Light-directed generation of the actin-activated ATPase activity of caged heavy meromyosin

Recent Advances in Protein Caging Tools for Protein Photoactivation

In biosciences and biotechnologies, it is recently critical to promote research regarding the regulation of the dynamic functions of proteins of interest. Light-induced control of protein activity is a strong tool for a wide variety of applications because light can be spatiotemporally irradiated in high resolutions. Therefore, synthetic, semi-synthetic, and genetic engineering techniques for photoactivation of proteins have been actively developed. In this review, the conventional approaches will be outlined. As a solution for overcoming barriers in conventional ones, our recent approaches in which proteins were chemically modified with biotinylated caging reagents are introduced to photo-activate a variety of proteins without genetic engineering and elaborate optimization. This review mainly focuses on protein caging and describes the concepts underlying the development of reported approaches that can contribute to the emergence of both novel protein photo-regulating methods and their killer applications.

Multimerization of Small G-protein H-Ras Induced by Chemical Modification at Hyper Variable Region with Caged Compound

The lipid-anchored small G protein Ras is a central regulator of cellular signal transduction processes, thereby functioning as a molecular switch. Ras forms a nanocluster on the plasma membrane by modifying lipids in the hypervariable region (HVR) at the C-terminus to exhibit physiological functions. In this study, we demonstrated that chemical modification of cysteine residues in HVR with caged compounds (instead of lipidation) induces multimerization of H-Ras. The sulfhydryl-reactive caged compound, 2-nitrobenzyl bromide (NBB), was stoichiometrically incorporated into the cysteine residue of HVR and induced the formation of the Ras multimer. Light irradiation induced the elimination of the 2-nitrobenzyl group, resulting in the conversion of the multimer to a monomer. SEC-HPLC and small-angle X-ray scattering (SAXS) analysis revealed that H-Ras forms a pentamer. Electron microscopic observation of the multimer showed a circular ring shape, which is consistent with the structure estimated from X-ray scattering. The shape of the multimer may reflect the physiological state of Ras. It was suggested that the multimerization and monomerization of H-Ras were controlled by modification with a caged compound in HVR under light irradiation.

Cysteine protecting groups: applications in peptide and protein science

Protecting group chemistry for the cysteine thiol group has enabled a vast array of peptide and protein chemistry over the last several decades. Increasingly sophisticated strategies for the protection, and subsequent deprotection, of cysteine have been developed, facilitating synthesis of complex disulfide-rich peptides, semisynthesis of proteins, and peptide/protein labelling in vitro and in vivo. In this review, we analyse and discuss the 60+ individual protecting groups reported for cysteine, highlighting their applications in peptide synthesis and protein science.

Selective immunocapture and light-controlled traceless release of transiently caged proteins

The 4,5-dimethoxy-2-nitrobenzyl (DMNB) photocaging group introduced into small biomolecules, peptides, oligonucleotides, and proteins is commonly used for spatiotemporal control of chemical and biological processes. Here, we describe the use of a DMNB-selective monoclonal antibody for non-covalent capture of chemically or biosynthetically produced proteins containing surface-exposed DMNB caging groups followed by light-controlled traceless decaging and release of the bound proteins into solution for a variety of downstream applications. For complete details on the use and execution of this protocol, please refer to Rakauskaitė et al. (2020).

Hydrophobic Tags for Highly Efficient Light-Activated Protein Release

We have previously described the photoactivated depot (PAD) approach for the light-stimulated release of therapeutic proteins such as insulin. The aim of this method is to release insulin from a shallow dermal depot in response to blood glucose information, using transcutaneous irradiation. Our first approach utilized a photocleavable group that linked insulin to an insoluble but injectable polymer bead. The bead conferred insolubility, ensuring that the injected material stayed at the site of injection, until light cleaved the link, and allowed insulin to be absorbed systemically. While this proved to be effective, the use of a polymer to ensure insolubility introduces two major design problems: (1) low concentration of insulin, as a majority of the material is composed of polymer, and (2) upon release of the insulin, the polymer has to be cleared from the system. To address these two problems, in this work, we have pursued "hydrophobic tags", photocleavable small nonpolar molecules that confer insolubility to the modified insulin prior to irradiation without the bulk or need for biodegradation required of polymers. We developed a combined solid- and solution-phase synthetic approach that allowed us to incorporate a range of small nonpolar moieties, including peptides, into the final depot materials. The resulting materials are >90% w/w insulin and have sharply decreased solubilities relative to unmodified insulin (≤1000 × lower). We demonstrated that they can be milled into low micron-sized particles that can be readily injected through a 31G needle. These suspensions can be prepared at an effective concentration of 20 mM insulin, a concentration at which 120 μL contains 7 days of insulin for a typical adult. Finally, upon photolysis, the insoluble particles release soluble, native insulin in a predictable fashion. These combined properties make these new modified insulins nearly ideal as candidates for PAD materials.

