A new class of microtubule-associated proteins in plants.

In plants there are three microtubule arrays involved in cellular morphogenesis that have no equivalent in animal cells. In animals, microtubules are decorated by another class of proteins - the structural MAPS - which serve to stabilize microtubules and assist in their organization. The best-studied members of this class in plants are the MAP-65 proteins that can be purified together with plant microtubules after several cycles of polymerization and depolymerization. Here we identify three similar MAP-65 complementary DNAs representing a small gene family named NtMAP65-1, which encode a new set of proteins, collectively called NtMAP65-1. We show that NtMAP65-1 protein localizes to areas of overlapping microtubules, indicating that it may function in the behaviour of antiparallel microtubules in the mitotic spindle and the cytokinetic phragmoplast.

Cell model systems in plant cytoskeleton studies.

Cytoskeletons play an essential role in cellular functions in both animal and plant cells. In studies of the molecular mechanisms of their functions, a variety of cell model systems, mainly of animal cells, have yielded much information. With plant cells, cell model systems have mostly been restricted to studies on the mechanism of cytoplasmic streaming. Recently, however, there have been several reports of studies employing plant cell model systems to investigate plant cytoskeletons that have revealed new concepts about their structure and functions. To promote and support a general understanding of cell model systems, this review attempts to categorize them, present currently known information on the structure and function of plant cytoskeletons, and offer a possible role of cell model systems in future studies of plant cytoskeletons.

Biochemical and immunocytochemical characterization of two types of myosins in cultured tobacco bright yellow-2 cells

We have isolated a myosin (referred to as 170-kD myosin) from lily pollen tubes, which consists of 170-kD heavy chain and calmodulin (CaM) light chain and is responsible for cytoplasmic streaming. A 170-kD polypeptide that has similar antigenicity to the 170-kD myosin heavy chain of lily pollen tubes was also present in cultured tobacco (Nicotiana tabacum) Bright Yellow-2 (BY-2) cells, and possessed the ability to interact with F-actin in an ATP-dependent manner. In addition to this myosin, we identified biochemically another kind of myosin in BY-2 cells. This myosin consisted of a CaM light chain and a 175-kD heavy chain with antigenicity different from the 170-kD myosin heavy chain. In the present study, we referred to this myosin as 175-kD myosin. This myosin was able to translocate rhodamine-phalloidin (RP)-labeled F-actin at an average velocity of about 9 &mgr;m/s in the motility assay in vitro. In contrast, the sliding velocity of RP-labeled F-actin translocated by fractions containing the 170-kD myosin was 3 to 4 &mgr;m/s. The velocity of cytoplasmic streaming in living BY-2 cells ranged from 2 to 9 &mgr;m/s. The motile activity of 175-kD myosin in vitro was inhibited by Ca(2+) at concentrations higher than 10(-6) M. Immunoblot analyses using an antiserum against the heavy chain of 170- or 175-kD myosin revealed that in tobacco plants, the 175-kD myosin was expressed in leaf, stem, and root, but not in germinating pollen, while 170-kD myosin was present in all of these plant parts and in germinating pollen. These results suggest that the two types of myosins, 170 and 175 kD, presumably participate in cytoplasmic streaming in BY-2 cells and other somatic cells of tobacco plants.

Effects of brefeldin A on the formation of the cell plate in tobacco BY-2 cells.

Treatment with 20 microM brefeldin A (BFA) for 60 min caused the disassembly of the Golgi apparatus in tobacco BY-2 cells, and the effect of BFA was reversible. Connections between Golgi cisternae and the ER were observed in cells that had been treated for 15 min with BFA. BFA applied to cells at metaphase allowed the cells to form aniline blue-positive cell plates but not to complete cytokinesis. BFA seems to inhibit cytokinesis by shutting off the supply of cell-plate materials by disassembling the Golgi apparatus.

A 32-kDa protein associated with phospholipase A2-inhibitory activity from human placenta.

Two monomeric 32-kDa proteins, termed 32K-I (pI 5.8) and 32K-II (pI 5.1), were isolated from human placenta, which was solubilized by a Ca2+-chelator. Only 32K-I was associated with PLA2-inhibitory activity. CNBr peptide mapping indicated that 32K-I was distinct from 32K-II and two 36-kDa proteins, called calpactin I and II or lipocortin II and I, which have been shown to possess PLA2-inhibitory activity. 32K-I bound to PS in a Ca2+-dependent manner. 32K-I was detected in many tissues except brain, cardiac and skeletal muscle.

