Organization of interdoublet links in tetrahymena cilia

Ciliary Motility: Regulation of Axonemal Dynein Motors

Ciliary motility is crucial for the development and health of many organisms. Motility depends on the coordinated activity of multiple dynein motors arranged in a precise pattern on the outer doublet microtubules. Although significant progress has been made in elucidating the composition and organization of the dyneins, a comprehensive understanding of dynein regulation is lacking. Here, we focus on two conserved signaling complexes located at the base of the radial spokes. These include the I1/f inner dynein arm associated with radial spoke 1 and the calmodulin- and spoke-associated complex and the nexin-dynein regulatory complex associated with radial spoke 2. Current research is focused on understanding how these two axonemal hubs coordinate and regulate the dynein motors and ciliary motility.

The nexin link and B-tubule glutamylation maintain the alignment of outer doublets in the ciliary axoneme

We developed quantitative assays to test the hypothesis that the N-DRC is required for integrity of the ciliary axoneme. We examined reactivated motility of demembranated drc cells, commonly termed "reactivated cell models." ATP-induced reactivation of wild-type cells resulted in the forward swimming of ∼90% of cell models. ATP-induced reactivation failed in a subset of drc cell models, despite forward motility in live drc cells. Dark-field light microscopic observations of drc cell models revealed various degrees of axonemal splaying. In contrast, >98% of axonemes from wild-type reactivated cell models remained intact. The sup-pf4 and drc3 mutants, unlike other drc mutants, retain most of the N-DRC linker that interconnects outer doublet microtubules. Reactivated sup-pf4 and drc3 cell models displayed nearly wild-type levels of forward motility. Thus, the N-DRC linker is required for axonemal integrity. We also examined reactivated motility and axoneme integrity in mutants defective in tubulin polyglutamylation. ATP-induced reactivation resulted in forward swimming of >75% of tpg cell models. Analysis of double mutants defective in tubulin polyglutamylation and different regions of the N-DRC indicate B-tubule polyglutamylation and the distal lobe of the linker region are both important for axonemal integrity and normal N-DRC function. © 2016 Wiley Periodicals, Inc.

DRC3 connects the N-DRC to dynein g to regulate flagellar waveform

The nexin-dynein regulatory complex (N-DRC), which is a major hub for the control of flagellar motility, contains at least 11 different subunits. A major challenge is to determine the location and function of each of these subunits within the N-DRC. We characterized a Chlamydomonas mutant defective in the N-DRC subunit DRC3. Of the known N-DRC subunits, the drc3 mutant is missing only DRC3. Like other N-DRC mutants, the drc3 mutant has a defect in flagellar motility. However, in contrast to other mutations affecting the N-DRC, drc3 does not suppress flagellar paralysis caused by loss of radial spokes. Cryo-electron tomography revealed that the drc3 mutant lacks a portion of the N-DRC linker domain, including the L1 protrusion, part of the distal lobe, and the connection between these two structures, thus localizing DRC3 to this part of the N-DRC. This and additional considerations enable us to assign DRC3 to the L1 protrusion. Because the L1 protrusion is the only non-dynein structure in contact with the dynein g motor domain in wild-type axonemes and this is the only N-DRC-dynein connection missing in the drc3 mutant, we conclude that DRC3 interacts with dynein g to regulate flagellar waveform.

Tubulin polyglutamylation regulates flagellar motility by controlling a specific inner-arm dynein that interacts with the dynein regulatory complex

The tpg1 mutant of Chlamydomonas lacks the tubulin polyglutamylase TTLL9 and is deficient in flagellar tubulin polyglutamylation. It exhibits slow swimming, whereas the double mutant with oda2 (a slow-swimming mutant that lacks outer-arm dynein) is completely nonmotile. Thus, tubulin polyglutamylation must be important for the functioning of inner-arm dynein(s). In this study, we show that the tpg1 mutation only slightly affects the motility of mutants that lack dynein "e," one of the seven species of major inner-arm dyneins, whereas it greatly reduces the motility of mutants lacking other inner-arm dynein species. This suggests that dynein e is the main target of motility regulation by tubulin polyglutamylation. Furthermore, the motility of various mutants in the background of the tpg1 mutation raises the possibility that tubulin polyglutamylation also affects the dynein regulatory complex, a dynein e-associated key regulator of flagellar motility, which possibly constitutes the interdoublet (nexin) link. Tubulin polyglutamylation thus may play a central role in the regulation of ciliary and flagellar motility. © 2012 Wiley Periodicals, Inc.

