Super helix formation of actin filaments in an in vitro motile system.

Muscle contraction results from relative sliding of actin and myosin filaments. However, the possibility that actin filaments twist or rotate during sliding has not yet been experimentally investigated. We found that a super helix of an actin filament is formed in an in vitro motile system. This fact suggests that an actin filament twists and rotates due to a torque component of a sliding force generated at cross-bridges.     

Tubular actin filaments in tobacco guard cells

The dynamic remodeling of actin filaments in guard cells functions in stomatal movement regulation. In our previous study, we found that the stochastic dynamics of guard cell actin filaments play a role in chloroplast movement during stomatal movement. In our present study, we further found that tubular actin filaments were present in tobacco guard cells that express GFP-mouse talin; approximately 2.3 tubular structures per cell with a diameter and height in the range of 1–3 µm and 3–5 µm, respectively. Most of the tubular structures were found to be localized in the cytoplasm near the inner walls of the guard cells. Moreover, the tubular actin filaments altered their localization slowly in the guard cells of static stoma, but showed obvious remodeling, such as breakdown and re-formation, in moving guard cells. Tubular actin filaments were further found to be colocalized with the chloroplasts in guard cells, but their roles in stomatal movement regulation requires further investigation.Key words: actin dynamics, tubular actin filaments, chloroplast, guard cell, stomatal movementStomatal movement responses to surrounding environment are mediated by guard cell signaling.^1,2 Actin filaments within guard cells are dynamic cytoarchitectures and function in stomatal development and movement.³ Arrays of actin filaments in guard cells that are dependent on different stomatal apertures have also been reported in references ^4–7. For example, the random or longitudinal orientations of actin filaments in closed stomata change to a radial orientation or ring-like array after stomata opening.^5,6,8 The reorganization of the actin architecture during stomatal movement depends on the depolymerization and repolymerization of actin filaments in guard cells. In contrast to the traditional treadmill model of actin dynamic mechanisms, stochastic dynamics of actin have been revealed in plant cells, such as in the epidermal cells of hypocotyl and root, the pavement cells of Arabidopsis cotyledons, and the guard cells of tobacco (Nicotiana tabacum).^9–11 In this alternative system, the short actin fragments generated from severed long filaments can link with each other to form longer filaments by end-joining activity. The actin regulatory proteins, Arp2/3 complex, capping protein and actin depolymerizing factor (ADF)/cofilin, may also be involved in the stochastic dynamics of actin filaments.^12,13Using tobacco GFP-mouse talin expression lines, we have previously analyzed the stochastic dynamics of guard cell actin filaments and their roles in chloroplast displacement during stomatal movement.^6,11 We found from these analyses that another arrangement of actin filaments, i.e., tubular actin filaments, exists in the guard cells of these tobacco lines. We first found the circle-like actin filaments in 82% of the guard cells (counting 320 cells) in tobacco expressing GFPmouse talin when analyzing a single optical section (Fig. 1A). In a previous study of BY-2 cells expressing GFP-Lifeact labeled actin filaments, Smertenko et al. found similar structures, i.e., quoit-like structures or acquosomes in all of the plant tissues examined except growing root hairs.¹⁰ However, in our present analysis of serial sections, we determined that the circle-like actin filaments in the tobacco guard cells were long tubes (Fig. 1A), as the lengths (about 3–5 µm) of these structures were greater than their diameter (about 1–3 µm). Hence, we denoted these structures as tubular actin filaments to distinguish them from the circular conformations of actin filaments observed previously in other plant cell tissues.^10,14–19 About 2.3 of these tubular actin filaments were found per guard cell, which is less than the number of acquosomes reported in BY-2 cells (about 6.7 per cell).¹⁰ Analysis of serial optical sections at the z-axis revealed that the tubular actin filaments localize in the cytoplasm near the inner walls of the guard cells (Fig. 1B), which is similar to the distribution of chloroplasts in guard cells.¹¹ Longitudinal sections further revealed a colocalization of tubular actin filaments and chloroplasts (Fig. 1B).Open in a separate windowFigure 1Tubular actin filaments in the guard cells of a tobacco (Nicotiana tabacum) line expressing GFP-mouse talin. (A) Optical-sections (interval, 1.5 µm) of guard cells in a moving stoma showing tubular actin filaments (arrow heads). Frames (a1) and (a2) are cross sections of 1.5-µm-picture through the yellow and red lines, respectively, revealing the cross section of the circle structures are parallel lines (arrows). (B) Optical-sections of a stoma from the outer periclinal walls to the inner walls of the guard cells (interval, 1 µm). The tubular actin filaments (arrow heads) are localized in the cytoplasm near to the inner periclinal walls of guard cells. Frame (b1) is the guard cell on the right of the frame “4 µm”; (b2) is the cross section of b1 through the red line; and (b3) is a higher magnification image of the area encompassed by the white square in b2. Arrows indicate the colocalization between the tubular actin filaments and the chloroplast (indicated using a red pseudocolor). (C) Time-series imaging showing the movement of tubular actin filaments in the guard cells of static stomata. Frame (c1) comprises three images colored red (0 S), green (40 S) and blue (80 S), that are merged in a single frame to show the translocation of the tubular actin filaments (arrows). (D) Time-series images of the opening stomata showing the breakdown (arrows) and re-formation (arrowheads) of the tubular actin filaments. All images were captured using a Zeiss LSM 510 META confocal laser scanning microscope, as described by Wang et al.¹¹ Bars, 10 µm.We performed time-lapse imaging and found that the translocation of tubular actin filaments is slow in static stomata in which the distance between two tubular actin filaments typically increased from 2.22 to 2.50 µm after 80 sec (Fig. 1C). In moving stomata, however, the tubular actin filaments showed an obvious dynamic reorganization whereby they could be processed into short fragments and also reemerged after they had disintegrated (Fig. 1D). These results indicate that tubular actin filaments have stochastic dynamics that are similar to the long actin filaments of guard cells.¹¹ In our previous study, we found that the stochastic dynamics of actin filaments correlate with light-induced chloroplast movement in guard cells.¹¹ However, whether the dynamics of the tubular actin filaments are also involved in chloroplast movement during stomatal movement remains to be investigated. In cultured mesophyll cells which had been mechanically isolated from Zinnia elegans, Wilsen et al. previously found a close association between fully closed actin rings and chloroplasts.¹⁸ These authors further found that the average percentage of cells with free actin rings increased at the initial culture stage, and then decreased, which indicates that the formation of actin rings might be a response of the actin cytoskeleton to cellular stress or disturbance.¹⁸ The turgor pressure of guard cells is the fundamental basis of stomatal movement leading to changes in the shape, volume, wall structure, and membrane surface of guard cells.^20–24 We speculate from our current data that there is a relationship between tubular actin filaments and the shape changes of guard cells during stomatal movement.     

