeletal/adhesion associated proteins play fundamental roles in the dynamic regulation of leukocyte migration. Furthermore, we will discuss molecular details of the adhesion dynamics controlled by Ezh2-mediated talin methylation and the potential implications of this novel regulatory mechanism for leukocyte migration, immune responses, and pathogenic processes, such as allergic contact dermatitis and tumorigenesis.

STK16 regulates actin dynamics to control Golgi organization and cell cycle

STK16 is a ubiquitously expressed, myristoylated, and palmitoylated serine/threonine protein kinase with underexplored functions. Recently, it was shown to be involved in cell division but the mechanism remains unclear. Here we found that human STK16 localizes to the Golgi complex throughout the cell cycle and plays important roles in Golgi structure regulation. STK16 knockdown or kinase inhibition disrupts actin polymers and causes fragmented Golgi in cells. In vitro assays show that STK16 directly binds to actin and regulates actin dynamics in a concentration- and kinase activity-dependent way. In addition, STK16 knockdown or kinase inhibition not only delays mitotic entry and prolongs mitosis, but also causes prometaphase and cytokinesis arrest. Therefore, we revealed STK16 as a novel actin binding protein that resides in the Golgi, which regulates actin dynamics to control Golgi structure and participate in cell cycle progression.

There is more than one way to model an elephant. Experiment-driven modeling of the actin cytoskeleton

Mathematical modeling has established its value for investigating the interplay of biochemical and mechanical mechanisms underlying actin-based motility. Because of the complex nature of actin dynamics and its regulation, many of these models are phenomenological or conceptual, providing a general understanding of the physics at play. But the wealth of carefully measured kinetic data on the interactions of many of the players in actin biochemistry cries out for the creation of more detailed and accurate models that could permit investigators to dissect interdependent roles of individual molecular components. Moreover, no human mind can assimilate all of the mechanisms underlying complex protein networks; so an additional benefit of a detailed kinetic model is that the numerous binding proteins, signaling mechanisms, and biochemical reactions can be computationally organized in a fully explicit, accessible, visualizable, and reusable structure. In this review, we will focus on how comprehensive and adaptable modeling allows investigators to explain experimental observations and develop testable hypotheses on the intracellular dynamics of the actin cytoskeleton.

Regulation of levels of actin threonine phosphorylation during life cycle of Physarum polycephalum

Under various environmental stresses, the true slime mold Physarum polycephalum converts into dormant forms, such as microcysts, sclerotia, and spores, which can survive in adverse environments for a considerable period of time. In drought-induced sclerotia, actin is threonine phosphorylated, which blocks its ability to polymerize into filaments. It is known that fragmin and actin-fragmin kinase (AFK) mediate this phosphorylation event. In this work, we demonstrate that high levels of actin threonine phosphorylation are also found in other dormant cells, including microcysts and spores. As the threonine phosphorylation of actin in microcysts and sclerotia were induced by drought stress but not by other stresses, we suggest that drought stress is essential for actin phosphorylation in both cell types. Although characteristic filamentous actin structures (dot- or rod-like structures) were observed in microcysts, sclerotia, and spores, actin phosphorylation was not required for the formation of these structures. Prior to the formation of both microcysts and sclerotia, AFK mRNA expression was activated transiently, whereas fragmin mRNA levels decreased. Our results suggest that drought stress and AFK might be involved in the threonine phosphorylation of actin.

Calyculin A-induced actin phosphorylation and depolymerization in renal epithelial cells

This study reports actin phosphorylation and coincident actin cytoskeleton alterations in renal epithelial cell line, LLC-PK1. Serine phosphorylation of actin was first observed in vitro after the cell lysate was incubated with phosphatase inhibitors and ATP. Both the phosphorylated actin and actin kinase activities were found in the cytoskeletal fraction. Actin phosphorylation was later detected in living LLC-PK1 cells after incubation with the phosphatase inhibitor calyculin A. Calyculin A-induced actin phosphorylation was associated with reorganization of the actin cytoskeleton, including net actin depolymerization, loss of cell-cell junction and stress fiber F-actin filaments, and redistribution of F-actin filaments in the periphery of the rounded cells. Actin phosphorylation was abolished by 3-h ATP depletion but not by the non-specific kinase inhibitor staurosporine. These results demonstrate that renal epithelial cells contain kinase/phosphatase activities and actin can be phosphorylated in LLC-PK1 cells. Actin phosphorylation may play an important role in regulating the organization of the actin cytoskeleton in renal epithelium.

