Post-translational incorporation of actin into myofibrils in vitro: evidence for isoform specificity

Myofibrillar protein turnover: the proteasome and the calpains

Metabolic turnover of myofibrillar proteins in skeletal muscle requires that, before being degraded to AA, myofibrillar proteins be removed from the myofibril without disrupting the ability of the myofibril to contract and develop tension. Skeletal muscle contains 4 proteolytic systems in amounts such that they could be involved in metabolic protein turnover: 1) the lysosomal system, 2) the caspase system, 3) the calpain system, and 4) the proteasome. The catheptic proteases in lysosomes are not active at the neutral pH of the cell cytoplasm, so myofibrillar proteins would have to be degraded inside lysosomes if the lysosomal system were involved. Lysosomes could not engulf a myofibril without destroying it, so the lysosomal system is not involved to a significant extent in metabolic turnover of myofibrillar proteins. The caspases are not activated until initiation of apoptosis, and, therefore, it is unlikely that the caspases are involved to a significant extent in myofibrillar protein turnover. The calpains do not degrade proteins to AA or even to small peptides and do not catalyze bulk degradation of the sarcoplasmic proteins, so they cannot be the only proteolytic system involved in myofibrillar protein turnover. Research during the past 20 yr has shown that the proteasome is responsible for 80 to 90% of total intracellular protein turnover, but the proteasome degrades peptide chains only after they have been unfolded, so that they can enter the catalytic chamber of the proteasome. Thus, although the proteasome can degrade sarcoplasmic proteins, it cannot degrade myofibrillar proteins until they have been removed from the myofibril. It remains unclear how this removal is done. The calpains degrade those proteins that are involved in keeping the myofibrillar proteins assembled in myofibrils, and it was proposed over 30 yr ago that the calpains initiated myofibrillar protein turnover by disassembling the outer layer of proteins from the myofibril and releasing them as myofilaments. Such myofilaments have been found in skeletal muscle. Other studies have indicated that individual myofibrillar proteins can exchange with their counterparts in the cytoplasm; it is unclear whether this can be done to an extent that is consistent with the rate of myofibrillar protein turnover in living muscle. It seems that both the calpains and the proteasome are responsible for myofibrillar protein turnover, but the mechanism is still unknown.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

Substitution of flight muscle-specific actin by human (beta)-cytoplasmic actin in the indirect flight muscle of Drosophila

The human (beta)-cytoplasmic actin differs by only 15 amino acids from Act88F actin which is the only actin expressed in the indirect flight muscle (IFM) of Drosophila melanogaster. To test the structural and functional significance of this difference, we ectopically expressed (beta)-cytoplasmic actin in the IFM of Drosophila that lack endogenous Act88F. When expression of the heterologous actin was regulated by approximately 1.5 kb of the 5′ promoter region of the Act88F gene, little (beta)-cytoplasmic actin accumulated in the IFM of the flightless transformants. Including Act88F-specific 5′ and 3′ untranslated regions (UTRs) yielded transformants that expressed wild-type amounts of (beta)-cytoplasmic actin. Despite the assembly of (beta)-cytoplasmic actin containing thin filaments to which endogenous myosin crossbridges attached, sarcomere organization was deficient, leaving the transformants flightless. Rather than affecting primarily actin-myosin interactions, our findings suggest that the (beta)-cytoplasmic actin isoform is not competent to interact with other actin-binding proteins in the IFM that are involved in the organization of functional myofibrils.

Defining actin filament length in striated muscle: rulers and caps or dynamic stability?

Actin filaments (thin filaments) are polymerized to strikingly uniform lengths in striated muscle sarcomeres. Yet, actin monomers can exchange dynamically into thin filaments in vivo, indicating that actin monomer association and dissociation at filament ends must be highly regulated to maintain the uniformity of filament lengths. We propose several hypothetical mechanisms that could generate uniform actin filament length distributions and discuss their application to the determination of thin filament length in vivo. At the Z line, titin may determine the minimum extent and tropomyosin the maximum extent of thin filament overlap by regulating alpha-actinin binding to actin, while a unique Z filament may bind to capZ and regulate barbed end capping. For the free portion of the thin filament, we evaluate possibilities that thin filament components (e.g. nebulin or the tropomyosin/troponin polymer) determine thin filament lengths by binding directly to tropomodulin and regulating pointed end capping, or alternatively, that myosin thick filaments, together with titin, determine filament length by indirectly regulating tropomodulin's capping activity.