Diagnostic surface expression of SWAP-70 on HIV-1 infected T cells

Following immunization with HIV-1 infected cells, a hybridoma cell line termed 9F11 was established from the P3U-1 myeloma line fused with lymphocytes from a trans-chromosome (TC) mouse, that harbors human chromosomes containing immunoglobulin genes. The 9F11 human IgM monoclonal antibody (9F11 Ab) reacts with HIV-1 infected MOLT4 cells but not with uninfected MOLT4 cells, and causes immune cytolysis with homologous human complement at a concentration as low as 0.4 microg/ml. This Ab was used to perform immunoscreening of a cDNA expression library derived from HIV-1 infected cells. All positive cDNA clones contained SWAP-70 cDNA. SWAP-70 RNA and protein expression are much stronger in HIV-1 infected cells. SWAP-70 was also detected on the surface of HIV-1 infected cells by flow cytometric analysis. The monocyte cell line U937 cells expresses SWAP-70 on its cell surface regardless of whether it was infected with HIV-1. Furthermore, among PBMCs surface expression of SWAP-70 was detected on CD21+, CD56+ and CD14+ cells. Although CD3+ cells scarcely express SWAP-70 on their surface, once activated, they become positive. SWAP-70 may therefore serve as a marker for T cell differentiation as well as for HIV-1 infection.     