Light-triggered release of photocaged therapeutics - Where are we now?

The current available therapeutics face several challenges such as the development of ideal drug delivery systems towards the goal of personalized treatments for patients benefit. The application of light as an exogenous activation mechanism has shown promising outcomes, owning to the spatiotemporal confinement of the treatment in the vicinity of the diseased tissue, which offers many intriguing possibilities. Engineering therapeutics with light responsive moieties have been explored to enhance the bioavailability, and drug efficacy either in vitro or in vivo. The tailor-made character turns the so-called photocaged compounds highly desirable to reduce the side effects of drugs and, therefore, have received wide research attention. Herein, we seek to highlight the potential of photocaged compounds to obtain a clear understanding of the mechanisms behind its use in therapeutic delivery. A deep overview on the progress achieved in the design, fabrication as well as current and possible future applications in therapeutics of photocaged compounds is provided, so that novel formulations for biomedical field can be designed.

Photodegradable hydrogels for external manipulation of cellular microenvironments with real-time monitoring

A designed hydrogel whose stiffness could not only be controlled but also monitored in situ by fluorescence.

Remote control of myosin and kinesin motors using light-activated gearshifting

Cytoskeletal motors perform critical force generation and transport functions in eukaryotic cells. Engineered modifications of motor function provide direct tests of protein structure-function relationships and potential tools for controlling cellular processes or for harnessing molecular transport in artificial systems. Here, we report the design and characterization of a panel of cytoskeletal motors that reversibly change gears--speed up, slow down or switch directions--when exposed to blue light. Our genetically encoded structural designs incorporate a photoactive protein domain to enable light-dependent conformational changes in an engineered lever arm. Using in vitro motility assays, we demonstrate robust spatiotemporal control over motor function and characterize the kinetics of the optical gearshifting mechanism. We have used a modular approach to create optical gearshifting motors for both actin-based and microtubule-based transport.

The path to visualization of walking myosin V by high-speed atomic force microscopy

The quest for understanding the mechanism of myosin-based motility started with studies on muscle contraction. From numerous studies, the basic frameworks for this mechanism were constructed and brilliant hypotheses were put forward. However, the argument about the most crucial issue of how the actin-myosin interaction generates contractile force and shortening has not been definitive. To increase the "directness of measurement", in vitro motility assays and single-molecule optical techniques were created and used. Consequently, detailed knowledge of the motility of muscle myosin evolved, which resulted in provoking more arguments to a higher level. In parallel with technical progress, advances in cell biology led to the discovery of many classes of myosins. Myosin V was discovered to be a processive motor, unlike myosin II. The processivity reduced experimental difficulties because it allowed continuous tracing of the motor action of single myosin V molecules. Extensive studies of myosin V were expected to resolve arguments and build a consensus but did not necessarily do so. The directness of measurement was further enhanced by the recent advent of high-speed atomic force microscopy capable of directly visualizing biological molecules in action at high spatiotemporal resolution. This microscopy clearly visualized myosin V molecules walking on actin filaments and at last provided irrefutable evidence for the swinging lever-arm motion propelling the molecules. However, a peculiar foot stomp behavior also appeared in the AFM movie, raising new questions of the chemo-mechanical coupling in this motor and myosin motors in general. This article reviews these changes in the research of myosin motility and proposes new ideas to resolve the newly raised questions.

Small photoactivatable molecules for controlled fluorescence activation in living cells

The search for chemical probes which allow a controlled fluorescence activation in living cells represent a major challenge in chemical biology. To be useful, such probes have to be specifically targeted to cellular proteins allowing thereof the analysis of dynamic aspects of this protein in its cellular environment. The present paper describes different methods which have been developed to control cellular fluorescence activation emphasizing the photochemical activation methods known to be orthogonal to most cellular components and, in addition, allowing a spatio-temporal controlled triggering of the fluorescent signal.