Flagellar oscillation: a commentary on proposed mechanisms

Eukaryotic flagella and cilia have a remarkably uniform internal 'engine' known as the '9+2' axoneme. With few exceptions, the function of cilia and flagella is to beat rhythmically and set up relative motion between themselves and the liquid that surrounds them. The molecular basis of axonemal movement is understood in considerable detail, with the exception of the mechanism that provides its rhythmical or oscillatory quality. Some kind of repetitive 'switching' event is assumed to occur; there are several proposals regarding the nature of the 'switch' and how it might operate. Herein I first summarise all the factors known to influence the rate of the oscillation (the beating frequency). Many of these factors exert their effect through modulating the mean sliding velocity between the nine doublet microtubules of the axoneme, this velocity being the determinant of bend growth rate and bend propagation rate. Then I explain six proposed mechanisms for flagellar oscillation and review the evidence on which they are based. Finally, I attempt to derive an economical synthesis, drawing for preference on experimental research that has been minimally disruptive of the intricate structure of the axoneme. The 'provisional synthesis' is that flagellar oscillation emerges from an effect of passive sliding direction on the dynein arms. Sliding in one direction facilitates force-generating cycles and dynein-to-dynein synchronisation along a doublet; sliding in the other direction is inhibitory. The direction of the initial passive sliding normally oscillates because it is controlled hydrodynamically through the alternating direction of the propulsive thrust. However, in the absence of such regulation, there can be a perpetual, mechanical self-triggering through a reversal of sliding direction due to the recoil of elastic structures that deform as a response to the prior active sliding. This provisional synthesis may be a useful basis for further examination of the problem.

The counterbend phenomenon in dynein-disabled rat sperm flagella and what it reveals about the interdoublet elasticity

Rat sperm that have been rendered passive by disabling the dynein motors with 50 muM sodium metavanadate and 0.1 mM ATP exhibit an interesting response to imposed bending. When the proximal flagellum is bent with a microprobe, the portion of the flagellum distal to the probe contact point develops a bend in the direction opposite the imposed bend. This "counterbend" is not compatible with a simple elastic beam. It can be satisfactorily explained by the sliding tubule model of flagellar structure but only if there are permanent elastic connections between the outer doublets of the axoneme. The elastic component that contributes the bending torque for the counterbend does not reset to a new equilibrium position after an imposed bend but returns the flagellum to a nearly straight or slightly curved final position after release from the probe. This suggests it is based on fixed, rather than mobile, attachments. It is also disrupted by elastase or trypsin digestion, confirming that it is dependent on a protein linkage. Adopting the assumption that the elasticity is attributed to the nexin links that repeat at 96 nm intervals, we find an apparent elasticity for each link that ranges from 1.6 to 10 x 10(-5) N/m. However, the elasticity is nonlinear and does not follow Hooke's law but appears to decrease with increased stretch. In addition, the responsible elastic elements must be able to stretch to more than 10 times their resting length without breakage to account for the observed counterbend formation. Elasticity created by some type of protein unfolding may be the only viable explanation consistent with both the extreme capacity for extension and the nonlinear character of the restoring force that is observed.

