玉米根细胞中类整合素蛋白与α-微管蛋白的共定位及其可能的相互作用

采用间接免疫荧光标记法对玉米根细胞中的类整合素蛋白和细胞骨架主要组分之一的α-微管蛋白进行了荧光定位。结果表明：类整合素蛋白主要分布在质膜上。与对照相比,用与类整合素蛋白特异结合的5肽GRGDS处理后,质膜上类整合素的分布更为均匀,微管的排列密度降低,而用不与类整合素蛋白特异结合的GRGDS类似物SDGRG处理则对类整合素蛋白分布和微管蛋白的排列均无明显影响。微管蛋白解聚剂或稳定剂处理改变类整合素在质膜上的分布。这些结果表明类整合素蛋白与微管蛋白间有复杂的相互作用。     

The filament-forming protein Pil1 assembles linear eisosomes in fission yeast

The cortical cytoskeleton mediates a range of cellular activities such as endocytosis, cell motility, and the maintenance of cell rigidity. Traditional polymers, including actin, microtubules, and septins, contribute to the cortical cytoskeleton, but additional filament systems may also exist. In yeast cells, cortical structures called eisosomes generate specialized domains termed MCCs to cluster specific proteins at sites of membrane invaginations. Here we show that the core eisosome protein Pil1 forms linear cortical filaments in fission yeast cells and that purified Pil1 assembles into filaments in vitro. In cells, Pil1 cortical filaments are excluded from regions of cell growth and are independent of the actin and microtubule cytoskeletons. Pil1 filaments assemble slowly at the cell cortex and appear stable by time-lapse microscopy and fluorescence recovery after photobleaching. This stability does not require the cell wall, but Pil1 and the transmembrane protein Fhn1 colocalize and are interdependent for localization to cortical filaments. Increased Pil1 expression leads to cytoplasmic Pil1 rods that are stable and span the length of cylindrical fission yeast cells. We propose that Pil1 is a novel component of the yeast cytoskeleton, with implications for the role of filament assembly in the spatial organization of cells.     

Short-Term Boron Deprivation Induces Increased Levels of Cytoskeletal Proteins in Arabidopsis Roots

Abstract: Although boron is known to be an essential element for the growth of all higher plants, the links between primary responses to boron deprivation and the expression of visual symptoms are yet unknown. Western blots with anti-actin and anti-tubulin antibodies revealed an increase of both proteins upon 20 - 40 min of boron deprivation in roots of hydroponically grown Arabidopsis thaliana. Moreover, actin depolymerizing factor and myosin VIII showed a less pronounced but similiar response to boron deficiency. In contrast, no increase in higher molecular mass ubiquitin was observed, indicating an absence of intensive protein degradation during the experimental time span. This is the first report of cytoskeletal responses of plants to short-term boron removal. Rapid elevation of cytoskeletal proteins after boron deprivation is discussed in relation to the cell wall-plasma membrane-cytoskeleton continuum.     

Cytoskeletal organization during xylem cell differentiation

The water and mineral conductive tube, the xylem vessel and tracheid, is a highly conspicuous tissue due to its elaborately patterned secondary-wall deposition. One constituent of the xylem vessel and tracheid, the tracheary element, is an empty dead cell that develops secondary walls in the elaborate patterns. The wall pattern is appropriately regulated according to the developmental stage of the plant. The cytoskeleton is an essential component of this regulation. In fact, the cortical microtubule is well known to participate in patterned secondary cell wall formation. The dynamic rearrangement of the microtubules and actin filaments have also been recognized in the cultured cells differentiating into tracheary elements in vitro. There has recently been considerable progress in our understanding of the dynamics and regulation of cortical microtubules, and several plant microtubule associated proteins have been identified and characterized. The microtubules have been observed during tracheary element differentiation in living Arabidopsis thaliana cells. Based on this recently acquired information on the plant cytoskeleton and tracheary element differentiation, this review discusses the role of the cytoskeleton in secondary cell wall formation.     