Heavy-meromyosin-decorated actin filaments: a simple method to preserve actin filaments for rotary shadowing

It has become accepted that deep-freeze-drying at or below -90 degrees C is necessary to preserve the structure of supramolecular assemblies such as actin filaments (AFs) for metal shadowing. This has kept the metal shadowing technique from widespread use in the study of proteins complexed with AFs because of the limited availability of the apparatus for deep-freeze-drying. I report here that adsorption to freshly cleaved mica, treatment with buffered uranyl acetate in glycerol solution, rinsing, and removal of liquid eliminate the need of freeze-drying to preserve the structure of AFs. This technique, in combination with metal shadowing, was applied to the study of AFs decorated with heavy meromyosin (HMM). It was observed that (1) when HMM molecules are associated with single AFs in the majority of cases only one head of each HMM molecule makes contact at the point furthest from the neck region; (2) binding of HMM causes bundling of AFs, probably by the two heads of each molecule binding different filaments; and (3) the binding of HMM to the bundled AFs appears to be more stable than that to a single AF. This method of specimen preparation requires no freeze-drying and is therefore easily applicable to other large protein complexes.     

The presence of ring formed actin filaments in plant cells

Summary Ring formed actin filaments were observed in tobacco BY-2 cells. The change of this structure during culture was followed by fluorescence microscopy.     

SWAP-70: A New Type of Oncogene

SWAP-70 is a protein that has been suggested to be involved in regulation of actin rearrangement. Having discovered that an artificially-derived mutant of SWAP-70 can transform mouse embryo fibroblasts, we searched for naturally-occurring mutations in the SWAP-70 gene, finding listings for several on the Web at www.sanger.ac.uk/genetics/CGP/cosmic/, including three mutations found in ovarian cancers. (The number of such mutations has now reached 13 out of 228 tumors). We created expression vectors for the mutant SWAP-70 proteins and introduced these into NIH3T3 cells. The cells expressing the mutant SWAP-70 constructs exhibited faster growth than the parental or wild-type SWAP-70-expressing cells. In most instances, cells that are able to grow in soft agar will form tumors in nude mice. While SWAP-70-transformed cells grew in soft agar, they failed to form tumors in nude mice. This result implies that transformation by the SWAP-70 mutants is unique. The cells bearing the mutant SWAP-70 genes were sensitive to nutrient starvation, supporting the idea that they are transformed cells. However, they failed to pile up and demonstrated contact inhibition, unlike most normal transformed cells. Upon expression of human SWAP-70 genes, MEK1 was activated. This activation appeared to contribute to the saturation density of the cells. As SWAP-70 has been shown to be the last protein to receive signals from cytokines, it is likely that there is a putative feedback signaling pathway, and that disorder of this signaling pathway can transform cells. Accordingly, this may explain why SWAP-70-transformed cells have different characteristics than most transformed cells.     

Interactions between actin filaments and between actin filaments and membranes in quick-frozen and deeply etched hair cells of the chick ear

Replicas of the apical surface of hair cells of the inner ear (vestibular organ) were examined after quick freezing and rotary shadowing. With this technique we illustrate two previously undescribed ways in which the actin filaments in the stereocilia and in the cuticular plate are attached to the plasma membrane. First, in each stereocilium there are threadlike connectors running from the actin filament bundle to the limiting membrane. Second, many of the actin filaments in the cuticular plate are connected to the apical cell membrane by tiny branched connecting units like a "crow's foot." Where these "feet" contact the membrane there is a small swelling. These branched "feet" extend mainly from the ends of the actin filaments but some connect the lateral surfaces of the actin filaments as well. Actin filaments in the cuticular plate are also connected to each other by finer filaments, 3 nm in thickness and 74 +/- 14 nm in length. Interestingly, these 3-nm filaments (which measure 4 nm in replicas) connect actin filaments not only of the same polarity but of opposite polarities as documented by examining replicas of the cuticular plate which had been decorated with subfragment 1 (S1) of myosin. At the apicolateral margins of the cell we find two populations of actin filaments, one just beneath the tight junction as a network, the other at the level of the zonula adherens as a ring. The latter which is quite substantial is composed of actin filaments that run parallel to each other; adjacent filaments often show opposite polarities, as evidenced by S1 decoration. The filaments making up this ring are connected together by the 3-nm connectors. Because of the polarity of the filaments this ring may be a "contractile" ring; the implications of this is discussed.     