Isoform sorting and the creation of intracellular compartments

The generation of isoforms via gene duplication and alternative splicing has been a valuable evolutionary tool for the creation of biological diversity. In addition to the formation of molecules with related but different functional characteristics, it is now apparent that isoforms can be segregated into different intracellular sites within the same cell. Sorting has been observed in a wide range of genes, including those encoding structural molecules, receptors, channels, enzymes, and signaling molecules. This results in the creation of intracellular compartments that (a) can be independently controlled and (b) have different functional properties. The sorting mechanisms are likely to operate at the level of both proteins and mRNAs. Isoform sorting may be an important consequence of the evolution of isoforms and is likely to have contributed to the diversity of functional properties within groups of isoforms.

ADP-ribosylation of actins in fibroblasts and myofibroblasts by botulinum C2 toxin: influence on microfilament morphology and migratory behavior

Actins comprise six isoforms of which the nonmuscle isoforms beta-/gamma-actins are expressed by all eukaryotic cells. The expression pattern of one of the muscle actin isoforms, alpha-sm actin, previously believed to be restricted to smooth muscle, has been broadened to encompass activated fibroblasts (myofibroblasts) as well. The significance of this molecular conversion has remained largely unknown. We have recently shown that a reduction in filamentous alpha-sm actin by electroinjected specific antibodies or antisense oligodeoxynucleotides leads to increased motility in breast myofibroblasts (Rønnov-Jessen, L., Petersen, O. W. J. Cell Biol. 1996, 134, 67-80). In the present study we have expanded on the functional significance of actin isotypes in fibroblasts from the opposite point of view, namely filamentous nonmuscle actin. Nonmuscle actins in fibroblasts and myofibroblasts were ADP-ribosylated by Clostridium botulinum C2 toxin. The substrate for C2 toxin is globular actin, which upon ribosylation cannot incorporate into microfilaments. The pattern of actin ADP-ribosylation in (myo)fibroblasts in the presence of [32P]NAD was analyzed by isoelectric focusing, fluorography and immunoblotting. The influence of C2 toxin on microfilaments in intact cells was further assessed by immunofluorescence, and motility was measured in a mass migration assay and by computerized video time-lapse microscopy. We show here that C2 toxin specifically ribosylates beta- and gamma-actin in both fibroblasts and myofibroblasts. Whereas fibroblasts rapidly round up and stop migrating when filamentous beta-/gamma-actin is reduced by short-term ADP-ribosylation, myofibroblasts maintain their flattened morphology and a basic low motility.

A function for filamentous alpha-smooth muscle actin: retardation of motility in fibroblasts

Actins are known to comprise six mammalian isoforms of which beta- and gamma-nonmuscle actins are present in all cells, whereas alpha-smooth muscle (alpha-sm) actin is normally restricted to cells of the smooth muscle lineages. alpha-Sm actin has been found also to be expressed transiently in certain nonmuscle cells, in particular fibroblasts, which are referred to as myofibroblasts. The functional significance of alpha-sm actin in fibroblasts is unknown. However, myofibroblasts appear to play a prominent role in stromal reaction in breast cancer, at the site of wound repair, and in fibrotic reactions. Here, we show that the presence of alpha-sm actin is a signal for retardation of migratory behavior in fibroblasts. Comparison in a migration assay of fibroblast cell strains with and without alpha-sm actin revealed migratory restraint in alpha-sm actin-positive fibroblasts. Electroporation of monoclonal antibody (mAb) 1A4, which recognizes specifically the NH2-terminal Ac-EEED sequence of alpha-sm actin, significantly increased the frequency of migrating cells over that obtained with an unrelated antibody or a mAb against beta-actin. Time-lapse video microscopy revealed migratory rates of 4.8 and 3.0 microns/h, respectively. To knock out the alpha-sm actin protein, several antisense phosphorothioate oligodeoxynucleotide (ODNs) were tested. One of these, 3'UTI, which is complementary to a highly evolutionary conserved 3' untranslated (3'UT) sequence of alpha-sm actin mRNA, was found to block alpha-sm actin synthesis completely without affecting the synthesis of any other proteins as analyzed by two-dimensional gel electrophoresis. Targeting by antisense 3'UTI significantly increased motility compared with the corresponding sense ODN. alpha-Sm actin inhibition also led to the formation of less prominent focal adhesions as revealed by immunofluorescence staining against vinculin, talin, and beta1-integrin. We propose that an important function of filamentous alpha-sm actin is to immobilize the cells.