Photochemical tools to study dynamic biological processes

Light-responsive biologically active compounds offer the possibility to study the dynamics of biological processes. Phototriggers and photoswitches have been designed, providing the capability to rapidly cause the initiation of wide range of dynamic biological phenomena. We will discuss, in this article, recent developments in the field of light-triggered chemical tools, specially how two-photon excitation, "caged" fluorophores, and the photoregulation of protein activities in combination with time-resolved x-ray techniques should break new grounds in the understanding of dynamic biological processes.

Caged molecules: principles and practical considerations

A caged molecule is an inert but photosensitive molecule that is transformed by photolysis into a biologically active molecule at high speed (typically 1 msec). The process is referred to as photorelease. The spatial resolution of photorelease is limited by the properties of light; submicrometer resolution is potentially achievable. Therefore, focal photorelease of caged molecules enables one to control biological processes with high spatio-temporal precision. The principles underlying caged molecules as well as practical considerations for their use are discussed in this unit.

Alterations to myofibrillar protein function in nonischemic regions of the heart early after myocardial infarction

Remote-zone left ventricular dysfunction (LVD) contributes to global reductions in contractile function after localized myocardial infarction (MI). However, the molecular mechanisms underlying this form of LVD are not clear. This study tested the hypothesis that myofibrillar protein function is directly affected in remote-zone LVD early after MI. Cardiac myosin and native thin filaments were purified from mouse myocardium taken from both the nonnecrotic zone adjacent to and the nonischemic zone remote from an infarct induced by 1 h of coronary occlusion followed by 24 h of reperfusion. Thin filament velocities were measured using the in vitro motility assay. Results showed that overall function was significantly reduced in samples from both the adjacent (43 +/- 12% of control, n = 7) and remote (53 +/- 8% of control, n = 13) zones when compared with control proteins (P < 0.05). Myosin from the remote zone propelled control thin filaments at reduced velocities similar to those measured above. In contrast, the Ca(2+) sensitivity of remote-zone thin filaments over control myosin was unchanged from control thin filaments (half-maximal at pCa 6.32 +/- 0.08 and 6.27 +/- 0.06, respectively) but showed a 20% increase in velocity at saturating Ca(2+) that parallels an increase in tropomyosin phosphorylation. Myosin dysfunction may be related to oxidation of cysteines in the myosin heavy chains or carbonylation of myosin binding protein-C. We hypothesize that phosphorylation of tropomyosin may serve a compensatory role, augmenting contraction during periods of oxidative stress when myosin function is compromised.

Reversible inactivation of the CG specific SssI DNA (cytosine-C5)-methyltransferase with a photocleavable protecting group

Caging of proteins by conjugation with a photocleavable group is a powerful approach for reversibly blocking enzymatic activity. Here we describe the covalent modification of the bacterial SssI DNA methyltransferase (M.SssI) with the cysteine-specific reagent 4,5-dimethoxy-2-nitrobenzylbromide (DMNBB). M.SssI contains two cysteine residues; replacement of the active-site Cys141 with Ser resulted in an approximately 100-fold loss of enzymatic activity; this indicates an important role for this residue in catalysis. However, replacement of Cys368 with Ala did not affect methyltransferase activity. Treatment of the Cys368Ala mutant enzyme with DMNBB led to an almost complete loss of activity. Irradiation of the inactivated enzyme with near-ultraviolet light (320-400 nm) restored 60 % of the catalytic activity. This indicates that caging by DMNBB can be used for the reversible inactivation of M.SssI.

Family of site-selective molecular optical switches

[reaction: see text] We describe the design, synthesis, and characterization of a family of thiol-reactive optical switches for labeling proteins and other biomolecules. Site-selective introduction of photochromic probes within biomolecules is being used as part of a new approach for optical control of biomolecular interactions and activities within cells. The thiol-reactive photochromic probes described in this report include a spironaphthoxazine and five spirobenzopyrans. The location of the thiol-reactive group on the spirobenzopyran is different for each probe; this feature can be used to control the geometry of the optical switch within a bioconjugate. The photochromes undergo rapid and reversible, optically driven transitions between a colorless spiro (SP) state and a brightly colored merocyanine (MC) state. The MC absorption of a spironaphthoxazine conjugate is red shifted by more than 100 nm compared to the equivalent spirobenzopyran, which may be exploited for the independent control of the MC to SP transition for up to two different spironaphthoxazine and spirobenzopyran conjugates within the same sample.

A three-component photoreversible tag for thiols

[structure: see text] A one-pot coupling of a 1,3-diketone, an aldehyde, and an alkanethiol has been developed to produce a protected sulfide. Through use of an o-nitrophenylbenzaldehyde, this method provides a one-step route to a photochemically reversible thiol-protecting group. The kinetics of photolysis were established using (1)H NMR analysis, which allows for the rate to be based on the entire reaction scheme.