Integrin-like Protein Is Involved in the Osmotic Stress-induced Abscisic Acid Biosynthesis in Arabidopsis thaliana

We studied the perception of plant cells to osmotic stress that leads to the accumulation of abscisic acid （ABA） in stressed Arabidopsis thaliana L. cells. A significant difference was found between protoplasts and cells in terms of their responses to osmotic stress and ABA biosynthesis, implying that cell wall and/or cell wall-plasma membrane interaction are essential in identifying osmotic stress. Western blotting and immunofluorescence localization experiments, using polyclonal antibody against human integrin β1, revealed the existence of a protein similar to the integrin protein of animals in the suspension-cultured cells located in the plasma membrane fraction. Treatment with a synthetic pentapeptide, Gly-Arg-Gly-Asp-Ser （GRGDS）, which contains an RGD domain and interacts specifically with integrin protein and thus blocks the cell wall-plasma membrane interaction, significantly inhibited osmotic stress-induced ABA biosynthesis in cells, but not in protoplasts. These results demonstrate that cell wall and/or cell wall-plasma membrane interaction mediated by integrin-Iike proteins played important roles in osmotic stress-induced ABA biosynthesis in Arabidopsis thaliana.     

Emerging role for ERM proteins in cell adhesion and migration

The highly related ERM (Ezrin, Radixin, Moesin) proteins provide a regulated linkage between the membrane and the underlying actin cytoskeleton. They also provide a platform for the transmission of signals in responses to extracellular cues. Studies in different model organisms and in cultured cells have highlighted the importance of ERM proteins in the generation and maintenance of specific domains of the plasma membrane. A central question is how do ERM proteins coordinate actin filament organization and membrane protein transport/stability with signal transduction pathways to build up complex structures? Through their interaction with numerous partners including membrane proteins, actin cytoskeleton and signaling molecules, ERM proteins have the ability to organize multiprotein complexes in specific cellular compartments. Likewise, ERM proteins participate in diverse functions including cell morphogenesis, endocytosis/exocytosis, adhesion and migration. This review focuses on aspects still poorly understood related to the function of ERM proteins in epithelial cell adhesion and migration.Key words: epithelial cells, membrane-cytoskeleton interface, morphogenesis, ERM proteins, cell adhesion     

PIP5K-dependent production of PIP2 sustains microtubule organization to establish polarized transport in the Drosophila oocyte

The attachment of the cytoskeleton to the plasma membrane is crucial in controlling the polarized transport of cell-fate-determining molecules. Attachment involves adaptor molecules, which have the capacity to bind to both the plasma membrane and elements of the cytoskeleton, such as microtubules and actin filaments. Using the Drosophila oocyte as a model system, we show that the type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K), Skittles, is necessary to sustain the organization of microtubules and actin cytoskeleton required for the asymmetric transport of oskar, bicoid and gurken mRNAs and thereby controls the establishment of cell polarity. We show that Skittles function is crucial to synthesize and maintain phosphatidylinositol 4,5 bisphosphate (PIP2) at the plasma membrane in the oocyte. Reduction of Skittles activity impairs activation at the plasma membrane of Moesin, a member of the ERM family known to link the plasma membrane to the actin-based cytoskeleton. Furthermore, we provide evidence that Skittles, by controlling the localization of Bazooka, Par-1 and Lgl, but not Lkb1, to the cell membrane, regulates PAR polarity proteins and the maintenance of specific cortical domains along the anteroposterior axis.     

Microtubular and actin cytoskeletons and ultrastructural characteristics of the potentially pathogenic basidiomycetous yeast Malassezia pachydermatis

Microtubular and actin cytoskeletons were investigated in the lipophilic yeast Malassezia pachydermatis by fluorescence and electron microscopy. To detect microtubules by indirect immunofluorescence using monoclonal anti-tubulin antibody, a prolonged incubation with lysing enzymes was necessary due to its very thick cell wall. Cytoplasmic microtubules were detected in interphase and a spindle with astral microtubules was seen in M-phase. The disintegration of cytoplasmic microtubules and migration of the nucleus to the bud before mitosis were characteristic features of the basidiomycetous yeast Malassezia pachydermatis. The visualisation of F-actin structures (patches, cables and cytokinetic rings) by fluorescence microscopy using both monoclonal anti-actin antibody and rhodamine-phalloidin failed, but actin was detected by electron microscopy with immunogold labelling. Clusters of gold particles indicating actin structures were detected at the plasma membrane of cells with unique cortical ultrastructural features characteristic of the genus Malassezia. A possible association of these with the actin cytoskeleton is suggested.     