The G1556S-type tuberin variant suppresses tumor formation in tuberous sclerosis 2 mutant (Eker) rats despite its deficiency in mTOR inhibition.

Tuberin, a tumor-suppressor protein produced by the tuberous sclerosis gene TSC2, downregulates the Rheb-mTOR-S6K pathway (mTOR axis). Comparison of the effects of human tuberin mutations, such as G1556S, suggests that pathways other than the mTOR axis might also be involved in the pathogenesis of tuberous sclerosis. Here we test this possibility using the rat G1556S-type mutation (GSM) and a transgenic Eker (Tsc2 mutant) rat system. Cells expressing GSM-tuberin failed to downregulate the mTOR axis. GSM-tuberin had an altered localization, which underlie its reduced ability to form a complex with hamartin, and a site-specific alteration in phosphorylation status indicating diverse regulation by Akt. GSM-transgenic (GSM-Tg) rats exhibited suppression of macroscopic renal tumors following N-ethyl-N-nitrosourea treatment. Intriguingly, rats with weaker GSM-Tg expression showed microscopic cystic and pre-tumorous lesions that were restricted in size and expansion, although they had hyper-phosphorylation of ribosomal protein S6. These results highlight a novel pathway involving tuberin that regulates tumor suppression independently of the mTOR inhibitory function. Identification of such a novel pathway will provide clear implications for generation of new therapeutic targets in the treatment of these tumors.

New aspects of membrane dynamics of Amoeba proteus contractile vacuole revealed by vital staining with FM 4-64.

The contractile vacuole (CV) cycle of Amoeba proteus has been studied by phase contrast and electron microscopy. However, the understanding of membrane dynamics in this cycle is still poor. In this study, we used live imaging by fluorescence microscopy to obtain new insights. We succeeded in staining the CV with a styryl dye, FM 4-64 (N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide), and obtained the following results. (1) The CV membrane was directly stained with the dye in the external medium when the CV pore opened upon contraction. This indicates that transfer of plasma membrane to the CV does not occur. (2) The membrane dynamics during the CV cycle were elucidated. In particular, the fluorescent CV membrane was maintained as an aggregate just after contraction and the vacuole re-formed from the aggregate. Staining was maintained during continued contraction cycles. We conclude that the CV membrane is maintained during the CV cycle.

Clathrin is involved in organization of mitotic spindle and phragmoplast as well as in endocytosis in tobacco cell cultures.

We previously identified a 175 kDa polypeptide in Lilium longiflorum germinating pollen using a monoclonal antibody raised against myosin II heavy chain from Physarum polycephalum. In the present study, the equivalent polypeptide was also found in cultured tobacco BY-2 cells. Analysis of the amino acid sequences revealed that the 175 kDa polypeptide is clathrin heavy chain and not myosin heavy chain. After staining of BY-2 cells, punctate clathrin signals were distributed throughout the cytoplasm at interphase. During mitosis and cytokinesis, clathrin began to accumulate in the spindle and the phragmoplast and then was intensely concentrated in the cell plate. Expression of the C-terminal region of clathrin heavy chain, in which light chain binding and trimerization domains reside, induced the suppression of endocytosis and the formation of an aberrant spindle, phragmoplast, and cell plate, the likely cause of the observed multinucleate cells. These data strongly suggest that clathrin is intimately involved in the formation of the spindle and phragmoplast, as well as in endocytosis.

In situ synthesis of beta-glucan microfibrils on tobacco plasma membrane sheets.

A major concern in plant morphogenesis is whether cortical microtubules are responsible for the arrangement and action of beta-glucan synthases in the plasma membrane. We prepared isolated plasma membrane sheets with cortical microtubules attached and tested whether beta-glucan synthases penetrated through the membrane to form microfibrils and whether these synthases moved in the fluid membrane along the cortical microtubules. This technique enabled us to examine synthesis of beta-glucan as a fiber with a two-dimensional structure. The synthesis of beta-glucan microfibrils was directed in arrays by cortical microtubules at many loci on the membrane sheets. The microfibrils were mainly arranged along the microtubules, but the distribution of microfibrils was not always parallel to that of the microtubules. The rate of beta-glucan elongation as determined directly on the exoplasmic surface was 620 nm per min. When the assembly of microtubules was disrupted by treatment with propyzamide, the beta-glucans were not deposited in arrays but in masses. This finding shows that the arrayed cortical microtubules are not required for beta-glucan synthesis but are required for the formation of arranged microfibrils on the membrane sheet.