Optical switching of dipolar interactions on proteins

This work shows that optical switching between the spiro (SP) and merocyanine (MC) states of different photochromes specifically labeled to G-actin can be used to rapidly and reversibly modulate specific dipolar interactions within the conjugate. Members of a common spirobenzopyran photochrome and a related spironaphthoxazine that differ only in the locations of their alkylating groups were selectively labeled to Cys-374 on G-actin. The nature of MC and SP interactions within G-actin was investigated by using optical spectroscopy. The average absorption energy of the highly polarized MC is sensitive to interactions with polar groups on solvents and G-actin; the average absorption energy of the corresponding SP state was found to be relatively constant, consistent with its lower dipole moment compared with MC (5 and 20 D, respectively). Alternate excitation of spirobenzopyran G-actin conjugates with 365 and 546 nm leads to rapid transitions from the SP to MC states and MC to SP states, respectively; optical switching within spirobenzopyran-G-actin occurs with high fidelity and the recovery of specific dipolar interactions between the protein and the MC and SP states. The difference in the free energy for specific dipolar interactions between different MC states within G-actin (6 kcal/mol) is similar to that found for complexes of G-actin and its regulatory proteins. We propose, therefore, that optical switching between SP and MC within an appropriately labeled conjugate could be used to inhibit a functional interaction with a ligand in the MC, but not the SP, state.

Design and Synthesis of Photochemically Controllable Restriction EndonucleaseBamHI by Manipulating the Salt-Bridge Network in the Dimer Interface

The strategy for the design of photochemically controllable enzymes by manipulating the dimer interface is described. Employing a restriction endonuclease BamHI, the selective incorporation of amino acids having a photoremovable 6-nitroveratryl group into the specific position (Lys132) in the dimer interface of the BamHI mutant (H133A) was performed. The activity of the photofunctionalized BamHI mutant was significantly suppressed, and the following photoirradiation induced the recovery of the activity. In addition, uncaging of the 6-nitroveratryl group introduced to Lys132 did not seriously reduce the catalytic activity and affinity for the substrate. These results indicate that the activity of the enzyme can be effectively regulated by caging and uncaging of the specific amino acid in the dimer interface using the photoremovable group.

Conformational dynamics of the SH1-SH2 helix in the transition states of myosin subfragment-1

The alpha-helix containing the thiols, SH1 (Cys-707) and SH2 (Cys-697), has been proposed to be one of the structural elements responsible for the transduction of conformational changes in the myosin head (subfragment-1 (S1)). Previous studies, using a method that isolated and measured the rate of the SH1-SH2 cross-linking step, showed that this helix undergoes ligand-induced conformational changes. However, because of long incubation times required for the formation of the transition state complexes (S1.ADP.BeF(x), S1.ADP.AlF(4)-, and S1.ADP.V(i)), this method could not be used to determine the cross-linking rate constants for such states. In this study, kinetic data from the SH1-SH2 cross-linking reaction were analyzed by computational methods to extract rate constants for the two-step mechanism. For S1.ADP.BeF(x), the results obtained were similar to those for S1.ATPgammaS. For reactions involving S1.ADP.AlF(4)- and S1.ADP.V(i), the first step (SH1 modification) is rate limiting; consequently, only lower limits could be established for the rate constants of the cross-linking step. Nevertheless, these results show that the cross-linking rate constants in the transition state complexes are increased at least 20-fold for all the reagents, including the shortest one, compared with nucleotide-free S1. Thus, the SH1-SH2 helix appears to be destabilized in the post-hydrolysis state.

Catalytic subunit of protein kinase A caged at the activating phosphothreonine

Caged reagents are photoactivatable molecules with applications in biological research. While a great deal of work has been carried out on small caged molecules, less has been done on caged macromolecules, such as proteins. Caged proteins would be especially useful in signal transduction research. Since most proteins involved in cell signaling are regulated by phosphorylation, a means to cage phosphorylated proteins would be generally applicable. Here we show that the catalytic subunit of protein kinase A can be activated by thiophosphorylation at Thr-197. The modified protein can then be caged with 4-hydroxyphenacyl bromide to yield a derivative with a specific catalytic activity that is reduced by approximately 17-fold. Upon photolysis at near UV wavelengths, an approximately 15-fold increase in activity is observed, representing an approximately 85-90% yield of uncaged product with a quantum yield phi(P) = 0.21. Because protein kinases belong to a superfamily with structurally related catalytic domains, the protein chemistry demonstrated here should be applicable to a wide range of signaling proteins.