Geometry drives the ?deviated-bending? of the bi-tubular structures of the 9+2 axoneme in the flagellum

The axoneme "9 + 2" is basically a system constituted of a cylinder of 9 microtubule doublets surrounding a central pair of microtubules. These bi-tubular structures are considered as the support system of the active molecular complexes that generate and regulate the axonemal movement. Schoutens has calculated their moments of inertia [Schoutens, 1994: Journal of Theoretical Biology 171:163-177]. The results obtained allowed us to assume that these bi-tubular systems are endowed with dynamic properties that could be involved in the regulation of the axonemal machinery. For the first time, using the finite elements methods and the resistance of material principles, we have now calculated that the curvature of the axoneme induces the deviated-bending of the bi-tubular structures of the axoneme, because of their geometry only; they behave as beams in a framework. This approach is similar to the one used to measure the deflection of a single microtubule [Kasas et al., 2004: Chem Phys Chem 5:252-257]. These behaviors induce internal movement or constraints of either couples or triplets of doublets within the axonemal cylinder that could be directly involved in a constrained or a spontaneous "convergence/divergence" equilibrium of the cylindrical generatrices that they draw along the axonemal cylinder, which could apparently regulate the activity of the axonemal motors (the dynein arms). These results are discussed here, taking into consideration the dynamic propagation of the wave train along the flagellar axoneme, and the regulated balance between the activities of the two opposite sides of the axoneme during the beat. This study raises a few questions about the architecture-activity duo of the axonemal doublets.

Rib72, a conserved protein associated with the ribbon compartment of flagellar A-microtubules and potentially involved in the linkage between outer doublet microtubules

Ciliary and flagellar axonemes are basically composed of nine outer doublet microtubules and several functional components, e.g. dynein arms, radial spokes, and interdoublet links. Each A-tubule of the doublet contains a specialized "ribbon" of three protofilaments composed of tubulin and other proteins postulated to specify the three-dimensional arrangement of the various axonemal components. The interdoublet links hold the doublet microtubules together and limit their sliding during the flagellar beat. In this study on Chlamydomonas reinhardtii, we cloned a cDNA encoding a 71,985-Da polypeptide with three DM10 repeats, two C-terminal EF-hand motifs, and homologs extending to humans. This polypeptide, designated as Rib72, is a novel component of the ribbon compartment of flagellar microtubules. It remained associated with 9-fold arrays of doublet tubules following extraction under high and low ionic conditions, and anti-Rib72 antibodies revealed an approximately 96-nm periodicity along axonemes, consistent with Rib72 associating with interdoublet links. Following proteolysis- and ATP-dependent disintegration of axonemes, the rate of cleavage of Rib72 correlated closely with the rate of sliding disintegration. These observations identify a ribbon-associated protein that may function in the structural assembly of the axoneme and in the mechanism and regulation of ciliary and flagellar motility.

Elastic extension and jump of the flagellar nexin links: a theoretical mechanical cycle

The functions of the nexin links of a flagellar axoneme have not been clearly demonstrated. Taking into account both the elastic (Hookean) characteristics and the possible jump of the nexin links, we calculated the sliding to bending conversion of a theoretical model in a tip-ward direction step by step, according to the essential principles proposed by the geometric clutch hypothesis [Lindemann, 1994: J Theoret Biol 168:175-189]: the activity of the dynein arms depends on the transverse forces induced by the axonemal curvature. In our calculations, however, the transverse forces that are involved in the regulation of the activities of the dynein arms were due to the extension of the nexin links located upstream of a given abscissa. This allowed us to define a bent segment as the axonemal portion at whose proximal and distal ends the nexin links were relaxed, and as fully extended as possible, respectively. The model creates an apparent disorder in the orientation of the nexin links as already observed [Bozkurt and Wooley, 1993: Cell Motil Cytoskeleton 24:109-118; Wooley, 1997: J Cell Sci 110:85-94]. We propose that the nexin links are involved in a mechanical cycle, whose 3 stages are (1) rapid extension, (2) slow relaxation, and (3) stand-by. The rapid extension is compatible with the mechanical interactions between the nexin links and the inner dynein arms with which they form the dynein regulatory complex. This was correlated with the oscillating properties of the nexin links along the axoneme that allow them to be one of the regulatory elements of the local ATPase activity of the dynein arms.

