Calcium Levels Affect the Ability to Immunolocalize Calmodulin to Cortical Microtubules

Review: Microtubules monitor calcium and reactive oxygen species signatures in signal transduction

Signal transductions require calcium (Ca²⁺) or reactive oxygen species (ROS) signatures, which act as chemical and electrical signals in response to various biotic and abiotic stresses. Calcium as an ion or second messenger affects the membrane potential and microtubules (MTs) dynamicity, while MTs can modulate auto-propagating waves of calcium and ROS signatures in collaboration with ion channels depending on the stimulus type. Thus, in the current review, we highlight advances in research focused on the relationship between dynamic MTs and calcium and ROS signatures in short-distance transmission. The challenges of Ca²⁺-MTs-ROS crosstalk in cold sensing are addressed, which could suggest the prioritization of ROS or Ca²⁺ in signalling.

Cold sensing in grapevine-Which signals are upstream of the microtubular "thermometer"

Plants can acquire freezing tolerance in response to cold but non-freezing temperatures. To efficiently activate this cold acclimation, low temperature has to be sensed and processed swiftly, a process that is linked with a transient elimination of microtubules. Here, we address cold-induced microtubules elimination in a grapevine cell line stably expressing a green fluorescent protein fusion of Arabidopsis TuB6, which allows to follow their response in vivo and to quantify this response by quantitative image analysis. We use time-course studies with several specific pharmacological inhibitors and activators to dissect the signalling events acting upstream of microtubules elimination. We find that microtubules disappear within 30 min after the onset of cold stress. We provide evidence for roles of calcium influx, membrane rigidification, and activation of NAD(P)H oxidase as factors in signal susception and amplification. We further conclude that a G-protein in concert with a phospholipase D convey the signal towards microtubules, whereas calmodulin seems to be not involved. Moreover, activation of jasmonate pathway in response to cold is required for an efficient microtubule response. We summarize our findings in a working model on a complex signalling hub at the membrane-cytoskeleton interphase that assembles the susception, perception and early transduction of cold signals.

The Cytoskeleton and Its Regulation by Calcium and Protons

Calcium and protons exert control over the formation and activity of the cytoskeleton, usually by modulating an associated motor protein or one that affects the structural organization of the polymer.

Modulation of Root Skewing in Arabidopsis by Apyrases and Extracellular ATP

When plant primary roots grow along a tilted surface that is impenetrable, they can undergo a slanted deviation from the direction of gravity called skewing. Skewing is induced by touch stimuli which the roots experience as they grow along the surface. Touch stimuli also induce the release of extracellular ATP (eATP) into the plant's extracellular matrix, and two apyrases (NTPDases) in Arabidopsis, APY1 and APY2, can help regulate the concentration of eATP. The primary roots of seedlings overexpressing APY1 show less skewing than wild-type plants. Plants suppressed in their expression of APY1 show more skewing than wild-type plants. Correspondingly, chemical inhibition of apyrase activity increased skewing in mutants and wild-type roots. Exogenous application of ATP or ATPγS also increased skewing in wild-type roots, which could be blocked by co-incubation with a purinergic receptor antagonist. These results suggest a model in which gradients of eATP set up by differential touch stimuli along roots help direct skewing in roots growing along an impenetrable surface.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

Altered patterns of tubulin polymerization in dividing leaf cells of Chlorophyton comosum after a hyperosmotic treatment

•  Microtubule organization and tubulin polymerization in meristematic leaf cells of Chlorophyton comosum treated with an aqueous solution of 1 M mannitol, inducing plasmolysis, were examined with immunofluorescence and transmission electron microscopy. •  Hyperosmotic treatment induced disintegration of the interphase microtubule systems. Free tubulin, either liberated from the depolymerized microtubules or pre-existing as a nonassembled pool, was incorporated into a network of paracrystals. In most of the dividing cells, mitotic and cytokinetic microtubule systems were replaced by atypical spindle-like structures displaying bipolarity and atypical phragmoplasts, respectively. These atypical mitotic and cytokinetic structures consisted of large densely packed bundles of macrotubules (32 nm diameter) or macrotubules and paracrystals. Tubulin paracrystals also occurred in ectopic positions in plasmolysed mitotic and cytokinetic cells. Dividing cells displaying paracrystals only did not form atypical mitotic and cytokinetic apparatuses. •  Short hyperosmotic stress causes disintegration of all microtubule arrays in dividing cells of C. comosum. Free tubulin is incorporated into macrotubules and tubulin paracrystals. The latter exhibit definite periodicity and characteristic fine structure.