Identification and characterization of a cathepsin B-like protease in Physarum sclerotium

In response to dry stress the plasmodium of a true slime mold, Physarum polycephalum, undergoes formation of sclerotium, which is a dormant body resistant to desiccation. The sclerotium can germinate within several hours after addition of water, followed by generation of the plasmodium. In the early phase of the germination many enzymes and other proteins of the sclerotium are required for formation of the plasmodium. As dehydration of proteins often leads to destruction of their structure or reduction in their activity, it is important to elucidate whether the dehydrated enzymes are present as the intact in the sclerotium. In this study three peaks of protease activity were detected with anion exchange column chromatography of the extract from the sclerotia. From among them, an acid protease was purified to homogeneity by gel filtration column chromatography, hydroxyapatite column chromatography, acid treatment, and cation-exchange column chromatography. Treatment of the protease fractions with pH 4.0 resulted in approximately 20-fold activation of the activity. The purified protease was a monomer with a molecular mass of 35 kDa. The optimum pH and temperature were 6.3 and 40 degrees C, respectively. Beta-casein, histone H1, and H2B were degraded by the 35 kDa protease, but human hemoglobin and human serum albumin were very poor substrates. In addition, the enzyme was sensitive to the cysteine protease inhibitors chymostatin, E-64, and leupeptin. These results indicate that, in the sclerotium, a premature form of a cathepsin B-like protease remains non-denatured under dehydrated conditions.

The MEK kinase Ssk2p promotes actin cytoskeleton recovery after osmotic stress

Saccharomyces cerevisiae adapts to osmotic stress through the activation of a conserved high-osmolarity growth (HOG) mitogen-activated protein (MAP) kinase pathway. Transmission through the HOG pathway is very well understood, yet other aspects of the cellular response to osmotic stress remain poorly understood, most notably regulation of actin organization. The actin cytoskeleton rapidly disassembles in response to osmotic insult and is induced to reassemble only after osmotic balance with the environment is reestablished. Here, we show that one of three MEK kinases of the HOG pathway, Ssk2p, is specialized to facilitate actin cytoskeleton reassembly after osmotic stress. Within minutes of cells' experiencing osmotic stress or catastrophic disassembly of the actin cytoskeleton through latrunculin A treatment, Ssk2p concentrates in the neck of budding yeast cells and concurrently forms a 1:1 complex with actin. These observations suggest that Ssk2p has a novel, previously undescribed function in sensing damage to the actin cytoskeleton. We also describe a second function for Ssk2p in facilitating reassembly of a polarized actin cytoskeleton at the end of the cell cycle, a prerequisite for efficient cell cycle completion. Loss of Ssk2p, its kinase activity, or its ability to localize and interact with actin led to delays in actin recovery and a resulting delay in cell cycle completion. These unique capabilities of Ssk2p are activated by a novel mechanism that does not involve known components of the HOG pathway.

Involvement of actin dephosphorylation in germination of Physarum sclerotium

The plasmodium of Physarum polycephalum grows without cytokinesis and shows an active cytoplasmic streaming under wet and nutritious conditions. It can undergo reversible differentiation into several types of dormancy to survive in adverse environments. Temperature change or osmotic stress leads to cytoplasmic division of the plasmodium into cells containing one or more nuclei: these form a macrocyst, the spherule. Desiccation also induces cell division of the plasmodium followed by formation of a sclerotium, a dormant body resistant to dry stress. More than half of the actin in a sclerotium is phosphorylated at a single site, threonine 203, resulting in loss of its ability to polymerize into actin filaments. In the present study, actin phosphorylation was found in the sclerotium but not in either the plasmodium or in the spherule. This result suggests that phosphorylation of sclerotium actin may be related to the mechanism associated with desiccation resistance rather than morphological changes through cell compartmentalization in the macrocyst formation. Moreover. dephosphorylation of the phosphorylated form of sclerotium actin began within 5 min after addition of water. Dephosphorylation was not affected by sucrose and sorbitol sugars, but was inhibited by ammonium bicarbonate, ammonium phosphate, sodium phosphate, NaCl, and KCl in a dose-dependent manner. On the other hand, in germination of the sclerotium there was measurable sensitivity to both carbohydrates and salts. Actin dephosphorylation seems to be one of the important processes in the early phase of sclerotium germination.