A study of helical and planar waves on sea urchin sperm flagella, with a theory of how they are generated

When the spermatozoon of Echinus esculentus swims in sea water containing methyl cellulose (viscosity 1.5-4 Pa s), its flagellum may generate either a helical or a planar waveform, each type being stable. The helical wave, which is dextral, is complicated by the concurrent passage of miniature waves along it. These miniature waves have a pulsatile origin in the neck region of the spermatozoon. Our videotape analysis indicates that there are two pulses of mechanical activity for each true cycle of the helical wave. (The true helical frequency was obtained from the apparent wave frequency and the roll frequency of the sperm head, the latter being detectable in some sperm when lit stroboscopically.) The planar wave has a meander shape. During the propagation of planar waves, the sliding displacements are adjustable in either direction; moribund flagella can undergo unrestricted sliding. The planar waves are, in fact, exactly planar only at interfaces. Otherwise, there tend to be torsions in the interbend segments between planar bends. Mechanical stimulation of the flagellum can cause a sudden transition from the helical to the planar waveform. To account for the two modes of beating, we advance the hypothesis that circumferential linkages yield beyond a threshold strain. Whether this yield point is exceeded, we suggest, depends upon the balance between the active shear force and the external viscosity (among other factors). We propose that a subthreshold force originates in one array and then triggers the other dynein arrays circumferentially, but unidirectionally, around the base of the flagellum; whereas a suprathreshold force provokes bi-directional circumferential triggering. These may be the two patterns of activation that result in helical and planar waveforms, respectively. The transition from helical to planar bending may result from an increment in the force produced by the dynein motors. The pulsatile origin of the helical wave resembles behaviour described previously for spermatozoa of Ciona intestinalis and of the quail Coturnix coturnix.

Transient disruptions of axonemal structure and microtubule sliding during bend propagation by Ciona sperm flagella

Demembranated sperm flagella of Ciona were reactivated at increased salt concentrations (0.45 to 0.5 M K acetate). In addition to a decrease in amplitude of propagated bends, some flagella switch between "stable" and "transient" bending cycles. In the transient bending cycles, there is increased intermicrotubule sliding, in the direction that forms a new principal bend at the base of the flagellum, during the first half of a bending cycle. The magnitude of this increased sliding may be as much as 1 radian, or 0.06 micron between adjacent doublet microtubules. Most transient bending patterns also show a characteristic disruption of axonemal structure, involving separation between strands of microtubule doublets over a distance of up to 5 microns, occurring within a principal bend, typically about 16 microns from the base of the flagellum. The disruptions usually disappear after the principal bend propagates beyond the region of the disruption. Formation of these disruptions requires additional sliding, in the direction that would form a principal bend at the base of the flagellum, of up to about 0.3 micron. Formation of these disruptions may be explained by weakening of structural interactions by increased salt concentration and transverse forces, proportional to curvature and transmitted force, that will tend to separate doublets in a bend. These observations indicate that an actively beating flagellum possesses active sliding capability that is activated but not expressed during normal bend initiation and propagation. The initiation and propagation of flagellar bends may not be explicable solely in terms of local activation and inactivation of dynein-driven sliding.

Direct measurement of inter-doublet elasticity in flagellar axonemes

The outer doublet microtubules in ciliary and flagellar axonemes are presumed to be connected with each other by elastic links called the inter-doublet links or the nexin links, but it is not known whether there actually are such elastic links. In this study, to detect the elasticity of the putative inter-doublet links, shear force was applied to Chlamydomonas axonemes with a fine glass needle and the longitudinal elasticity was determined from the deflection of the needle. Wild-type axonemes underwent a high-frequency, nanometer-scale vibration in the presence of ATP. When longitudinal shear force was applied, the average position of the needle tip attached to the axoneme moved linearly with the force applied, yielding an estimate of spring constant of 2.0 (S.D.: 0.8) pN/nm for 1 microm of axoneme. This value did not change in the presence of vanadate, i.e., when dynein does not form strong cross bridges. In contrast, it was at least five times larger when ATP was absent, i.e., when dynein forms strong cross bridges. The measured elasticity did not significantly differ in various mutant axonemes lacking the central-pair microtubules, a subset of inner-arm dynein, outer-arm dynein, or the radial spokes, although it was somewhat smaller in the latter two mutants. It was also observed that the shear displacement in an axoneme in the presence of ATP often took place in a stepwise manner. This suggests that the inter-doublet links can reversibly detach from and reattach to the outer doublets in a cooperative manner. This study thus provides the first direct measure of the elasticity of inter-doublet links and also demonstrates its dynamic nature.