Deletion of amino acids from the carboxy-terminal end of actin

A series of deletions was made from the C-terminal end of actin by inserting termination codons into a full length cDNA of human alpha-skeletal muscle actin. These included deletions of 2, 3, 10, 20, 30, and 40 amino acids. The cDNA clones were transcribed and the resulting mRNAs were translated in vitro using 35S-labeled methionine. The 35S-labeled actin and actin mutants were then tested for the ability to coassemble with carrier actin, bind DNAse I, bind myosin S-1, bind a 27 kDa proteolytic fragment of alpha-actinin, and incorporate into myofibrils in vitro. Removal of the C-terminal two or three amino acids did not grossly alter the properties of actin tested. Deletion of an additional 7 amino acids (10 amino acids total) significantly decreased coassembly, binding to DNAse I, and incorporation into myofibrils, but did not dramatically reduce binding to myosin S-1 or the 27 kDa fragment of alpha-actinin. Deletion of 20 or more amino acids virtually abolished all normal actin function tested. By examining the structure of actin, we propose that the effect of removing residues 356-365 is due to the important role Trp356 plays in maintaining hydrophobic bonds between three non-contiguous segments of actin. We also suggest that removal of residues 366-372 adversely affected the structure or orientation of the DNAse I binding loop and that this change can account for defects in actin binding to DNAse I, coassembly with wild type actin, and incorporation into myofibrils.

Tropomodulin caps the pointed ends of actin filaments

Many proteins have been shown to cap the fast growing (barbed) ends of actin filaments, but none have been shown to block elongation and depolymerization at the slow growing (pointed) filament ends. Tropomodulin is a tropomyosin-binding protein originally isolated from red blood cells that has been localized by immunofluorescence staining to a site at or near the pointed ends of skeletal muscle thin filaments (Fowler, V. M., M. A., Sussman, P. G. Miller, B. E. Flucher, and M. P. Daniels. 1993. J. Cell Biol. 120: 411-420). Our experiments demonstrate that tropomodulin in conjunction with tropomyosin is a pointed end capping protein: it completely blocks both elongation and depolymerization at the pointed ends of tropomyosin-containing actin filaments in concentrations stoichiometric to the concentration of filament ends (Kd < or = 1 nM). In the absence of tropomyosin, tropomodulin acts as a "leaky" cap, partially inhibiting elongation and depolymerization at the pointed filament ends (Kd for inhibition of elongation = 0.1-0.4 microM). Thus, tropomodulin can bind directly to actin at the pointed filament end. Tropomodulin also doubles the critical concentration at the pointed ends of pure actin filaments without affecting either the rate of extent of polymerization at the barbed filament ends, indicating that tropomodulin does not sequester actin monomers. Our experiments provide direct biochemical evidence that tropomodulin binds to both the terminal tropomyosin and actin molecules at the pointed filament end, and is the long sought-after pointed end capping protein. We propose that tropomodulin plays a role in maintaining the narrow length distributions of the stable, tropomyosin-containing actin filaments in striated muscle and in red blood cells.