Calcium regulation of the actin-myosin interaction of Physarum polycephalum

Plasmodia of Physarum polycephalum show vigorous cytoplasmic streaming, the motive force of which is supported by the actin-myosin interaction. Calcium is not required for the interaction but inhibits it. This calcium inhibition, a regulatory mode first discovered in Physarum, is the overwhelming mode of regulation of cytoplasmic streaming of plant cells and lower eukaryotes, and it is diametrically opposite to calcium activation of the interaction found in muscle and nonmuscle cells of the animal kingdom. Myosin, myosin II in myosin superfamily, is the most important protein for Ca2+ action. Its essential light chain, called calcium-binding light chain, is the sole protein that binds Ca2+. Although phosphorylation and dephosphorylation of myosin modify its properties, regulation of physiological significance is shown to be Ca-binding to myosin. The actin-binding protein of Physarum amplifies calcium inhibition when Ca2+ binds to calmodulin and other calcium-binding proteins. This review also includes characterization of this and other calcium-binding proteins of Physarum.

Dry stress-induced phosphorylation of Physarum actin

Protein phosphorylation plays important roles in a variety of stress responses. Although plasmodium of Physarum polycephalum rapidly grows and shows an active cytoplasmic streaming under nutrient and wet conditions, dry stress transforms plasmodium into a dormant state called sclerotium. Sclerotium can change into plasmodium within several hours after addition of water. We herein report that more than half of actin in sclerotium was in a phosphorylated state. The in vivo phosphorylation site was identified to be Thr-203 which is in contact with another actin molecule upon polymerization. The phosphorylated from of actin showed no polymerizing activity, while the unphosphorylated form possessed the ability to polymerize into F-actin. These results suggest that phosphorylation of Physarum actin is involved in reorganization and/or preservation of the actin molecules in the process of sclerotium formation.

In vivo phosphorylation of actin in Physarum polycephalum. Study of the substrate specificity of the actin-fragmin kinase

Actin-fragmin is a heterodimeric protein complex from Physarum polycephalum microplasmodia that is phosphorylated in vitro at residues Thr203 and Thr202 of the actin subunit by the endogenous actin-fragmin kinase. Following phosphorylation, the F-actin capping activity of the complex becomes Ca(2+)-dependent, suggesting a fundamental regulatory role in controlling F-actin growth [Gettemans, J., De Ville, Y., Waelkens E. and Vandekerckhove, J. (1995) J. Biol. Chem. 270, 2644-2651]. In this study we analysed actin phosphorylation in vivo. We demonstrate that the actin-fragmin complex constitutes the only substrate of the actin-fragmin kinase in plasmodia. Monomeric actin is not phosphorylated. Immunoprecipitation of actin-fragmin reveals that approximately 40% of the actin subunit of the complex is phosphorylated in vivo. However, using purified substrate and kinase, the complex can be quantitatively phosphorylated as judged by two-dimensional gel electrophoresis. Through comparative phosphopeptide fingerprinting, we show that the phosphorylation sites in vivo are identical to those identified in vitro. We additionally characterized a complex of actin and the NH2-terminal half of fragmin (residues 1-168) that is also phosphorylated by the same kinase. In contrast to actin-fragmin, phosphorylation of the complex between actin and residues 1-168 of fragmin is independent of Ca2+ because the second Ca(2+)-dependent regulatory actin-binding domain is missing. By artificially varying the actin-fragmin concentration or the actin-fragmin kinase activity present in microplasmodia cytosolic extracts, we attempted to detect alternative protein substrates for the actin-fragmin kinase. The fact that none could be identified suggests that the control and properties of actin-fragmin phosphorylation observed in vitro may stand as a model for F-actin growth control in Physarum cells.