Identification and preliminary characterization of a 65 kDa higher-plant microtubule-associated protein.

Microtubules in plant cells, as in animal cells, are dynamic structures. However, our lack of knowledge about the constituents of microtubules in plant cells has prevented us from understanding the mechanisms that control microtubule dynamics. To characterize some of these constituents, a cytoplasmic extract was prepared from evacuolated protoplasts (miniprotoplasts) of tobacco BY-2 cells, and microtubules were assembled in the presence of taxol and disassembled by cold treatment in the presence of Ca2+ and a high concentration of NaCl. SDS-PAGE analysis of triple-cycled microtubule protein revealed the presence of 120 kDa, 110 kDa and a group of 60-65 kDa polypeptides in addition to tubulin. Since these polypeptides had copolymerized with tubulin, through the three cycles of assembly and disassembly, and they bundle microtubules, we tentatively identified the three polypeptides as microtubule-associated proteins (MAPs). To characterize these factors further, triple-cycled microtubule protein was fractionated by Mono-Q anion-exchange chromatography and the microtubule-bundling activity of each fraction was examined. Fractions having microtubule-bundling activity contained only the 65 kDa MAP, an indication that the 65 kDa MAP is responsible for the bundling of microtubules. Purified 65 kDa MAP formed cross-bridge structures between adjacent microtubules in vitro. Polyclonal antibodies were raised in mice against the 65 kDa MAP. Immunofluorescence microscopy revealed that the 65 kDa MAP colocalized with microtubules in BY-2 cells throughout the cell cycle. Western blotting analysis of extracts from several species of plants suggested that the 65 kDa MAP and/or related peptides are widely distributed in the plant kingdom.

Possible involvement of 65 kda MAP in elongation growth of azuki bean epicotyls.

Although regulation of the dynamics of plant microtubules (MTs) by microtubule-associated proteins (MAPs) has been suggested, the mechanism has not yet been elucidated. As one candidate, a MAP composed of a 65 kDa polypeptide (65 kDa MAP) has been isolated from tobacco cultured cells [Jiang and Sonobe (1993), J. Cell Sci 105: 8911. To investigate the physiological role of the 65 kDa MAP in situ, we analyzed the changes in content and colocalization of this MAP with cortical MTs in relation to elongation growth, using azuki bean epicotyls (Vigna angularis Ohwi et Ohashi). All apical, intermediate, and basal segments prepared from 6 d seedlings showed high growth activity. In 12 d seedlings, growth activity of intermediate and basal segments was low, although that of apical segments was high. The relationship between the growth activity and the orientation of cortical MTs in the epidermal cells was analyzed. Cells could be classified into four types with respect to orientation of cortical MTs: transverse (T), oblique (O), longitudinal (L) to the vertical axis of cells, and random (R). In rapidly growing segments, three types of cells, T, O, L, were observed at similar ratios. In such segments, significant amounts of the 65 kDa MAP were expressed, and it colocalized well with cortical MTs. In segments showing low growth activity, most of the cells showed oblique and longitudinal orientation of cortical MTs. In such segments, the content of the 65 kDa MAP was low. These results suggested involvement of this 65 kDa MAP in regulation of the elongation growth of this epicotyl.

Phosphorylation of Amoeba G-actin and its effect on actin polymerization.

Mass culture of Amoeba proteus enabled us to do biochemical studies on this organism. Actin and profilin were purified from Amoeba to examine actin phosphorylation and polymerization. The apparent molecular weight of Amoeba actin was 44,000, and its isoelectric point was 5.8. The apparent molecular weight of Amoeba profilin was 12,000, and its isoelectric point was 4.9. It reduced the rate of actin polymerization as reported in the cases of profilins from other organisms. A protein of Mr = 44,000 (44 K protein) was phosphorylated in a Ca2+-dependent manner in cell homogenate of Amoeba without being inhibited by calmodulin antagonists. Using the homogenate as a kinase, purified Amoeba G-actin could be phosphorylated in proportion to the amount of actin. However, neither Amoeba F-actin nor rabbit skeletal muscle G-actin was phosphorylated. The phosphorylation of Amoeba actin with a kinase partially purified from A. proteus increased with dilution of the actin concentration. When Amoeba profilin was added, more than 80% of the actin was phosphorylated. By viscometry, electron microscopy, and ultracentrifugation analysis it was demonstrated that Amoeba G-actin phosphorylated in the presence of profilin and kinase did not polymerize in this solution. High-performance liquid chromatography analysis showed that phosphorylated Amoeba actin remained in a monomeric state even under conditions favorable for actin polymerization.