Calcium-calmodulin suppresses the filamentous actin-binding activity of a 135-kilodalton actin-bundling protein isolated from lily pollen tubes

We have isolated a 135-kD actin-bundling protein (P-135-ABP) from lily (Lilium longiflorum) pollen tubes and have shown that this protein is responsible for bundling actin filaments in lily pollen tubes (E. Yokota, K. Takahara, T. Shimmen [1998] Plant Physiol 116: 1421-1429). However, only a few thin actin-filament bundles are present in random orientation in the tip region of pollen tubes, where high concentrations of Ca(2+) have also been found. To elucidate the molecular mechanism for the temporal and spatial regulation of actin-filament organization in the tip region of pollen tubes, we explored the possible presence of factors modulating the filamentous actin (F-actin)-binding activity of P-135-ABP. The F-actin-binding activity of P-135-ABP in vitro was appreciably reduced by Ca(2+) and calmodulin (CaM), although neither Ca(2+) alone nor CaM in the presence of low concentrations of Ca(2+) affects the activity of P-135-ABP. A micromolar order of Ca(2+) and CaM were needed to induce the inhibition of the binding activity of P-135-ABP to F-actin. An antagonist for CaM, W-7, cancelled this inhibition. W-5 also alleviated the inhibition effect of Ca(2+)-CaM, however, more weakly than W-7. These results suggest the specific interaction of P-135-ABP with Ca(2+)-CaM. In the presence of both Ca(2+) and CaM, P-135-ABP organized F-actin into thin bundles, instead of the thick bundles observed in the absence of CaM. These results suggest that the inhibition of the P-135-ABP activity by Ca(2+)-CaM is an important regulatory mechanism for organizing actin filaments in the tip region of lily pollen tubes.

Bundling of microtubules by motor and tail domains of a kinesin-like calmodulin-binding protein from Arabidopsis: regulation by Ca(2+)/Calmodulin

Kinesin-like calmodulin-binding protein (KCBP), a novel kinesin-like protein from plants, is unique among kinesins and kinesin-like proteins in having a calmodulin-binding domain adjacent to its motor domain. KCBP localizes to mitotic microtubule (MT) arrays including the preprophase band, the spindle apparatus, and the phragmoplast, suggesting a role for KCBP in establishing these MT arrays by bundling MTs. To determine if KCBP bundles MTs, we expressed C-terminal motor and N-terminal tail domains of KCBP, and used the purified proteins in MT bundling assays. The 1.5 C protein with the motor and calmodulin-binding domains induced MT bundling. The 1.5 C-induced bundles were dissociated in the presence of Ca(2+)/calmodulin. Similar results were obtained with a 1.4 C protein, which lacks much of the coiled-coil region present in 1.5 C protein and does not form dimers. The N-terminal tail of KCBP, which contains an ATP-independent MT binding site, is also capable of bundling MTs. These results, together with the KCBP localization data, suggest the involvement of KCBP in establishing mitotic MT arrays during different stages of cell division and that Ca(2+)/calmodulin regulates the formation of these MT arrays.