Orientational order of the lamellipodial actin network as demonstrated in living motile cells

Lamellipodia of crawling cells represent both the motor for cell advance and the primary building site for the actin cytoskeleton. The organization of actin in the lamellipodium reflects actin dynamics and is of critical importance for the mechanism of cell motility. In previous structural studies, the lamellipodial actin network was analyzed primarily by electron microscopy (EM). An understanding of lamellipodial organization would benefit significantly if the EM data were complemented and put into a kinetic context by establishing correspondence with structural features observable at the light microscopic level in living cells. Here, we use an enhanced phase contrast microscopy technique to visualize an apparent long-range diagonal actin meshwork in the advancing lamellipodia of living cells. Visualization of this meshwork permitted a correlative light and electron microscopic approach that validated the underlying organization of lamellipodia. The linear features in the light microscopic meshwork corresponded to regions of greater actin filament density. Orientation of features was analyzed quantitatively and compared with the orientation of actin filaments at the EM level. We infer that the light microscopic meshwork reflects the orientational order of actin filaments which, in turn, is related to their branching angle.     

Correlated waves of actin filaments and PIP3 in Dictyostelium cells

Chemotaxis-deficient amiB-null mutant Dictyostelium cells show two distinct movements: (1) they extend protrusions randomly without net displacements; (2) they migrate persistently and unidirectionally in a keratocyte-like manner. Here, we monitored the intracellular distribution of phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)) to gain insight into roles PIP(3) plays in those spontaneous motilities. In keratocyte-like cells, PIP(3) showed convex distribution over the basal membrane, with no anterior enrichment. In stalled cells, as well as in wild type cells, PIP(3) repeated wave-like changes, including emergence, expansion and disappearance, on the basal membrane. The waves induced lamellipodia when they approached the cell edge, and the advancing speed of the waves was comparable to the migration speed of the keratocyte-like cells. LY294002, an inhibitor of PI3 kinase, abolished PIP(3) waves in stalled cells and stopped keratocyte-like cells. These results together suggested that keratocyte-like cells are "surfing" on the PIP(3) waves by coupling steady lamellipodial protrusions to the PIP(3) waves. Simultaneous live observation of actin filaments and PIP(3) in wild type or stalled amiB(-) cells indicated that the PIP(3) waves were correlated with wave-like distributions of actin filaments. Most notably, PIP(3) waves often followed actin waves, suggesting that PIP(3) induces local depolymerization of actin filaments. Consistent with this idea, cortical accumulation of PIP(3) was often correlated with local retraction of the periphery. We propose that the waves of PIP(3) and actin filaments are loosely coupled with each other and play important roles in generating spontaneous cell polarity.     

Banding and polarity of actin filaments in interphase and cleaving cells

Heavy meromyosin (HMM) decoration of actin filaments was used to detect the polarity of microfilaments in interphase and cleaving rat kangaroo (PtK2) cells. Ethanol at -20 degrees C was used to make the cells permeable to HMM followed by tannic acid-glutaraldehyde fixation for electron microscopy. Uniform polarity of actin filaments was observed at cell junctions and central attachment plaques with the HMM arrowheads always pointing away from the junction or plaque. Stress fibers were banded in appearance with their component microfilaments exhibiting both parallel and antiparallel orientation with respect to one another. Identical banding of microfilament bundles was also seen in cleavage furrows with the same variation in filament polarity as found in stress fibers. Similarly banded fibers were not seen outside the cleavage furrow in mitotic cells. By the time that a mid-body was present, the actin filaments in the cleavage furrow were no longer in banded fibers. The alternating dark and light bands of both the stress fibers and cleavage furrow fibers are approximately equal in length, each measuring approximately 0.16 micrometer. Actin filaments were present in both bands, and individual decorated filaments could sometimes be traced through four band lengths. Undecorated filaments, 10 nm in diameter, could often be seen within the light bands. A model is proposed to explain the arrangement of filaments in stress fibers and cleavage furrows based on the striations observed with tannic acid and the polarity of the actin filaments.     

Orientation and three-dimensional organization of actin filaments in dividing cultured cells