Structural and functional reconstitution of thin filaments in skeletal muscle

Thin filaments were reconstituted by incorporating exogenous actin, tropomyosin and troponin into glycerinated skeletal muscle fibres or myofibrils. Firstly, thin filaments except short fragments at the Z line were selectively removed by treatment with plasma gelsolin, an actin severing protein. As a result, the fibres (or fibrils) lost the ability to generate active tension. Next, actin filaments were reconstituted by adding purified G-actin which polymerizes onto the actin fragments which remained at the Z line. Rhodamine phalloidin staining of myofibrils showed that exogenous actin was incorporated into the position where the intrinsic thin filaments located. Thin section electron micrographs of fibres showed that reconstituted actin filaments ran from the Z line to the inside of the A band, with some reaching the H zone. The number density of reconstituted actin filaments in the A band was about 20% of that found in intact fibres. The actin filament-reconstituted fibres (or fibrils) generated active tension in a Ca(2+)-insensitive manner and the tension was reversibly suppressed by 2,3-butanedione 2-monoxime. The recovered active tension was about 20% of tension developed by intact fibres. These results indicate that reconstituted actin filaments bear active tension similar to that borne by intact thin filaments. Thin filament-reconstituted fibres, which were prepared by adding purified tropomyosin-troponin complexes into actin filament-reconstituted fibres, showed Ca(2+)-sensitive tension generation. The maximum tension generated was not affected by the presence of tropomyosin and troponin. SDS-PAGE analysis showed that more than 25% of actin and 20% of tropomyosin and troponin was incorporated into the reconstituted fibres. These results indicate that the structure and function of thin filaments are substantially reconstituted by self-assembly of actin, tropomyosin and troponin. The reconstituted fibres and fibrils will be useful for studying the molecular mechanism of muscle contraction and its regulation.

Isoform-specific 3'-untranslated sequences sort alpha-cardiac and beta-cytoplasmic actin messenger RNAs to different cytoplasmic compartments

We demonstrate that in differentiating myoblasts, the mRNAs encoding two actin isoforms, beta-cytoplasmic, and alpha-cardiac, can occupy different cytoplasmic compartments within the same cytoplasm. beta-actin mRNA is localized to the leading lamellae and alpha-actin mRNA is associated with a perinuclear compartment. This was revealed by co-hybridizing, in situ, fluorochrome-conjugated oligonucleotide probes specific for each isoform. To address the mechanism of isoform-specific mRNA localization, molecular chimeras were constructed by insertion of actin sequences between the Lac Z coding region and SV-40 3'UTR in a reporter plasmid. These constructs were transiently expressed in a mixed culture of embryonic fibroblasts, myoblasts and myotubes, beta-galactosidase activity within transfectants was revealed by a brief incubation with its substrate (X-gal). Since the blue-insoluble reaction product co-localized with the specific mRNAs expressed from each construct, it was used as a bioassay for mRNA localization. Transfectants were scored as either perinuclear, peripheral or nonlocalized with respect to the distribution of the blue product. The percentage of transfectants within those categories was quantitated as a function of the various constructs. This analysis revealed that for each actin mRNA its 3'UTR is necessary and sufficient to direct reporter transcripts to its appropriate compartment; beta-actin peripheral and alpha-actin perinuclear. In contrast, sequences from the 5'UTR through the coding region of either actin gene did not localize the blue product. Therefore, 3'UTR sequences play a key role in modulating the distribution of actin mRNAs in muscle cells. We propose that the mechanism of mRNA localization facilitates actin isoform sorting in the cytoplasm.

Beta and gamma actin mRNAs are differentially located within myoblasts

Actin is of fundamental importance to all eukaryotic cells. Of the six mammalian actins, beta (beta) and gamma (gamma) cytoplasmic are the isoforms found in all nonmuscle cells and differ by only four amino acids at the amino-terminal region. Both genes are regulated temporally and spatially, though no differences in protein function have been described. Using fluorescent double in situ hybridization we describe the simultaneous intracellular localization of both beta and gamma actin mRNA. This study shows that myoblasts differentially segregate the beta and gamma actin mRNAs. The distribution of gamma actin mRNA, only to perinuclear and nearby cytoplasm, suggests a distribution based on diffusion or restriction to nearby cytoplasm. The distribution of beta actin mRNA, perinuclear and at the cell periphery, implicates a peripheral localizing signal which is unique to the beta isoform. The peripheral beta actin mRNA corresponded to cellular morphologies, extending processes, and ruffling edges that reflect cell movement. Total actin and gamma actin protein steady-state distributions were identified by specific antibodies. gamma actin protein was found in both stress fibers and at the cell plasma membrane and does not correspond to its mRNA distribution. We suggest that localized protein synthesis rather than steady-state distribution functionally differentiates between the actin isoforms.