Isolation of cortical MTs from tobacco BY-2 cells.

We isolated the cortical microtubules (CMTs) from tobacco BY-2 cells to identify their components. By centrifugation of protoplasts homogenized in the presence of taxol, a MT-stabilizing reagent, in a density gradient of Percoll, we obtained membranous vesicles to which MTs forming a sheet-like bundle were attached. Rhodamine-conjugated Ricinus communis agglutinin I (RCA-I), a lectin that bound to the surface of protoplasts, stained these vesicles, indicating that they were plasma membrane (PM) vesicles that retained CMTs. CMTs were released by solubilization of PM vesicles with Triton X-100. A sheet-like array of CMTs was retained even after solubilization of PM vesicles. Immunoblot analysis of the isolated CMTs demonstrated the presence of tubulin, actin, the 65 kDa microtubule-associated protein (MAP) and a 130 kDa RCA-I binding protein. Purification of the isolated CMTs by the temperature dependent disassembly-reassembly cycling method revealed four polypeptides, 190, 120, 85 and 65 kDa, co-assembling with CMTs.

Involvement of microtubules in rhizoid differentiation of Spirogyra species.

Some species of Spirogyra form rosette-shaped or rod-shaped rhizoids in the terminal cell of the filaments. In the present study, we analyzed an involvement of microtubules (MTs) in rhizoid differentiation. Before rhizoid differentiation, cortical MTs were arranged transversely to the long axis of cylindrical cells, reflecting the diffuse growth. At the beginning of rhizoid differentiation, MTs were absent from the extreme tip of the terminal cell. In the other area of the cell, however, MTs were arranged transversely to the long axis of the cell. In the fully differentiated rosette-shaped rhizoid, MTs were randomly organized. However, at a younger stage of rosette-shaped rhizoids, MTs were sometimes arranged almost transversely in the lobes of the rosette. In the rod-shaped rhizoid, MTs were arranged almost transversely. MT-destabilizing drugs (oryzalin and propyzamide) induced swelling of rhizoids, and neither rosette-shaped nor rod-shaped rhizoids were formed. The role of MTs in rhizoid differentiation was discussed.

Interaction of actin filaments with the plasma membrane in Amoeba proteus: studies using a cell model and isolated plasma membrane.

We prepared a cell model of Amoeba proteus by mechanical bursting to study the interaction between actin filaments (AFs) and plasma membrane (PM). The cell model prepared in the absence of Ca2+ showed remarkable contraction upon addition of ATP. When the model was prepared in the presence of Ca2+, the cytoplasmic granules formed an aggregate in the central region, having moved away from PM. Although this model showed contraction upon addition of ATP in the presence of Ca2+, less contraction was noted. Staining with rhodamine-phalloidin revealed association of AFs with PM in the former model, and a lesser amount of association in the latter model. The interaction between AFs and PM was also studied using the isolated PM. AFs were associated with PM isolated in the absence of Ca2+, but were not when Ca2+ was present. These results suggest that the interaction between AFs and PM is regulated by Ca2+.

Isolation and characterization of three forms of 36-kDa Ca2+-dependent actin- and phospholipid-binding proteins from human placenta membrane.

We purified three forms of 36-kDa proteins, two monomeric 36-kDa proteins, which had pIs of 7.5 (36K-I) and 6.4 (36K-II), and one 36-kDa complex (36K-C) consisting of two subunits, 36-kDa (pI 7.5) and 12-kDa (pI 5.8), from human placenta membrane. The 36-kDa subunit of 36K-C was identical to 36K-I as judged by pI, cyanogen bromide peptide mapping and immunological cross-reactivity. The three proteins showed F-actin- and phosphatidylserine-binding abilities dependent on Ca2+ concentrations at millimolar and micromolar levels, respectively. They all had phospholipase A2 inhibitory activity. Only 36K-II was phosphorylated extensively at tyrosine residue in Ca2+- and EGF- dependent manners in the membrane fraction of A431 cells. 36K-I was the best substrate for src kinase, whereas 36K-II was the best for fps kinase. However, 36K-C was not phosphorylated by any kinases used here.

Characterizations of two distinct Ca2+-dependent phospholipid-binding proteins of 68-kDa isolated from human placenta.