The current hypothesis of cytokinesis suggests that contractile forces in the cleavage furrow are generated by a circumferential band of actin filaments. However, relatively little is known about the global organization of actin filaments in dividing cells. To approach this problem we have used fluorescence-detected linear dichroism (FDLD) microscopy to measure filament orientation, and digital optical sectioning microscopy to perform three-dimensional reconstructions of dividing NRK cells stained with rhodamine-phalloidin. During metaphase, actin filaments in the equatorial region show a slight orientation along the spindle axis, while those in adjacent regions appear to be randomly distributed. Upon anaphase onset and through cytokinesis, the filaments become oriented along the equator in the furrow region, and along the spindle axis in adjacent regions. The degree of orientation appears to be dependent on cell-cell and cell-substrate adhesions. By performing digital optical sectioning microscopy on a highly spread NRK subclone, we show that actin filaments organize as a largely isotropic cortical meshwork in metaphase cells and convert into an anisotropic network shortly after anaphase onset, becoming more organized as cytokinesis proceeds. The conversion is most dramatic on the adhering ventral surface which shows little or no cleavage activity, and results in the formation of large bundles along the equator. On the dorsal surface, where cleavage occurs actively, actin filaments remain isotropic, showing only subtle alignment late in cytokinesis. In addition, stereo imaging has led to the discovery of a novel set of filaments that are associated with the cortex and traverse through the cytoplasm. Together, these studies provide important insights into the process of actin remodeling during cell division and point to possible additional mechanisms for force generation.     

Distributionof actin filaments in differentiating cells of Equisetum hyemale root tips

The distribution of F-actin was studied in various cell types in root tips of Equisetum hyemale using rhodamine labelled phalloidin as a probe. In dividing cells no cables could be observed, although a strong diffuse staining, which has the same distribution as the microtubules, occurs in the phragmoplast. During cell development the cables become longer and form meshworks but no relationship with cell expansion and microtubule patterns was observed. The distribution apparently follows the pattern of plasma streaming. In expanded cells in particular the nucleus is surrounded by a web of filaments connected with the larger cables in the cell.     

Dynamic motion of paxillin on actin filaments in living endothelial cells

Our three-dimensional (3-D) images showed that paxillin co-localized on actin filaments as fibrous structures, as well as clusters, in endothelial cells (ECs). In living ECs under flow condition, we monitored concurrently the intracellular dynamics of DsRed2-paxillin and GFP-actin by time-lapse video recording and dual-color fluorescence imaging. The results showed that the dynamic motion of paxillin as fibrous structures was associated with actin filaments, but not with microtubules. Our findings suggest that the actin network plays an important role not only in the assembly/disassembly of paxillin at focal adhesions, but also as a track for the intracellular transport of paxillin, which is involved in signaling pathway.     

Stress fibers are generated by two distinct actin assembly mechanisms in motile cells

Stress fibers play a central role in adhesion, motility, and morphogenesis of eukaryotic cells, but the mechanism of how these and other contractile actomyosin structures are generated is not known. By analyzing stress fiber assembly pathways using live cell microscopy, we revealed that these structures are generated by two distinct mechanisms. Dorsal stress fibers, which are connected to the substrate via a focal adhesion at one end, are assembled through formin (mDia1/DRF1)-driven actin polymerization at focal adhesions. In contrast, transverse arcs, which are not directly anchored to substrate, are generated by endwise annealing of myosin bundles and Arp2/3-nucleated actin bundles at the lamella. Remarkably, dorsal stress fibers and transverse arcs can be converted to ventral stress fibers anchored to focal adhesions at both ends. Fluorescence recovery after photobleaching analysis revealed that actin filament cross-linking in stress fibers is highly dynamic, suggesting that the rapid association-dissociation kinetics of cross-linkers may be essential for the formation and contractility of stress fibers. Based on these data, we propose a general model for assembly and maintenance of contractile actin structures in cells.     

Drosophila quail, a villin-related protein, bundles actin filaments in apoptotic nurse cells

Drosophila Quail protein is required for the completion of fast cytoplasm transport from nurse cells to the oocyte, an event critical for the production of viable oocytes. The abundant network of cytoplasmic filamentous actin, established at the onset of fast transport, is absent in quail mutant egg chambers. Previously, we showed that Quail is a germline-specific protein with sequence homology to villin, a vertebrate actin-regulating protein. In this study, we combined biochemical experiments with observations in egg chambers to define more precisely the function of this protein in the regulation of actin-bundle assembly in nurse cells. We report that recombinant Quail can bind and bundle filamentous actin in vitro in a manner similar to villin at a physiological calcium concentration. In contrast to villin, Quail is unable to sever or cap filamentous actin, or to promote nucleation of new actin filaments at a high calcium concentration. Instead, Quail bundles the filaments regardless of the calcium concentration. In vivo, the assembly of nurse-cell actin bundles is accompanied by extensive perforation of the nurse-cell nuclear envelopes, and both of these phenomena are manifestations of nurse-cell apoptosis. To investigate whether free calcium levels are affected during apoptosis, we loaded egg chambers with the calcium indicator Indo-1. Our observations indicate a rise in free calcium in the nurse-cell cytoplasm coincident with the permeabilization of the nuclear envelopes. We also show that human villin expressed in the Drosophila germline could sense elevated cytoplasmic calcium; in nurse cells with reduced levels of Quail protein, villin interfered with actin-bundle stability. We conclude that Quail efficiently assembles actin filaments into bundles in nurse cells and maintains their stability under fluctuating free calcium levels. We also propose a developmental model for the fast phase of cytoplasm transport incorporating findings presented in this study.     