The roles of the cytoskeleton during cellulose deposition at the secondary cell wall

During secondary cell wall formation, developing xylem vessels deposit cellulose at specific sites on the plasma membrane. Bands of cortical microtubules mark these sites and are believed to somehow orientate the cellulose synthase complexes. We have used live cell imaging on intact roots of Arabidopsis to explore the relationship between the microtubules, actin and the cellulose synthase complex during secondary cell wall formation. The cellulose synthase complexes are seen to form bands beneath sites of secondary wall synthesis. We find that their maintenance at these sites is dependent upon underlying bundles of microtubules which localize the cellulose synthase complex (CSC) to the edges of developing cell wall thickenings. Thick actin cables run along the long axis of the cells. These cables are essential for the rapid trafficking of complex-containing organelles around the cell. The CSCs appear to be delivered directly to sites of secondary cell wall synthesis and it is likely that transverse actin may mark these sites.     

Membrane-cytoskeleton associations during myogenesis deviate from traditional definitions

Plasma membrane-cytoskeleton associations involving four membrane proteins (A5, H58, H36, and I20) were studied in developing L8E63 rat skeletal muscle cells using immunofluorescence microscopy and photometry on the basis of three criteria: Triton-insolubility, colocalization with cytoskeletal components, and sensitivity to cytoskeleton-directed drugs. The results presented demonstrate that there are developmental stage-specific associations between membrane proteins and the cytoskeleton during skeletal myogenesis. Several inconsistencies were found with traditional expectations of membrane-cytoskeleton associations. For example, although A5 is Triton-insoluble and sensitive to cytochalasin, its distribution generally does not correspond with any known cytoskeletal structure. Furthermore, the topography of A5 is dependent on the integrity of the plasma membrane. H36 and I20 are completely soluble in Triton and therefore by accepted definitions would not be expected to be associated with any cytoskeletal component. Yet H36 and actin codisrupt in the presence of cytochalasin, while I20, whose distribution does not correspond with microtubules, is uniquely sensitive to their disruption. These results demonstrate that (i) neither Triton-solubility nor colocalization alone predicts all membrane-cytoskeleton associations; some associations between the membrane and cytoskeleton are unstable in nonionic detergent; (ii) the native distribution of proteins in the membrane may not reflect their cytoskeletal associations; and (iii) the topography of some membrane proteins with no apparent association with the cytoskeleton may be greatly influenced by the cell cytoskeleton.     

Actin as a cytoskeletal basis for cell architecture and a protein essential for ecdysis in Prorocentrum minimum (Dinophyceae,Prorocentrales)

The specific cell architecture of prorocentroid dinoflagellates is reflected in the internal cell structure, particularly, in cytoskeleton organization. Cytoskeleton arrangement in a Prorocentrum minimum cell was investigated using fluorescent labeling approaches, electron‐microscopy and immunocytochemical methods. The absence of cortical microtubules was confirmed. Phalloidin – tetramethylrhodamine isothiocyanate conjugate staining demonstrated that F‐actin forms a dense layer in the cortical region of the cell; besides, it was detected in the ‘archoplasmic sphere’ adjacent to the nucleus. In some cells the rest of the cytoplasm and the nucleus were also slightly stained. In dividing cells, F‐actin was mainly distributed in the cortical region and in the cleavage furrow. Fluorescent deoxyribonuclease I staining demonstrated more evenly distributed cytoplasmic non‐polymerized actin; the basis of the nuclear actin pool is monomeric actin. It concentrates in the nucleoplasm and forms a meshwork around chromosomes. The significant amount of G‐actin is apparently localized in the P. minimum nucleolus. Assumed involvement of F‐actin in the process of stress‐induced ecdysis – cell cover shedding – was examined. A sharp decrease in the level of ecdysis was observed after treatment with actin‐depolymerizing agent latrunculin B. The fluorescent staining of treated cells demonstrated disturbance of the actin cytoskeleton and disappearance of the cortical F‐actin layer. Our results support the recent data on the actin involvement in fundamental nuclear processes: cytoplasmic F‐actin appears to participate in cell shape determination, cell cover rearrangement and development. Actin may play a substitute role in the absence of cortical microtubules, representing the cytoskeletal basis of P. minimum cell architecture.     