Microtubule-organizing centers and nucleating sites in land plants

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

A kinesin-like protein, KatAp, in the cells of arabidopsis and other plants

The kinesin-like proteins (KLPs) are a large family of plus- or minus-end-directed microtubule motors important in intracellular transport, mitosis, meiosis, and development. However, relatively little is known about plant KLPs. We prepared an antibody against two peptides in the microtubule binding domain of an Arabidopsis KLP (KatAp) encoded by the KatA gene, one of a family of genes encoding KLPs whose motor domain is located near the C terminus of the polypeptide. Such KLPs typically move materials toward the minus end of microtubules. An immunoreactive band (Mr of 140,000) corresponding to KatAp was demonstrated with this antibody on immunoblots of Arabidopsis seedling extracts. During immunofluorescence localizations, the antibody produced weak, variable staining in the cytoplasm and nucleus of interphase Arabidopsis suspension cells but much stronger staining of the mitotic apparatus during division. Staining was concentrated near the midzone during metaphase and was retained there during anaphase. The phragmoplast was also stained. Similar localization patterns were seen in tobacco BY-2 cells. The antibody produced a single band (Mr of 130,000) in murine brain fractions prepared according to procedures that enrich for KLPs (binding to microtubules in the presence of AMP-PNP but not ATP). A similar fraction from carrot suspension cells yielded a cross-reacting polypeptide of similar apparent molecular mass. When dividing BY-2 cells were lysed in the presence of taxol and ATP, antibody staining moved rapidly toward the poles, supporting the presence of a minus-end motor. Movement did not occur without ATP, with AMP-PNP, or with ATP plus antibody. Our results indicate that the protein encoded by KatA, KatAp, is expressed in Arabidopsis and is specifically localized to the midzone of the mitotic apparatus and phragmoplast. A similar protein is also present in other species.

A kinesin-like protein, KatAp, in the cells of arabidopsis and other plants

The kinesin-like proteins (KLPs) are a large family of plus- or minus-end-directed microtubule motors important in intracellular transport, mitosis, meiosis, and development. However, relatively little is known about plant KLPs. We prepared an antibody against two peptides in the microtubule binding domain of an Arabidopsis KLP (KatAp) encoded by the KatA gene, one of a family of genes encoding KLPs whose motor domain is located near the C terminus of the polypeptide. Such KLPs typically move materials toward the minus end of microtubules. An immunoreactive band (Mr of 140,000) corresponding to KatAp was demonstrated with this antibody on immunoblots of Arabidopsis seedling extracts. During immunofluorescence localizations, the antibody produced weak, variable staining in the cytoplasm and nucleus of interphase Arabidopsis suspension cells but much stronger staining of the mitotic apparatus during division. Staining was concentrated near the midzone during metaphase and was retained there during anaphase. The phragmoplast was also stained. Similar localization patterns were seen in tobacco BY-2 cells. The antibody produced a single band (Mr of 130,000) in murine brain fractions prepared according to procedures that enrich for KLPs (binding to microtubules in the presence of AMP-PNP but not ATP). A similar fraction from carrot suspension cells yielded a cross-reacting polypeptide of similar apparent molecular mass. When dividing BY-2 cells were lysed in the presence of taxol and ATP, antibody staining moved rapidly toward the poles, supporting the presence of a minus-end motor. Movement did not occur without ATP, with AMP-PNP, or with ATP plus antibody. Our results indicate that the protein encoded by KatA, KatAp, is expressed in Arabidopsis and is specifically localized to the midzone of the mitotic apparatus and phragmoplast. A similar protein is also present in other species.

Characterization of a monoclonal antibody prepared against plant actin

Anti-actin monoclonal antibodies were prepared using phalloidin-stabilized actin that was purified from pea roots by DNase I affinity chromatography. One monoclonal antibody, designated mAb3H11, bound plant actin in preliminary screenings and was further analyzed. Immunoblot analysis showed that this antibody had a high affinity for plant actin in crude and purified preparations but a low affinity for rabbit muscle actin. In immunoblots of plant extracts separated on two-dimensional gels it appeared to bind all actin isoforms recognized by the JLA20 anti-chicken actin antibody. Using immunofluorescent cytochemistry, the antibody was used to observe actin filaments in aldehyde-fixed and methanol-treated tobacco protoplasts. These results indicate that mAb3H11 should be a useful reagent for the study of plant actins.