A novel type of protein kinase phosphorylates actin in the actin-fragmin complex

Actin-fragmin kinase (AFK) from Physarum polycephalum specifically phosphorylates actin in the EGTA-resistant 1:1 actin-fragmin complex. The cDNA deduced amino acid sequence reveals two major domains of approximately 35 kDa each that are separated by a hinge-like proline/serine-rich segment of 50 residues. Whereas the N-terminal domain does not show any significant similarity to protein sequences from databases, there are six complete kelch repeats in the protein that comprise almost the entire C-terminal half of the molecule. To prove the intrinsic phosphorylation activity of AFK, full-length or partial cDNA fragments were expressed both in a reticulocyte lysate and in Escherichia coli. In both expression systems, we obtained specific actin phosphorylation and located the catalytic domain in the N-terminal half. Interestingly, this region did not contain any of the known protein kinase consensus sequences. The only known sequence motif present that could have been involved in nucleotide binding was a nearly perfect phosphate binding loop (P-loop). However, introduction of two different point mutations into this putative P-loop sequence did not alter the catalytic activity of the kinase, which indicates an as yet unknown mechanism for phosphate transfer. Our data suggest that AFK belongs to a new class of protein kinases and that this actin phosphorylation might be the first example of a widely distributed novel type of regulation of the actin cytoskeleton in non-muscle cells.

Zonula occludens toxin modulates tight junctions through protein kinase C-dependent actin reorganization, in vitro

The intracellular signaling involved in the mechanism of action of zonula occludens toxin (ZOT) was studied using several in vitro and ex vivo models. ZOT showed a selective effect among various cell lines tested, suggesting that it may interact with a specific receptor, whose surface expression on various cells differs. When tested in IEC6 cell monolayers, ZOT-containing supernatants induced a redistribution of the F-actin cytoskeleton. Similar results were obtained with rabbit ileal mucosa, where the reorganization of F-actin paralleled the increase in tissue permeability. In endothelial cells, the cytoskeletal rearrangement involved a decrease of the soluble G-actin pool (-27%) and a reciprocal increase in the filamentous F-actin pool (+22%). This actin polymerization was time- and dose-dependent, and was reversible. Pretreatment with a specific protein kinase C inhibitor, CGP41251, completely abolished the ZOT effects on both tissue permeability and actin polymerization. In IEC6 cells ZOT induced a peak increment of the PKC-alpha isoform after 3 min incubation. Taken together, these results suggest that ZOT activates a complex intracellular cascade of events that regulate tight junction permeability, probably mimicking the effect of physiologic modulator(s) of epithelial barrier function.

The actin-binding properties of the Physarum actin-fragmin complex. Regulation by calcium, phospholipids, and phosphorylation

The actin-binding properties of the actin-fragmin complex from Physarum polycephalum microplasmodia were investigated with respect to regulation by Ca2+, phospholipids, and phosphorylation of the actin subunit by the endogenous actin-fragmin kinase. Fragmin possesses two high affinity actin-binding sites and probably also a third, low affinity site. Its nucleating and F-actin severing activities are inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2). Actin-fragmin specifically binds PIP2 which competes with actin for the Ca(2+)-sensitive site. However, PIP2 cannot dissociate the actin-fragmin complex nor the actin2-fragmin trimer. Efficient F-actin nucleating activity by actin-fragmin is only observed with unphosphorylated actin-fragmin, in the absence of PIP2 and at high Ca2+ (> microM) concentrations. In the presence of PIP2, actin-fragmin only caps actin filaments when unphosphorylated. The results suggest that in the cell, hydrolysis of PIP2, concomitant with the increase of cytosolic Ca2+, could promote subcortical actin polymerization.