Two distinct 68-kDa proteins, named 68K-I (pI 6.4) and 68K-II (pI 5.6), were solubilized from human placenta by treatment with 5 mM EGTA. On DE52 cellulose column chromatography at pH 7.4, 68K-I in the EGTA eluate was recovered in the unadsorbed fractions, whereas 68K-II was retained on the column and eluted with 0.2 M NaCl. The 68K-I protein was obtained in more than 95% purity by further hydroxylapatite and cation exchange chromatographies, while the 68K-II protein was purified further by gel filtration and hydroxylapatite chromatographies. Partial amino acid sequence data showed that 68K-I protein was a novel protein which shared the same sequences as lipocortin I and that 68K-II was the same as human p68/67-kDa calelectrin (Crompton, M. R., Owens, R. J., Totty, N. F., Moss, S. E., Waterfield, M.D., and Crumpton, M. J. (1988) EMBO J. 7, 21-27; Südhof, T. C., Slaughter, C. A., Leznicki, I., Barjon, P., and Reynolds, G. A. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 664-668). The two proteins bound to acidic phospholipids, phosphatidylserine, and/or phosphatidylinositol, but not to phosphatidylcholine, in the presence of micromolar levels of Ca2+. 68K-I bound to phosphatidylinositol preferentially to phosphatidylserine, whereas 68K-II bound only to phosphatidylserine. Both 68K-I and 68K-II inhibited phospholipase A2 activity, and the inhibition by 68K-II was detectable only in the presence of 100 mM KCl. 68K-I, but not 68K-II, was found to bind to F-actin in a Ca2+-dependent (1 mM) manner. Moreover 68K-I, but not 68K-II, was phosphorylated in vitro at tyrosine residues by fps kinase and by epidermal growth factor receptor/kinase, the latter reaction being dependent on Ca2+ and epidermal growth factor. Western blot analysis with affinity purified anti-68K-I and anti-68K-II antibodies showed that 68K-I was located in only certain tissues, especially human placenta, whereas 68K-II was present in many human and rat tissues.

The role of plant villin in the organization of the actin cytoskeleton, cytoplasmic streaming and the architecture of the transvacuolar strand in root hair cells of Hydrocharis.

In many types of plant cell, bundles of actin filaments (AFs) are generally involved in cytoplasmic streaming and the organization of transvacuolar strands. Actin cross-linking proteins are believed to arrange AFs into the bundles. In root hair cells of Hydrocharis dubia (Blume) Baker, a 135-kDa polypeptide cross-reacted with an antiserum against a 135-kDa actin-bundling protein (135-ABP), a villin homologue, isolated from lily pollen tubes. Immunofluorescence microscopy revealed that the 135-kDa polypeptide co-localized with AF bundles in the transvacuolar strand and in the sub-cortical region of the cells. Microinjection of antiserum against 135-ABP into living root hair cells induced the disappearance of the transvacuolar strand. Concomitantly, thick AF bundles in the transvacuolar strand dispersed into thin bundles. In the root hair cells, AFs showed uniform polarity in the bundles, which is consistent with the in-vitro activity of 135-ABP. These results suggest that villin is a factor responsible for bundling AFs in root hair cells as well as in pollen tubes, and that it plays a key role in determining the direction of cytoplasmic streaming in these cells.

Isolation of a novel 190 kDa protein from tobacco BY-2 cells: possible involvement in the interaction between actin filaments and microtubules.

Interaction between actin filaments (AFs) and microtubules (MTs) has been reported in various plant cells, and the presence of a factor(s) connecting these two cytoskeletal networks has been suggested, but its molecular entity has not been elucidated yet. We obtained a fraction containing MT-binding polypeptides, which induced bundling of AFs and of MTs. A 190 kDa polypeptide which associated with AFs was selectively isolated from the fraction. This polypeptide was thought to have an ability to bind to both AFs and MTs. We raised a monoclonal antibody against the 190 kDa polypeptide. Immunostaining demonstrated the association of the 190 kDa polypeptide with AF bundles and with MT bundles formed in vitro. Immunocytochemical studies throughout the cell cycle revealed that the 190 kDa polypeptide was localized in the nucleus before nuclear envelope breakdown, and in the spindle and the phragmoplast during cell division. After the re-formation of the nuclear envelope, the 190 kDa polypeptide was sequestered to the daughter nuclei. Using the antibody, we succeeded in cloning a cDNA encoding the 190 kDa polypeptide.