Molecular genetics of actin function

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Actin isoforms

The actin supergene family encodes a number of structurally related, but perhaps functionally distinct, protein isoforms that regulate contractile potential in muscle tissues and help to control the shape as well as the motility of non-muscle cells. In spite of the documented conservation amongst isoactin genes and their encoded proteins, recent results of biochemical, antibody localization, molecular mutagenesis and isoactin gene replacement studies lend credence to the notion that functional differences amongst muscle and non-muscle actin isoforms exist. Furthermore, the discovery of a new class of actin isoforms, the actin-related proteins, reveals that the actin gene and protein isoform family is more complex than was previously believed.

Tropomodulin is associated with the free (pointed) ends of the thin filaments in rat skeletal muscle

The length and spatial organization of thin filaments in skeletal muscle sarcomeres are precisely maintained and are essential for efficient muscle contraction. While the major structural components of skeletal muscle sarcomeres have been well characterized, the mechanisms that regulate thin filament length and spatial organization are not well understood. Tropomodulin is a new, 40.6-kD tropomyosin-binding protein from the human erythrocyte membrane skeleton that binds to one end of erythrocyte tropomyosin and blocks head-to-tail association of tropomyosin molecules along actin filaments. Here we show that rat psoas skeletal muscle contains tropomodulin based on immunoreactivity, identical apparent mobility on SDS gels, and ability to bind muscle tropomyosin. Results from immunofluorescence labeling of isolated myofibrils at resting and stretched lengths using anti-erythrocyte tropomodulin antibodies indicate that tropomodulin is localized at or near the free (pointed) ends of the thin filaments; this localization is not dependent on the presence of myosin thick filaments. Immunoblotting of supernatants and pellets obtained after extraction of myosin from myofibrils also indicates that tropomodulin remains associated with the thin filaments. 1.2-1.6 copies of muscle tropomodulin are present per thin filament in myofibrils, supporting the possibility that one or two tropomodulin molecules may be associated with the two terminal tropomyosin molecules at the pointed end of each thin filament. Although a number of proteins are associated with the barbed ends of the thin filaments at the Z disc, tropomodulin is the first protein to be specifically located at or near the pointed ends of the thin filaments. We propose that tropomodulin may cap the tropomyosin polymers at the pointed end of the thin filament and play a role in regulating thin filament length.

Probing actin incorporation into myofibrils using Asp11 and His73 actin mutants

We used a cell free system Bouché et al.: J. Cell Biol. 107:587-596, 1988] to study the incorporation of actin into myofibrils. We used alpha-skeletal muscle actin and actins with substitutions of either His73 [Solomon and Rubenstein: J. Biol.Chem. 262:11382, 1987], or Asp11 [Solomon et al.: J. Biol. Chem. 263:19662, 1988]. Actins were translated in reticulocyte lysate and incubated with myofibrils. The incorporated wild type actin could be cross-linked into dimers using N,N'-1,4-phenylenebismaleimide (PBM), indicating that the incorporated actin is actually inserted into the thin filaments of the myofibril. The His73 mutants incorporated to the same extent as wild type actin and was also cross-linked with PBM. Although some of the Asp11 mutants co-assembled with carrier actin, only 1-3% of the Asp11 mutant actins incorporated after 2 min and did not increase after 2 hr. Roughly 17% of wild type actin incorporated after 2 min and 31% after 2 hr. ATP increased the release of wild type actin from myofibrils, but did not increase the release of Asp11 mutants. We suggest that (1) the incorporation of wild type and His73 mutant actins was due to a physiological process whereas association of Asp11 mutants with myofibrils was non-specific, (2) the incorporation of wild type actin involved a rapid initial phase, followed by a slower phase, and (3) since some of the Asp11 mutants can co-assemble with wild type actin, the ability to self-assemble was not sufficient for incorporation into myofibrils. Thus, incorporation probably includes interaction between actin and a thin filament associated protein. We also showed that incorporation occurred at actin concentrations which would cause disassembly of F-actin. Since the myofibrils did not show large scale disassembly but incorporated actin, filament stability and monomer incorporation are likely to be mediated by actin associated proteins of the myofibril.