Microfilament dynamics: regulation of actin polymerization by actin-fragmin kinase and phosphatases

Based on the phosphorylation of the purified actin-fragmin complex, an 80 kDa monomeric kinase (AFK) has been isolated from Physarum polycephalum. Protein chemical analysis and studies involving kinase inhibitors and effectors establish that the AFK is a unique kinase that cannot be classified so far in one of the conventional kinase families. The actin-fragmin kinase behaves as an "independent" kinase since its activity towards the actin-fragmin complex is apparently not regulated by the binding of a ligand (e.g., the cyclic-nucleotides, Ca2+, calmodulin, phosphatidylserine and diolein). Rigorous screening of the substrate specificity suggests that the actin-fragmin complex represents the only substrate for this kinase. This kinase phosphorylates the actin moiety of the actin-fragmin complex at two consecutive threonine residues which constitute one of the contact sites for DNase I (37) and which are also located at one of the proposed actin-actin contact sites along the long-pitch helix of F-actin (38, 39). The physiological importance of this phosphorylation was demonstrated by studying the effect of phosphorylation on the nucleation and the capping activity of the actin-fragmin complex using fluorescence enhancement analysis. As could be demonstrated, the nucleation of actin filaments by the actin-fragmin complex is completely abolished upon phosphorylation by the AFK. Phosphorylation of the complex also interferes with its capping activity, which becomes Ca(2+)-dependent. In addition, capping and nucleating activity is regulated in vitro by phosphoinositides, of which PIP2 displays the highest activity and specificity. PIP2 partially inhibits the nucleation and capping activity of the unphosphorylated actin-fragmin. The capping activity of the phosphorylated actin-fragmin complex was inhibited by PIP2 to a much greater extent as compared to the unphosphorylated actin-fragmin complex. Among all phospholipids tested, PIP2 displayed the highest specificity. Initial experiments with purified preparations of the PP-1, PP-2A, PP-2B, alkaline phosphatase and acid phosphatases showed that PP-1 and PP-2A phosphatases were capable of dephosphorylating the phospho actin-fragmin complex. These findings raised the question of whether these or other protein phosphatases were involved in the dephosphorylation of this substrate in vivo. To address this question, Physarum extracts were subjected to fractionation by ion exchange chromatography, and the column fractions were assayed in a variety of conditions, to identify the protein phosphatases involved in the dephosphorylation of this substrate and to identify the elution position of the major Ser/Thr protein phosphatases present in the Physarum extract.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of 6-dimethylaminopurine on the length of the cell cycle and on the state of phosphorylation of putative intermediate filament proteins in sea urchin embryos

The effects of 6-dimethylaminopurine (6-DMAP) on the length of the cell cycle and on the state of phosphorylation of a putative intermediate filament protein, p117, have been studied in sea urchin embryos. Embryos were transferred into sea water containing 600 microM 6-DMAP at 0.5, 2 or 5 min after insemination, and incubated for 30 or 90 min. The effects of 6-DMAP on cell cycle length were studied by determining the time required for completion of mitosis upon return of the embryos in normal sea water. In all instances, except for the embryos transferred 0.5 min after insemination (AI) and incubated for 30 min, the duration of the M phase was shortened compared to controls, being faster in the embryos incubated for 90 minutes compared to the 30 min incubation period. However, embryos transferred 0.5 min AI have a longer M-phase than those transferred 2 minutes or later after fertilization, suggesting that between 0.5 and 2 min after fertilization, critical phosphorylating events occur which affect the commitment of the cells to enter M-phase. To study the pattern of p117 phosphorylation during the cell cycle, the eggs were transferred 2 minutes after fertilization in presence of 600 microM 6-DMAP and with 200 microCi/ml of 32P-orthophosphate. Analyses of 32P-labelled proteins after exposure of SDS-PAGE gels and their corresponding blots suggested that phosphorylation of p117 greatly increases at the time of pronuclear fusion, and then declines slightly at prophase-metaphase. This decrease is markedly enhanced when the cells are treated with 6-DMAP during metaphase in order to induce a premature breakdown of the mitotic apparatus. A causal link is suggested between the level of phosphorylation of p117 and its state of assembly.

Purification and partial amino acid sequence of the actin-fragmin kinase from Physarum polycephalum

The 80-kDa actin-fragmin kinase (AFK) was purified from Physarum polycephalum microplasmodia to apparent homogeneity through a procedure involving six chromatographic steps. Taking the activity at the first purification step as 100%, the kinase was purified more than 1500-fold, with an overall yield of 8%. The specific activity of the purified enzyme was 700 U/mg. The total amount of AFK present could be estimated as 34 ng/mg extracted protein. One of the polyclonal antibodies raised against four tryptic peptides of the purified 80-kDa AFK recognized the 80-kDa band in an immunoblot and could inhibit the AFK activity up to 100%. A minor 78-kDa protein, also displaying AFK activity, appeared to be derived from the 80-kDa AFK. A partial amino acid sequence analysis, covering up to 20% of the protein (150 amino acids), was performed and confirmed the lack of similarity with any of the sequenced kinases. These data are in agreement with the unique physical and enzymatic properties of the AFK.

