Reconstitution of intermediate filaments from a higher plant

Cell cycle-dependent dynamics of a plant intermediate filament motif protein with intracellular localization related to microtubules

Although intermediate filaments (IFs) are biochemically and immunologically suggested to exist in plant cells, there are few molecular genetic studies related to the proteins that form these structures. In this study, Arabidopsis AT3G05270 was selected as a candidate gene for a protein constituting IF in plant cells. The protein encoded by AT3G05270 has a large α-helix as well as the IF protein motif indispensable for maintaining the structures of IF. Moreover, fluorescence signals of this protein fused with GFP exhibited cytoskeleton-like filamentous structures in plant cells. Thus, we named the protein encoded by AT3G05270 as Intermediate Filament Motif Protein 1 (IFMoP1). The structures composed of IFMoP1 and their localizations were examined in IFMoP1-GFP-expressing tobacco BY-2 cells whose cell cycle was synchronized using aphidicolin, a DNA synthesis inhibitor, and propyzamide, a microtubule-disrupting agent. The IFMoP1-GFP signals were present at the spindles and phragmoplasts in the mitotic phase. In addition, the frequency of cells with cytoskeleton-like filamentous structures composed of IFMoP1-GFP increased with the increase in cells that completed cell division, and then decreased after several hours. In terms of the relationship in intracellular localization between IFMoP1 and microtubules, the filamentous structures composed of IFMoP1 were present independently of microtubules during interphase. In living cells, these filamentous structures moved along with the nucleus. IFMoP1 co-localized with spindle and phragmoplast microtubules during mitosis, as well as with a part of the cortical microtubules in interphase.

Biochemical and immunological characterization of pea nuclear intermediate filament proteins

In immunoblot assays, at least three putative nuclear intermediate filament (NIF) proteins were detected in nuclear envelope-matrix (NEM) and lamin (L1) fractions of nuclei from plumules of dark-grown pea (Pisum sativum L.) seedlings. These NIF proteins had apparent molecular masses of ca. 65, 60, and 54 kDa (also referred to as p65, p60, and p54), and appeared as multiple isoelectric forms, with pIs ranging from ca. 4.8 to 6.0. Polyclonal and monoclonal antibodies were raised to the 65-kDa NIF protein bands excised from gels after electrophoresis. These anti-pea antibodies were specifically cross-reactive with the pea nuclear p65, p60, and p54 proteins and also with chicken lamins. Sequence alignment of peptide fragments obtained from the 65- and 60-kDa pea NIF proteins showed similarity with animal intermediate filament proteins such as lamins and keratins and with certain plant proteins predicted to have long coiled-coil domains. These pea NIF proteins were further purified and enriched from the NEM fraction using methods similar to those used for isolating animal lamins. When negatively stained and viewed by transmission electron microscopy, the filaments in the pea lamin (L1) fraction appeared to be 6-12 nm in diameter. As assayed by immunofluorescence cytochemistry using a confocal laser-scanning microscope, fixed pea plumule cells displayed uniform as opposed to peripheral nuclear staining by several of the antibody preparations, both polyclonal and monoclonal. This report describes the biochemical and immunological properties of these pea NIF proteins.

How I learned to love carrots: the role of the cytoskeleton in shaping plant cells

The carrot cell suspension was originally used because it provided a model system for studying directional cell expansion - a key process in plant morphogenesis. Early immunofluorescence studies of plant microtubules, using these cells, provided hints that the cortical array of microtubules was dynamic and this was later confirmed by microinjection studies on plant epidermal cells. A nonfixation approach for detecting F-actin was then developed on these cells and showed that, unlike animal cells, actin filaments remained associated with the nucleus throughout division and could have a role in aligning the plane of cell division. Currently, we are using detergent-extracted carrot cytoskeletons for isolating microtubule-associated proteins (MAPs). I discuss how MAPs may be involved in the oriented deposition of cellulose in the cell wall.