Dynein inner arm heavy chain identification in cAMP-activated flagella using class-specific polyclonal antibodies

While studying cAMP-dependent dynein alpha-heavy chain phosphorylation, we found previously [Stephens and Prior, 1992: J. Cell Sci. 103:999-1012] that high salt extraction of sperm flagella from the mussel Mytilus edulis or the clam Spisula solidissima removed most visible dynein arms, accompanied by an amount of Mg+2-ATPase that correlated with the mass of dynein alpha- and beta-heavy chains removed. However, although almost devoid of ATPase activity, such extracted axonemes retained one third of the heavy chain mass as two sets of electrophoretically-distinct, vanadate-cleavable, non-phosphorylated proteins. To explore the nature of these dynein-like proteins, antibodies to the alpha- and beta-heavy chains were blot affinity-purified from a rabbit antiserum raised against gradient-purified Spisula 18-20S flagellar outer arm dynein. Although able to recognize common epitopes of the opposite chain type, neither the alpha- nor the beta-heavy chain antibody recognized the tightly-bound proteins in either species, proving that they are immunologically distinct. While the beta-antibody recognized its heavy chain homolog in gill cilia, the alpha-antibody did not, demonstrating immunological distinction between flagellar and ciliary dynein alpha-heavy chains. Immunization of a mouse with nitrocellulose strips containing one of the two tightly-bound Spisula flagellar proteins produced an antiserum that cross-reacted with each tightly-bound protein in both species and also recognized alpha- and beta-heavy chains. The anti-molluscan serum cross-reacted strongly with sea urchin sperm flagellar dynein B-, C-, and D-bands, considered to be inner arm components, but not with sea urchin outer arm alpha- or beta-heavy chains. These data indicate that the electrophoretically and immunologically distinct, tightly-bound proteins of molluscan flagella are inner arm dynein heavy chains.

A model of flagellar and ciliary functioning which uses the forces transverse to the axoneme as the regulator of dynein activation

Ciliary and flagellar motion is driven by the dynein-tubulin interaction between adjacent doublets of the axoneme, and the resulting sliding displacements are converted into axonemal bends that are propagated. When the axoneme is bent in the normal beating plane, force develops across the axoneme in the plane of the bend. This transverse force (t-force) has maximal effect on the interdoublet spacing of outer doublets 2-4 on one side of the axoneme and doublets 7-9 on the opposite side. Episodes of sliding originates as the t-force brings these doublets into closer proximity (allowing dynein bridges to form) and are terminated when these doublets are separated from each other by the t-force. A second factor, the adhesive force of the dynein-tubulin attachments (bridges), also acts to pull neighboring doublets closer together. This force resists termination of a sliding episode once initiated, and acts locally to give the population of dynein bridges a type of excitability. In other words, as bridges form, the probability of nearby bridges attaching is increased by a positive feedback exerted through the interdoublet spacing. A conceptual working hypothesis explaining the behavior of cilia and flagella is proposed based on the above concepts. Additionally, the feasibility of this proposed mechanism is demonstrated using a computer simulation. The simulation uses a Monte Carlo-type algorithm for dynein attachment and adhesive force, together with a geometric evaluation of the t-force on the key microtubule pairs. This model successfully develops spontaneous oscillations from any starting configuration (including a straight position). It is compatible with the physical dimensions, mechanical properties and bridge forces measured in real cilia and flagella. In operation, it exhibits many of the observed actions of cilia and flagella, most notably wave propagation and the ability to produce both cilia-like and flagella-like waveforms.