A cytoskeletal mechanism for Ca2+ channel metabolic dependence and inactivation by intracellular Ca2+

Many different types of voltage-dependent Ca2+ channels inactivate when intracellular ATP declines or intracellular Ca2+ rises. An inside-out, patch-clamp technique was applied to the Ca2+ channels of Lymnaea neurons to determine the mechanism(s) underlying these two phenomena. Although no evidence was found for a phosphorylation mechanism, agents that act on the cytoskeleton were found to alter Ca2+ channel activity. The cytoskeletal disrupters colchicine and cytochalasin B were found to speed Ca2+ channel decline in ATP, whereas the cytoskeletal stabilizers taxol and phalloidin were found to prolong Ca2+ channel activity without ATP. In addition, cytoskeletal stabilizers reduced Ca(2+)-dependent channel inactivation, suggesting that both channel metabolic dependence and Ca(2+)-dependent inactivation result from a cytoskeletal interaction.

Neurofilament expression in human T lymphocytes

The expression of intermediate filaments in normal cells is mainly determined by their embryonal developmental origin. Flow cytometry using monoclonal antibody RT97 demonstrated that neurofilament was detectable in the human HuT 78 T-cell line and on resting T lymphocytes. Expression was greatly increased on lymphocytes activated for 3 days with phorbol ester. Western blotting confirmed the presence of the 200,000 MW form of neurofilament in T lymphocytes. Stimulation of peripheral blood T cells with phorbol myristate acetate (PMA) or with anti-CD3 monoclonal antibodies resulted in a marked increase in detection of phosphorylated neurofilament on Western blotting. Stimulation of HuT 78 cells with anti-LFA-1 resulted in redistribution of neurofilament from a perinuclear spheroid core into dendritic processes. These data indicate that T cells activated through the T-cell receptor associated complex express an intermediate filament usually associated with neurally derived cells. The finding that neurofilament expression and organization are regulated by T-cell surface molecules suggests a role for this intermediate filament in T-cell function.

Identification of actin kinase activity in purified fragmin-actin complex

Actin kinase phosphorylates actin of fragmin-actin complex, resulting in the inactivation of the nucleation and capping activities of the complex. Fragmin-actin complex was prepared by a new purification procedure. Incubation with ATP caused inactivation of the purified complex and phosphorylation of actin of fragmin-actin complex. The detailed analysis of the complex by SDS-gel electrophoresis showed that actin kinase was co-purified with the fragmin-actin complex. Formation of such an association between actin kinase and substrate suggests that the kinase is localized on the fragmin-actin complex to efficiently regulate actin cytoskeletons.

Changes in the lateral diffusion of fluorescent lipid analogues in the surface membrane of adult male Schistosoma mansoni

The effect of serotonin on the fluidity of the tegumental membranes of adult male Schistosoma mansoni was assessed by the fluorescence recovery after photobleaching technique. It was demonstrated that the translational diffusion of 5-N'-octadecanoyl aminofluorescein is reduced by a mechanism involving G-protein coupled activation of adenylate cyclase and lowering of intracellular calcium concentration. Furthermore, the lateral diffusion coefficient and the mobile fraction appear to be controlled by calcium and cAMP dependent pathways respectively. No change in the diffusion of the fluorescent phospholipid N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-phosphatidyl choline was observed, suggesting the two probes used here partition into two different domains that are under independent control. An increase in the amount of protein associating with a membrane cytoskeleton is also demonstrated.

Actin-binding proteins

Much new information on the sequence, structure, and function of filament crosslinking, capping, and severing proteins is now known. Other significant findings include identification of a new abundant monomer-sequestering protein in platelets, and evidence that many actin-binding proteins interact with phosphoinositides and that this interaction may have metabolic consequences.