Contemplating the plasmalemmal control center model

An abundant epidermal mechanosensory calcium-selective ion channel appears able not only to detect mechanical stimuli such as those that initiate gravitropism but also to detect thermal, electrical, and various chemical stimuli. Because it responds to multimodal input with a second messenger output, this channel system seems likely to be an integrator that can engage in feedbacks with many other systems of the cell--and feedback is the hallmark of regulation. In general, the mechanical tension required for channel activation is likely transmitted from the relatively rigid cell wall to the plasma membrane system via linkage or adhesion sites that display antigenicities recognized by antibodies to animal beta-1 integrin, vitronectin, and fibronectin and which have mechanical connections to the cytoskeleton. Thus, functionally, leverage exerted against any given adhesion site will tend to control channels within a surrounding domain. Reactions initiated by passage of calcium ions through the channels could presumably be more effectively regulated if channels within the domains were somewhat clustered and if appropriate receptors, kinases, porters, pumps, and some key cytoskeletal anchoring sites were in turn clustered about them. Accumulating evidence suggests not only that activity of clusters of channels may contribute to control of cytoskeletal architecture and of regulatory protein function within their domain, but also that both a variety of regulatory proteins and components of the cortical cytoskeleton may contribute to control of channel activity. The emerging capabilities of electronic optical microscopy are well suited for resolving the spatial distributions of many of these cytoskeletal and regulatory molecules in living cells, and for following some of their behaviors as channels are stimulated to open and cytosolic calcium builds in their vicinity. Such microscopy, coupled with biochemical and physiological probing, should help to establish the nature of the feedback loops putatively controlled by the linkage sites and their channel domains.

Immunological characterization of lamins in the nuclear matrix of onion cells

We have used polyclonal and monoclonal antibodies against different lamins from vertebrates, and the IFA antibody recognizing all kinds of intermediate filament proteins, to investigate the lamins of the nuclear matrix of Allium cepa meristematic root cells. All the antibodies react in the onion nuclear matrix with bands in the range of 60–65 kDa, which are enriched in the nuclear matrix after urea extraction, and do not crossreact with other antibodies recognizing intermediate filaments in plants (AFB, anti-vimentin and MAC 322), ruling out crossreaction with contaminating intermediate filaments of cytoplasmic bundles. In 2-D blots the chicken anti-lamin serum reacts with one spot at 65 kDa and pI 6.8 and the anti B-type lamin antibodies with another one at 64 kDa and pI 5.75. Both crossreact with IFA. The lamin is localized at the nuclear periphery and the lamina by indirect immunofluorescence. Immunogold labelling of nuclear matrix sections reveals that the protein is not only associated with the lamina, but also with the internal matrix. Taken together these results reveal that higher plants, which do not possess an organized network of cytoplasmic intermediate filaments, nevertheless present a well-organized lamina containing lamins in which at least one of them is immunologically related to vertebrate lamin B. Our data confirm that lamins are very old members of the intermediate filament proteins that have been better conserved in plants during evolution than their cytoplasmic counterparts.

Casein kinase II protein kinase is bound to lamina-matrix and phosphorylates lamin-like protein in isolated pea nuclei