Physical model of axonemal splitting

A physical model developed to explain microtubule sliding patterns in the trypsin-treated ciliary axoneme has been extended to investigate the generation of bending moments by microtubules sliding in an axoneme in which the doublets are anchored at one end. With sliding restricted, a bending moment is developed by the polarized shearing interaction between neighbouring doublets, effected by the activity of dynein arms on doublet N pushing N + 1 in a tipward (+) direction. In arrested axonemes in which arms on several contiguous doublets are active, the bending moment causes splitting of the 9 + 2 microtubule array into two or more sets of doublets. In the absence of special constraints, splitting depends only on breaking the circumferential interdoublet links most distorted by the bending moment. The analysis, which permits assignment of arm activity to specific microtubules in each of the observed patterns of splitting, indicates that the axoneme will split between doublet N and N + 1 if arms on doublet N are inactive and arms on either N + 1 or N-1 are active. To produce the observed major splits, dynein arms on the microtubules of roughly one-half of the axoneme are predicted to be active, in a manner consistent with the switch-point hypothesis of ciliary motion. Electron microscopic examination indicates that virtually every set of doublets in the split axonemes retains its cylindrical form. Maintenance of cylindrical symmetry can be ascribed to the mechanical properties of the unbroken links, which may resist both tensile and compressive stress, and to active dynein arms.

Structural and geometrical constraints on the outer dynein arm in situ

This study considers the relationship between two structural forms of the 22S dynein arm of Tetrahymena thermophila: the bouquet and the compact arm. The compact arm differs from the bouquet and from other proposed forms (e.g., the "toadstool") in that the globular domains are situated transversely across the interdoublet gap with one globular subunit, the head, proximal to the adjacent doublet microtubule. The other models place all three globular domains proximal to the neighboring doublet microtubule. When sliding of an isolated axoneme is induced, at least 57% of total attached arms on exposed doublets are in the compact form within dimensions of 24 x 24 x 12 nm, and only about 2% of the arms are bouquets. Toadstools are incompatible with the images seen. Bouquets are not found in regions of the doublet protected by a neighboring doublet. When axonemes with exposed doublets are treated with 0.5 M KCl for 30 min, the compact arms and the dynein heavy (H)-chains disappear, while isolated bouquets and dynein H-chains appear in the medium, suggesting that the compact arms give rise to the bouquets as they are solubilized. The bouquet is the predominant form of isolated 22S dynein molecules, which are found in two apparently enantiomorphic forms, within dimensions 45 x 39 x 13 nm; bouquets attached to doublets have dimensions similar to those of isolated bouquets. Computer modeling indicates that in an intact standard-diameter axoneme, these dimensions are incompatible with the interdoublet volume available for an arm; the bouquet therefore represents an unfolded compact arm. A plausible sequence of changes can be modeled to illustrate the conversion of an attached compact arm to an attached and then free bouquet. The toadstool is probably an artifact that arises after unfolding. Consistent with the conformational difference, H-chains of attached compact arms differ from those of isolated bouquets in their susceptibility to limited proteolysis. These results suggest that the compact arm, rather than the unfolded bouquet or the toadstool, is the functional form of the outer arm in the intact axoneme.

Morphology of nexin links in relation to interdoublet sliding in the sperm flagellum

In this work, we examine whether the "nexin" linkages of the flagellum can extend in length to accommodate interdoublet sliding. Flagellar bends of large angle were induced in bull spermatozoa by hypotonic treatment. It is argued that this produces large interdoublet displacements that are, nevertheless, still within physiological limits. Such flagella were examined by the rapid-freeze, deep-etch technique and the nexin linkages identified by their position in relation to the inner dynein arms and by their straplike, bipartite, morphology. They were found to bridge perpendicularly (or occasionally at an angle) between the A- and B-tubules of adjacent doublets. The nexin linkages were no more than approximately nm in length, even in regions in which approximately 200 nm of sliding could be inferred. Variable registration between adjacent nexin rows gave some further support to the assumption that sliding had indeed taken place. From this, it is concluded that elastic deformation of the links, such as would accommodate interdoublet sliding, does not occur; some form of displacement must occur between nexin and the adjacent B-tubule.