Sarcomeric myosin heavy chain expressed in nonmuscle cells forms thick filaments in the presence of substoichiometric amounts of light chains

Central to the function of myosin is its ability to assemble into thick filaments which interact precisely and specifically with other myofibrillar proteins. We have established a novel experimental system for studying myofibrillogenesis using transient transfections of COS cells, a monkey kidney cell line. We have expressed both full-length rat alpha cardiac myosin heavy chain (MHC) and a truncated heavy meromyosin-like alpha MHC (sHMM) and shown that immunoreactive MHC proteins of the expected sizes were detected in lysates of transfected cells. Surprisingly, the full-length MHC formed large spindle-shaped structures throughout the cytoplasm of transfected cells as determined by immunofluorescence microscopy. The structures were not found in cells expressing the sHMM construct, indicating that their formation required an MHC rod. The spindle-shaped structures ranged in length from approximately 1 micron to over 20 microns in length and were birefringent suggesting that they are ordered arrays of thick filaments. This was confirmed by electron microscopic analysis of the transfected cells which revealed arrays of filamentous structures approximately 12 nm in diameter at their widest point. In addition, the vast majority of transfected MHC did not associate with the endogenous nonmuscle myosin light chains, demonstrating that myosin thick filaments can form in the absence of stoichiometric amounts of myosin light chains.

Identification of a program of contractile protein gene expression initiated upon skeletal muscle differentiation

The functional diversity of skeletal muscle is largely determined by the combinations of contractile protein isoforms that are expressed in different fibers. Just how the developmental expression of this large array of genes is regulated to give functional phenotypes is thus of great interest. In the present study, we performed a comprehensive analysis of contractile protein isoform mRNA profiles in skeletal muscle systems representing each generation of fiber formed: primary, secondary, and regenerating fibers. We find that in each system examined there is a common pattern of isoform gene expression during early differentiation for 5 of the 6 gene families we have investigated: myosin light chain (MLC)1, MLC2, tropomyosin, troponin (Tn)C, and TnI. We suggest that the common isoform patterns observed together represent a genetic program of skeletal muscle differentiation that is independent of the mature fiber phenotype and is found in all newly formed myotubes. Within each of these contractile protein gene families the program is independent of the isoforms of myosin heavy chain (MHC) expressed. The maintenance of such a program may reflect a specific requirement of the initial differentiation process.

Actin filament structure probed with monoclonal antibodies

The interaction of two monoclonal antibodies (mAbs) with actin has been characterized to map the epitopes defined by these mAbs and to determine the accessibility of these sites in the actin filament (F-actin). Both mAbs react specifically with actin in radioimmunoassays and Western blot assays, and by immunoprecipitation. The location of the epitopes within the primary structure of actin has been determined using limited proteolysis of actin and Western blots, or using immunoprecipitation of truncated actin fragments synthesized in a cell free translation assay. Both mAbs bind to the C-terminal fragment of actin (residues 68-375) produced by chymotrypsin cleavage. One epitope is further localized to a 9.9 kD peptide corresponding to residues 5-93. Therefore, the epitope defined by this mAb (2G11.4) lies between residues Lys68 and Glu93 of actin. The location of the other epitope was determined by immunoprecipitation of actin fragments synthesized in vitro. Removal of residues 356-365 from the C-terminus of actin completely abolished the binding of mAb 4E3.adl. Therefore, this mAb defines an epitope that involves residues between Trp356 and Ala365. The accessibility of these epitopes in native F-actin was determined with solution binding assays and characterized by immunoelectron microscopy. Monoclonal antibody 4E3.adl binds strongly to filaments, resulting in bundling or decoration of F-actin depending on the valency of the mAb, and indicating that the epitope is readily accessible in F-actin. In contrast, mAb 2G11.4 disrupts F-actin structure, resulting in the formation of an amorphous immunoprecipitate. These results place constraints on models of actin filament structure.