Photoremovable protecting groups: reaction mechanisms and applications

Photolabile protecting groups enable biochemists to control the release of bioactive compounds in living tissue. 'Caged compounds' (photoactivatable bioagents) have become an important tool to study the events that follow chemical signalling in, e.g., cell biology and the neurosciences. The possibilities are by no means exhausted. Progress will depend on the development of photoremovable protecting groups that satisfy the diverse requirements of new applications--a challenging task for photochemists.

Effect of adenosine 5'-[beta,gamma-imido]triphosphate on myosin head domain movements

Conventional and saturation transfer electron paramagnetic resonance spectroscopy (EPR and ST EPR) was used to study the orientation of probe molecules in muscle fibers in different intermediate states of the ATP hydrolysis cycle. A separate procedure was used to obtain ST EPR spectra with precise phase settings even in the case of samples with low spectral intensity. Fibers prepared from rabbit psoas muscle were labeled with isothiocyanate spin labels at the reactive thiol sites of the catalytic domain of myosin. In comparison with rigor, a significant difference was detected in the orientation-dependence of spin labels in the ADP and adenosine 5'-[beta,gamma-imido]triphosphate (AdoPP[CH2]P) states, indicating changes in the internal dynamics and domain orientation of myosin. In the AdoPP[CH2]P state, approximately half of the myosin heads reflected the motional state of ADP-myosin, and the other half showed a different dynamic state with greater mobility.

A myosin II mutation uncouples ATPase activity from motility and shortens step size

It is thought that Switch II of myosin, kinesin and G proteins has an important function in relating nucleotide state to protein conformation. Here we examine a myosin mutant containing an S456L substitution in the Switch II region. In this protein, mechanical activity is uncoupled from the chemical energy of ATP hydrolysis so that its gliding velocity on actin filaments is only one-tenth of that of the wild type. The mutant spends longer in the strongly bound state and exhibits a shorter step size, which together account for the reduction in in vitro velocity. This is the first single point mutation in myosin that has been found to affect step size.

Synthesis of a spin-labeled photoaffinity ATP analogue, and its use to specifically photolabel myosin cross-bridges in skeletal muscle fibers

A spin-labeled photoaffinity ATP analogue 3'(2')-O-[4-[4-oxo-(4-amido-2,2,6,6-tetramethyl-piperidino-1-oxyl)]-benz oyl]benzoyl adenosine 5'-triphosphate (SL-Bz2ATP) was synthesized and used to photolabel myosin in muscle fibers. Previous work has shown that 3'(2')-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (Bz2ATP) photolabeled Ser-324 of the 50 kDa tryptic fragment of skeletal S1 heavy chain. In this work, [alpha-32P]SL-Bz2ATP was hydrolyzed and trapped as the diphosphate analogue with Co2+ and orthovanadate at the active site of myosin in rabbit psoas muscle fibers. After UV irradiation, the myosin heavy chain was the only protein band found to be significantly photolabeled as assayed by gel electrophoresis and radioactivity counting. The labeling was localized after brief trypsin digestion by SDS-PAGE to be on the 50 kDa tryptic fragment of the S1 heavy chain. Ca. 35% of the myosin in fibers was covalently photolabeled. The fibers photolabeled with SL-Bz2ATP had the same active tension and maximum shortening velocity as the control fibers. The resulting spin label on myosin was too mobile to report the orientation of the heads in fibers. Nonetheless, this is the first work to show the feasibility of utilizing active site binding and photoaffinity labeling to place covalent spectroscopic probes at the myosin active site in fibers with high specificity and yield without affecting mechanical function.

Effects of SH1 and SH2 modifications on myosin: similarities and differences

The properties of myosin modified at the SH2 group (Cys-697) were studied and compared with the previously reported properties of myosin modified at the SH1 group (Cys-707). 4-[N-[(iodoacetoxy)ethyl]-N methylamino]-7-nitrobenz-2-oxa-1, 3-diazole (IANBD) was used for selective modification of the SH2 group on myosin. SH2-labeled heavy meromyosin (SH2-HMM), similar to SH1-labeled HMM (SH1-HMM), did not propel actin filaments in the in vitro motility assays. SH1- and SH2-HMM produced similar amounts of load in the mixtures with unmodified HMM; the sliding speed of actin filaments gradually decreased with an increase in the fraction of either one of the modified HMMs in the mixture. In analogy to SH1-labeled myosin subfragment 1 (SH1-S1), SH2-labeled S1 (SH2-S1) activated regulated actin in the in vitro motility assays. SH2 modification inhibited Mg-ATPase of S1 and its activation by actin. The weak binding of S1 to actin was unaffected whereas the strong binding was weakened by SH2 modification. Overall, our results demonstrate similar behavior of SH1- and SH2-modified myosin heads in the in vitro motility assays despite some differences in their enzymatic properties. The effects of these modifications are ascribed to the location of the SH1-SH2 helix relative to other functional centers of S1.