Membrane tether formation from blebbing cells

Membrane tension has been proposed to be important in regulating cell functions such as endocytosis and cell motility. The apparent membrane tension has been calculated from tether forces measured with laser tweezers. Both membrane-cytoskeleton adhesion and membrane tension contribute to the tether force. Separation of the plasma membrane from the cytoskeleton occurs in membrane blebs, which could remove the membrane-cytoskeleton adhesion term. In renal epithelial cells, tether forces are significantly lower on blebs than on membranes that are supported by cytoskeleton. Furthermore, the tether forces are equal on apical and basolateral blebs. In contrast, tether forces from membranes supported by the cytoskeleton are greater in apical than in basolateral regions, which is consistent with the greater apparent cytoskeletal density in the apical region. We suggest that the tether force on blebs primarily contains only the membrane tension term and that the membrane tension may be uniform over the cell surface. Additional support for this hypothesis comes from observations of melanoma cells that spontaneously bleb. In melanoma cells, tether forces on blebs are proportional to the radius of the bleb, and as large blebs form, there are spikes in the tether force in other cell regions. We suggest that an internal osmotic pressure inflates the blebs, and the pressure calculated from the Law of Laplace is similar to independent measurements of intracellular pressures. When the membrane tension term is subtracted from the apparent membrane tension over the cytoskeleton, the membrane-cytoskeleton adhesion term can be estimated. In both cell systems, membrane-cytoskeleton adhesion was the major factor in generating the tether force.     

Actomyosin is Involved in the Organization of the Microtubule Preprophase Band in Arabidopsis Suspension Cultured Cells

The microtubule preprophase bands （PPBs） participate in the sequence of events to position cell plates in most plants. However, the mechanism of PPB formation remains to be clarified. In the present study, the organization of PPBs in Arabidopsis suspension cultured cells was investigated by confocal laser scanning microscopy combined with pharmacological treatments of reagents specific for the cytoskeleton elements. Double staining of F-actin and microtubules （MTs） showed that actin filaments were arranged randomly and no colocalization with cortical MTs was observed in the interphase cells. However, cortical actin filaments showed colocalization with MTs during the formation of PPBs. A broad actin band formed with the broad MT band in the initiation of PPB and narrowed down together with the MT band to form the PPB. Nevertheless, broad MT bands were formed but failed to narrow down in cells treated with the F-actin disruptor latrunculin A. In contrast, in the presence of the F-actin stabilizer phalloidin, PPB formation did not exhibit any abnormality. Therefore, the integrity, but not the dynamics, of the actin cytoskeleton is necessary for the formation of normal PPBs. Treatment with 2, 3-butanedine monoxime, a myosin inhibitor, also resulted in the formation of broad MT bands, indicating that actomyosin may be involved in the rearrangement of MTs to form the PPBs. Double staining of MTs and myosin revealed that myosin concentrated on the PPB region during PPB formation. It is suggested that the actin cytoskeleton at the PPB site may serve as a rack to transport cortical MTs by using myosin when the broad MT band narrows down to form the PPB.     

The<Emphasis Type="Italic"> Arabidopsis RHD3</Emphasis> gene is required for cell wall biosynthesis and actin organization

The Arabidopsis thaliana (L.) Heynh. ROOT HAIR DEFECTIVE3 (RHD3) gene has previously been shown to be essential for normal cell expansion [H. Wang et al. (1997) Genes Dev 11:799-811]. In this report, we demonstrated that mutation of the RHD3 gene in the Arabidopsis fragile fiber 4 (fra4) mutant caused a dramatic reduction in the wall thickness of fibers, vessels, and pith cells in the inflorescence stems and, concomitantly, a decrease in the mechanical strength of stems. The reduced wall thickness in the fra4 mutant was accompanied by an alteration in cell wall composition. Consistent with the defective fiber and vessel wall phenotypes, the RHD3 gene exhibited a strong expression in developing fiber and xylem cells. We showed that the Arabidopsis genome contains two additional RHD3-like genes, one of which was expressed specifically in flowers. In addition, we found that mutation of the RHD3 gene caused an alteration in the organization of the actin cytoskeleton but no effects on cortical microtubules. Our findings suggest an essential role of RHD3 in cell wall biosynthesis and actin organization, both of which are known to be important for cell expansion.     

Establishment of the circumferential actin filament network is a prerequisite for localization of the cadherin-catenin complex in epithelial cells.