Jasplakinolide reversibly disrupts actin filaments in suspension-cultured tobacco BY-2 cells

Summary. Jasplakinolide is potentially a useful pharmacological tool for the study of actin organization and dynamics in living cells, since it induces actin polymerization in vitro and, unlike phalloidin, is membrane permeative. In the present work, the effect of jasplakinolide on the actin cytoskeleton of living suspension-cultured Nicotiana tabacum ‘Bright Yellow 2’ cells was investigated. Actin filaments in the living cells were disrupted by jasplakinolide. The effect of jasplakionlide on the actin cytoskeleton was concentration and time dependent. When cells were treated with a moderate concentration (150 nM) of jasplakinolide, cortical actin filaments were disrupted preferentially, whereas actin aggregated at the perinuclear region. With concentrations higher than 400 nM and exposure times longer than 30 min, actin filaments in the cell disappeared completely. The effect of jasplakinolide on the actin cytoskeleton was reversible even at high concentration. Actin bundles appeared first in the perinuclear region within 5 min, and the cortical actin array was reestablished in 15 min, suggesting that actin filaments might be organized at this region. Received July 31, 2001 Accepted December 14, 2001     

Force-velocity measurements of a few growing actin filaments

The polymerization of actin in filaments generates forces that play a pivotal role in many cellular processes. We introduce a novel technique to determine the force-velocity relation when a few independent anchored filaments grow between magnetic colloidal particles. When a magnetic field is applied, the colloidal particles assemble into chains under controlled loading or spacing. As the filaments elongate, the beads separate, allowing the force-velocity curve to be precisely measured. In the widely accepted Brownian ratchet model, the transduced force is associated with the slowing down of the on-rate polymerization. Unexpectedly, in our experiments, filaments are shown to grow at the same rate as when they are free in solution. However, as they elongate, filaments are more confined in the interspace between beads. Higher repulsive forces result from this higher confinement, which is associated with a lower entropy. In this mechanism, the production of force is not controlled by the polymerization rate, but is a consequence of the restriction of filaments' orientational fluctuations at their attachment point.     

The organization of actin filaments in the stereocilia of cochlear hair cells

Within each tapering stereocilium of the cochlea of the alligator lizard is a bundle of actin filaments with > 3,000 filaments near the tip and only 18-29 filaments at the base where the bundle enters into the cuticular plate; there the filaments splay out as if on the surface of a cone, forming the rootlet. Decoration of the hair cells with subfragment 1 of myosin reveals that all the filaments in the stereocilia, including those that extend into the cuticular plate forming the rootlet, have unidirectional polarity, with the arrowheads pointing towards the cell center. The rest of the cuticular plate is composed of actin filaments that show random polarity, and numerous fine, 30 A filaments that connect the rootlet filaments to each other, to the cuticular plate, and to the membrane. A careful examination of the packing of the actin filaments in the stereocilia by thin sectin and by optical diffraction reveals that the filaments are packed in a paracrystalline array with the crossover points of all the actin helices in hear-perfect register. In transverse sections, the actin filaments are not hexagonally packed but, rather, are arranged in scalloped rows that present a festooned profile. We demonstrated that this profile is a product of the crossbridges by examining serial sections, sections of different thicknesses, and the same stereocilium at two different cutting angles. The filament packing is not altered by fixation in different media, removal of the limiting membrane by detergent extraction, or incubation of extracted hair cells in EGTA, EDTA, and Ca++ and ATP. From our results, we conclude that the stereocilia of the ear, unlike the brush border of intestinal epithelial cells, are not designed to shorten, nor do the filaments appear to slide past one another. In fact, the stereocilium is like a large, rigid structure designed to move as a lever.