The role of axonemal components in ciliary motility

1. The axoneme is the detergent-insoluble cytoskeleton of the cilium. 2. All axonemes generate movement by the same fundamental mechanism: microtubule sliding utilizing ATP hydrolysis during a mechanochemical cycling of dynein arms on the axonemal doublets. 3. Structure, fundamental biochemistry and physiology of the axoneme are conserved evolutionarily, but the phenotypes of beating movements and the responses to specific cytoplasmic signals differ greatly from organism to organism. 4. A model of asynchronous dynein arm activity--the switch point hypothesis--has been proposed to account for cyclic beating in the face of unidirectional sliding. The model suggests that the diversity of beat phenotype may be explicable by changes in the timing of switching between active and inactive states of doublet arm activity. Evidence of axonemal splitting in arrested axonemes provides new support for the hypothesis.

Mechanical stimulation activates beating in calcium-arrested lateral cilia of Mytilus edulis gill

Lateral cilia of Mytilus edulis gill arrest upon mechanical stimulation, the result of calcium influx. A mechanical stimulus that deflects these cilia toward the effective stroke, and is normally sufficient to cause transient arrest in beating lateral cilia or transient movement into the recovery stroke in quiescent cilia, initiates beating in Ca2+ ionophore-arrested cilia at 9-15 Hz, for periods as long as 30 s. This movement is restricted to the stimulated cilia and the beat pattern appears constrained in the first half of the beat cycle. Application of dopamine causes ciliary arrest in the presence (but not absence) of Ca2+ and mechanical stimulation will also activate such cilia to beat. In the presence of ATP, mechanical stimulation of detergent-permeabilized lateral cell models arrested in the presence of 50 microM Ca2+ will also cause activation comparable in frequency, duration, and beat pattern to that seen in Ca2+-arrested cells, but the initiation is more difficult. Upon application of ionophore in Ca2+-free (EGTA) seawater, the cilia become quiescent, stopped at the end of the recovery stroke. Mechanical stimulation will cause activation of beat, with a similar range of frequencies and duration as in Ca2+-arrested lateral cilia, but the beat pattern is normal and cilia of adjacent cells may also beat, presumably initiated by mechanical coupling. Gently lifting cilia at their basal ends, using small, slow movements of a mechanical probe, will initiate several beat cycles in quiescent lateral cilia but will cause Ca2+-arrested cilia to 'snap' into the effective stroke and back.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic AMP and calcium in the differential control of Mytilus gill cilia

Lateral (L) cilia of Mytilus gill are activated by serotonin which, in molluscan systems, is known to activate adenylate cyclase. Triton-extracted models of L-cells, arrested at greater than 10(-6) M Ca++, are stimulated to beat by the addition of 10(-5) M cAMP while still under Ca++ arrest conditions, suggesting that cAMP-activation is not mediated by alterations of Ca++ levels. Using isolated, permeabilized cilia, we find, independent of [Ca++], that cAMP-dependent protein phosphorylation in L-cilia occurs uniquely and reversibly on three low molecular weight polypeptides of 23,000, 18,000, and 14,000 daltons. Phosphorylation is maximal at cAMP concentrations above 0.5 microM. The phosphorylated chains partially co-extract at high salt with a 14S dynein fraction and have approximately the same molecular weights as reported for dynein light chains. Such conditions mainly extract the outer dynein arm, about 40% of the Mg++-ATPase activity, and a corresponding amount of the cAMP phosphorylated chains. However, the three polypeptides sediment together at 10-11S, clearly separable from the 14S dynein ATPase. Using a gel-overlay technique, we find that calmodulin binds to axonemal polypeptides of L-cilia with molecular weights of 18,000 and 13,000, independent of Ca++, while in mixed-population cilia, only a 12,000 dalton chain binds calmodulin, in a Ca++ dependent manner. In neither case are calmodulin binding proteins found in the high salt fraction containing the cAMP-dependent phosphorylated chains, indicating that, in spite of some similarity in molecular weight, the cAMP-phosphorylated and calmodulin binding polypeptides are different. Also, double-labelling indicates that only the 18,000 dalton chains co-migrate.(ABSTRACT TRUNCATED AT 250 WORDS)