With the adhesion molecules, the actin cytoskeleton controls cell-cell and cell-substrate interactions and participates in transmembrane signaling. The relationships between actin and adhesion complexes at the sites of adhesion have been well documented. Here we investigate by a series of studies whether a relationship exists between actin organization and the localization and function of the components of the cadherin-catenin complex (CCC) that participates in the cell-cell adherens junction. Reversible actin depolymerization reversibly affects the peripheral distribution of CCCs. Mutations in adenovirus E1A and the small GTPase rac1, but not Ha-ras, disrupt the circumferential, cortical actin filament (CAF) network and the targeting of CCC components to the cell surface. Disruption of actin stress fibers or microtubules does not interfere with CCC localization and function. Constitutive loss of the apical cortical actin ring results in epithelial cells in which components of the CCCs are found only in intracellular vesicles and never at the surface. A kinetic analysis of the de novo appearance of the CAF network and the CCCs at the cell surface was also conducted. When F-actin was dissolved, surface CCC components were internalized. Reestablishment of CAFs required about 4 h, during which time E-cadherin and alpha-catenin were found first in a juxtanuclear location and then in intracellular vesicles or post-Golgi carriers, similar to what was observed in cells expressing mutant E1A or rac1. Thus, disruption of preexisting CCCs resulted in their internalization and recycling to the Golgi. It was only after the regeneration of the filamentous actin ring beneath the cell surface that peripheral localization of CCCs was observed. A similar result was observed with dominant negative rac1. These data suggest that the status of cortical actin is assessed and transduced and thereby regulates the transport and delivery of cadherin and catenins to the cell surface.     

In vivo single-particle tracking of the aquaporin AtPIP2;1 in stomata reveals cell type-specific dynamics

Aquaporins such as the plasma membrane intrinsic proteins (PIPs) allow water to move through cell membranes and are vital for stomatal movement in plants. Despite their importance, the dynamic changes in aquaporins during water efflux and influx have not been directly observed in real time in vivo. Here, to determine which factors regulate these changes during the bidirectional translocation of water, we examined aquaporin dynamics during the stomatal immune response to the bacterial flagellin-derived peptide flg22. The Arabidopsis (Arabidopsis thaliana) aquaporin mutant pip2;1 showed defects in the flg22-induced stomatal response. Variable-angle total internal reflection fluorescence microscopy revealed that the movement dynamics and dwell times of AQ6]GFP-AtPIP2;1 in guard cells and subsidiary cells exhibited cell type-specific dependencies on flg22. The cytoskeleton, rather than the cell wall, was the major factor regulating AtPIP2;1 dynamics, although both the cytoskeleton and cell wall might form bounded domains that restrict the diffusion of AtPIP2;1 in guard cells and subsidiary cells. Finally, our analysis revealed the different roles of cortical actin and microtubules in regulating AtPIP2;1 dynamics in guard cells, as well as subsidiary cells, under various conditions. Our observations shed light on the heterogeneous mechanisms that regulate membrane protein dynamics in plants in response to pathogens.     

AHNAK interaction with the annexin 2/S100A10 complex regulates cell membrane cytoarchitecture

Remodelling of the plasma membrane cytoarchitecture is crucial for the regulation of epithelial cell adhesion and permeability. In Madin-Darby canine kidney cells, the protein AHNAK relocates from the cytosol to the cytosolic surface of the plasma membrane during the formation of cell-cell contacts and the development of epithelial polarity. This targeting is reversible and regulated by Ca(2+)-dependent cell-cell adhesion. At the plasma membrane, AHNAK associates as a multimeric complex with actin and the annexin 2/S100A10 complex. The S100A10 subunit serves to mediate the interaction between annexin 2 and the COOH-terminal regulatory domain of AHNAK. Down-regulation of both annexin 2 and S100A10 using an annexin 2-specific small interfering RNA inhibits the association of AHNAK with plasma membrane. In Madin-Darby canine kidney cells, down-regulation of AHNAK using AHNAK-specific small interfering RNA prevents cortical actin cytoskeleton reorganization required to support cell height. We propose that the interaction of AHNAK with the annexin 2/S100A10 regulates cortical actin cytoskeleton organization and cell membrane cytoarchitecture.     

Characterization of integrin-tetraspanin adhesion complexes: role of tetraspanins in integrin signaling.