Light-activated proteins

A wide assortment of caged compounds, which are species whose biological activity can be unleashed with light, have been synthesized and used to investigate a variety of biological phenomena. In contrast, the construction of caged proteins and their application to biological systems has lagged far behind. Recent advances in the synthesis of caged proteins, as well as the development of intracellular protein delivery systems, furnish a framework upon which light-activated proteins can be designed, synthesized and employed to address questions of biological significance.

Luminescence resonance energy transfer measurements in myosin

Myosin is thought to generate force by a rotation between the relative orientations of two domains. Direct measurements of distances between the domains could potentially confirm and quantify these conformational changes, but efforts have been hampered by the large distances involved. Here we show that luminescence resonance energy transfer (LRET), which uses a luminescent lanthanide as the energy-transfer donor, is capable of measuring these long distances. Specifically, we measure distances between the catalytic domain (Cys707) and regulatory light chain domain (Cys108) of the myosin head. An energy transfer efficiency of 21.2 +/- 1.9% is measured in the myosin complex without nucleotide or actin, corresponding to a distance of 73 A, consistent with the crystal structure of Rayment et al. Upon binding to actin, the energy transfer efficiency decreases by 4.5 +/- 1.0%, indicating a conformational change in myosin that involves a relative rotation and/or translation of Cys707 relative to the light chain domain. Addition of ADP also alters the energy transfer efficiency, likely through a rotation of the probe attached to Cys707. These results demonstrate that LRET is capable of making accurate measurements on the relatively large actomyosin complex, and is capable of detecting conformational changes between the catalytic and light chain domains of myosin.

Signaling pathways underlying eosinophil cell motility revealed by using caged peptides

Insights into structure-function relations of many proteins opens the possibility of engineering peptides to selectively interfere with a protein's activity. To facilitate the use of peptides as probes of cellular processes, we have developed caged peptides whose influence on specific proteins can be suddenly and uniformly changed by near-UV light. Two peptides are described which, on photolysis of a caging moiety, block the action of calcium-calmodulin or myosin light chain kinase (MLCK). The efficacy of theses peptides is demonstrated in vitro and in vivo by determining their effect before and after photolysis on activities of isolated enzymes and cellular functions known to depend on calcium-calmodulin and MLCK. These caged peptides each were injected into motile, polarized eosinophils, and when exposed to light promptly blocked cell locomotion in a similar manner. The results indicate that the action of calcium-calmodulin and MLCK, and by inference myosin II, are required for the ameboid locomotion of these cells. This methodology provides a powerful means for assessing the role of these and other proteins in a wide range of spatio-temporally complex functions in intact living cells.

Interaction of a Dictyostelium member of the plastin/fimbrin family with actin filaments and actin-myosin complexes

A protein purified from cytoskeletal fractions of Dictyostelium discoideum proved to be a member of the fimbrin/plastin family of actin-bundling proteins. Like other family members, this Ca(2+)-inhibited 67-kDa protein contains two EF hands followed by two actin-binding sites of the alpha-actinin/beta-spectrin type. Dd plastin interacted selectively with actin isoforms: it bound to D. discoideum actin and to beta/gamma-actin from bovine spleen but not to alpha-actin from rabbit skeletal muscle. Immunofluorescence labeling of growth phase cells showed accumulation of Dd plastin in cortical structures associated with cell surface extensions. In the elongated, streaming cells of the early aggregation stage, Dd plastin was enriched in the front regions. To examine how the bundled actin filaments behave in myosin II-driven motility, complexes of F-actin and Dd plastin were bound to immobilized heavy meromyosin, and motility was started by photoactivating caged ATP. Actin filaments were immediately propelled out of bundles or even larger aggregates and moved on the myosin as separate filaments. This result shows that myosin can disperse an actin network when it acts as a motor and sheds light on the dynamics of protein-protein interactions in the cortex of a motile cell where myosin II and Dd plastin are simultaneously present.