A casein kinase II (CK II)-like protein kinase was identified and partially isolated from a purified envelope-matrix fraction of pea (Pisum sativum L.) nuclei. When [gamma-32P]ATP was directly added to the envelope-matrix preparation, the three most heavily labeled protein bands had molecular masses near 71, 48, and 46 kDa. Protein kinases were removed from the preparation by sequential extraction with Triton X-100, EGTA, 0.3 M NaCl, and a pH 10.5 buffer, but an active kinase still remained bound to the remaining lamina-matrix fraction after these treatments. This kinase had properties resembling CK II kinases previously characterized from animal and plant sources: it preferred casein as an artificial substrate, could use GTP as efficiently as ATP as the phosphoryl donor, was stimulated by spermine, was calcium independent, and had a catalytic subunit of 36 kDa. Some animal and plant CK II kinases have regulatory subunits near 29 kDa, and a lamina-matrix-bound protein of this molecular mass was recognized on immunoblot by anti-Drosophila CK II polyclonal antibodies. Also found associated with the envelope-matrix fraction of pea nuclei were p34cdc2-like and Ca(2+)-dependent protein kinases, but their properties could not account for the protein kinase activity bound to the lamina. The 71-kDa substrate of the CK II-like kinase was lamin A-like, both in its molecular mass and in its cross-reactivity with anti-intermediate filament antibodies. Lamin phosphorylation is considered a crucial early step in the entry of cells into mitosis, so lamina-bound CK II kinases may be important control points for cellular proliferation.

Intermediate filament antigens of 60 and 65 kDa in the nuclear matrix of plants: their detection and localization

Although the presence of a matrix in plant nuclei has been reported, major questions remain about its structural and biochemical features. We have used an intermediate filament antibody of broad specificity to explore whether Daucus carota (carrot) nuclei and nuclear matrices contain intermediate filament/lamin antigens and, if so, where specifically they are localized. SDS-PAGE and Western blotting revealed two bands, at 60 and 65 kDa, that were highly immunoreactive with the intermediate filament antibody (IFA) of Pruss et al. (1981, Cell 27, 419-428). This pattern was observed consistently, not only with carrot cell-free nuclei and nuclear matrices, but also with nuclear preparations from Vicia faba (broad bean) and Pisum sativum (pea). Immunofluorescence studies with whole carrot nuclei localized the IFA antigens to the nucleoplasm and disclosed no accentuated peripheral labeling. Agarose-embedded nuclear matrices showed not only fluorescence throughout the nucleoplasm but also heavy labeling surrounding the nucleoli and suggestions of peripheral labeling. At the ultrastructural level, immunogold results from pre- and postembedment treatments supported the conclusion that IFA antigens occur throughout the nucleoplasm, with possibly a slight concentration at the periphery. These combined results provide substantial evidence that plant nuclei and their matrices possess at least two major intermediate filament antigens with molecular weights characteristic of animal lamins. Whether or not these antigens represent plant lamins, their nonperipheral localization hints at significant differences among the eukaryotic kingdoms in nuclear organization.

The plant cytoskeleton

Significant progress has been made in four areas: in appreciating the speed with which cortical microtubules reorient in response to environmental signals; in a consolidated understanding of the cytoskeletal nature of the phragmosome--the device that predicts and structures the division plane in vacuolated cells; in the description of new cytoskeletal proteins; and in reports that herald an attack on the cell cycle control of cytoskeletal organization.

The pollen tube cytoskeleton

In the last few years the role of pollen and the pollen tube in the fertilization process in higher plants has received considerable attention. By ultrastructural, biochemical and immunofluorescent investigations it has been shown that a cytoskeletal apparatus plays a central role in pollen tube growth. Microfilaments and microtubules, in which main components are, respectively, actin and tubulin, represent the most investigated cytoskeletal components. New information has been recently provided by the identification of myosin and also of a kinesin-like protein. The pollen tube cytoskeleton consists of two different cytoskeletal systems: the vegetative cell cytoskeleton, namely the cytoskeleton of the pollen grain and pollen tube, and the gamete cytoskeleton (generative cell and sperm cell cytoskeleton). The vegetative cell cytoskeleton plays a fundamental role in assuring the cytoplasmic movement of organelles, vesicles and gametes from the pollen grain to the pollen tube apex and consists mainly of microtubules and microfilaments. Also myosin and the kinesin-like protein are involved in the process of organelle and vesicle movement. The gamete cytoskeleton has a central role in sperm cell formation and in the reshaping process during gamete movement inside the pollen tube. It consists mostly of microtubules and partially characterized microtubule-associated structures. Actin filaments have recently also been identified.