Tetraspanins (or proteins from the transmembrane 4 superfamily, TM4SF) form membrane complexes with integrin receptors and are implicated in integrin-mediated cell migration. Here we characterized cellular localization, structural composition, and signaling properties of alpha3beta1-TM4SF adhesion complexes. Double-immunofluorescence staining showed that various TM4SF proteins, including CD9, CD63, CD81, CD82, and CD151 are colocalized within dot-like structures that are particularly abundant at the cell periphery. Differential extraction in conjunction with chemical cross-linking indicated that the cell surface fraction of alpha3beta1-TM4SF protein complexes may not be directly linked to the cytoskeleton. However, in cells treated with cytochalasin B alpha3beta1-TM4SF protein complexes are relocated into intracellular vesicles suggesting that actin cytoskeleton plays an important role in the distribution of tetraspanins into adhesion structures. Talin and MARCKS are partially codistributed with TM4SF proteins, whereas vinculin is not detected within the tetraspanin-containing adhesion structures. Attachment of serum-starved cells to the immobilized anti-TM4SF mAbs induced dephosphorylation of focal adhesion kinase (FAK). On the other hand, clustering of tetraspanins in cells attached to collagen enhanced tyrosine phosphorylation of FAK. Furthermore, ectopic expression of CD9 in fibrosarcoma cells affected adhesion-induced tyrosine phosphorylation of FAK, that correlated with the reorganization of the cortical actin cytoskeleton. These results show that tetraspanins can modulate integrin signaling, and point to a mechanism by which TM4SF proteins regulate cell motility.     

Association of the actin cytoskeleton with glass-adherent proteins in mouse peritoneal macrophages

Summary— When mouse peritoneal macrophages adherent to glass surface were removed by treatment with triethanolamine and Nonidet P-40, fine thread structures of unique loops were left behind on glass at the sites of cell adhesion. To examine the ultrastructural relationship between such looped threads and cytoskeletal components in glass-adherent macrophages, we successfully used the ‘zinc method’ to remove most of the cytoplasm including nuclei and to expose the cytoskeleton associated with the ventral plasma membrane. The cytoskeleton was seen to be mainly composed of actin filaments forming dense networks. The network contained scattered star-like foci from which actin filaments radiated. When the ventral plasma membrane-cytoskeleton complex was further treated with Nonidet P-40, the membrane was dissolved to expose the glass surface with actin foci persisting on glass. When the complex was removed by further treatment with Nonidet P-40 and DNase I, the looped threads became visible. Confocal laser microscopy of glass-adherent macrophages stained with fluorescent phalloidin showed the preferential distribution of F-actin in the ventral cytoplasm along the plasma membrane, where intense fluorescent spots were also scattered. Confocal interference reflection microscopy revealed densely populated dark dots and striae of focal contact, which corresponded in overall distribution to actin foci and looped threads. These observations suggest that actin cytoskeleton is closely associated with looped threads to reinforce cell adhesion to glass.     

Lectin receptor kinases participate in protein-protein interactions to mediate plasma membrane-cell wall adhesions in Arabidopsis

Interactions between plant cell walls and plasma membranes are essential for cells to function properly, but the molecules that mediate the structural continuity between wall and membrane are unknown. Some of these interactions, which are visualized upon tissue plasmolysis in Arabidopsis (Arabidopsis thaliana), are disrupted by the RGD (arginine-glycine-aspartic acid) tripeptide sequence, a characteristic cell adhesion motif in mammals. In planta induced-O (IPI-O) is an RGD-containing protein from the plant pathogen Phytophthora infestans that can disrupt cell wall-plasma membrane adhesions through its RGD motif. To identify peptide sequences that specifically bind the RGD motif of the IPI-O protein and potentially play a role in receptor recognition, we screened a heptamer peptide library displayed in a filamentous phage and selected two peptides acting as inhibitors of the plasma membrane RGD-binding activity of Arabidopsis. Moreover, the two peptides also disrupted cell wall-plasma membrane adhesions. Sequence comparison of the RGD-binding peptides with the Arabidopsis proteome revealed 12 proteins containing amino acid sequences in their extracellular domains common with the two RGD-binding peptides. Eight belong to the receptor-like kinase family, four of which have a lectin-like extracellular domain. The lectin domain of one of these, At5g60300, recognized the RGD motif both in peptides and proteins. These results imply that lectin receptor kinases are involved in protein-protein interactions with RGD-containing proteins as potential ligands, and play a structural and signaling role at the plant cell surfaces.