An atomic model of the thin filament in the relaxed and Ca2+-activated states

Contraction of striated muscles is regulated by tropomyosin strands that run continuously along actin-containing thin filaments. Tropomyosin blocks myosin-binding sites on actin in resting muscle and unblocks them during Ca2+-activation. This steric effect controls myosin-crossbridge cycling on actin that drives contraction. Troponin, bound to the thin filaments, couples Ca2+-concentration changes to the movement of tropomyosin. Ca2+-free troponin is thought to trap tropomyosin in the myosin-blocking position, while this constraint is released after Ca2+-binding. Although the location and movements of tropomyosin are well known, the structural organization of troponin on thin filaments is not. Its mechanism of action therefore remains uncertain. To determine the organization of troponin on the thin filament, we have constructed atomic models of low and high-Ca2+ states based on crystal structures of actin, tropomyosin and the "core domain" of troponin, and constrained by distances between filament components and by their location in electron microscopy (EM) reconstructions. Alternative models were also built where troponin was systematically repositioned or reoriented on actin. The accuracy of the different models was evaluated by determining how well they corresponded to EM images. While the initial low and high-Ca2+ models fitted the data precisely, the alternatives did not, suggesting that the starting models best represented the correct structures. Thin filament reconstructions were generated from the EM data using these starting models as references. In addition to showing the core domain of troponin, the reconstructions showed additional detail not present in the starting models. We attribute this to an extension of TnI linking the troponin core domain to actin at low (but not at high) Ca2+, thereby trapping tropomyosin in the OFF-state. The bulk of the core domain of troponin appears not to move significantly on actin, regardless of Ca2+ level. Our observations suggest a simple model for muscle regulation in which troponin affects the charge balance on actin and hence tropomyosin position.

Atomic model of a myosin filament in the relaxed state

Contraction of muscle involves the cyclic interaction of myosin heads on the thick filaments with actin subunits in the thin filaments. Muscles relax when this interaction is blocked by molecular switches on either or both filaments. Insight into the relaxed (switched OFF) structure of myosin has come from electron microscopic studies of smooth muscle myosin molecules, which are regulated by phosphorylation. These studies suggest that the OFF state is achieved by an asymmetric, intramolecular interaction between the actin-binding region of one head and the converter region of the other, switching both heads off. Although this is a plausible model for relaxation based on isolated myosin molecules, it does not reveal whether this structure is present in native myosin filaments. Here we analyse the structure of a phosphorylation-regulated striated muscle thick filament using cryo-electron microscopy. Three-dimensional reconstruction and atomic fitting studies suggest that the 'interacting-head' structure is also present in the filament, and that it may underlie the relaxed state of thick filaments in both smooth and myosin-regulated striated muscles over a wide range of species.

E93K charge reversal on actin perturbs steric regulation of thin filaments

Contraction in striated muscles is regulated by Ca2+-dependent movement of tropomyosin-troponin on thin filaments. Interactions of charged amino acid residues between the surfaces of tropomyosin and actin are believed to play an integral role in this steric mechanism by influencing the position of tropomyosin on the filaments. To investigate this possibility further, thin filaments were isolated from troponin-regulated, indirect flight muscles of Drosophila mutants that express actin with an amino acid charge reversal at residue 93 located at the interface between actin subdomains 1 and 2, in which a lysine residue is substituted for a glutamic acid. Electron microscopy and 3D helical reconstruction were employed to evaluate the structural effects of the mutation. In the absence of Ca2+, tropomyosin was in a position that blocked the myosin-binding sites on actin, as previously found with wild-type filaments. However, in the presence of Ca2+, tropomyosin position in the mutant filaments was much more variable than in the wild-type ones. In most cases (approximately 60%), tropomyosin remained in the blocking position despite the presence of Ca2+, failing to undergo a normal Ca2+-induced change in position. Thus, switching of a negative to a positive charge at position 93 on actin may stabilize negatively charged tropomyosin in the Ca2+-free state regardless of Ca2+ levels, an alteration that, in turn, is likely to interfere with steric regulation and consequently muscle activation. These results highlight the importance of actin's surface charges in determining the distribution of tropomyosin positions on thin filaments derived from troponin-regulated striated muscles.

Single Particle Analysis of Relaxed and Activated Muscle Thin Filaments

The movement of tropomyosin from actin's outer to its inner domain plays a key role in sterically regulating muscle contraction. This movement, from a low Ca2+ to a Ca2+-induced position has been directly demonstrated by electron microscopy and helical reconstruction. Solution studies, however, suggest that tropomyosin oscillates dynamically between these positions at all Ca2+ levels, and that it is the position of this equilibrium that is controlled by Ca2+. Helical reconstruction reveals only the average position of tropomyosin on the filament, and not information on the local dynamics of tropomyosin in any one Ca2+ state. We have therefore used single particle analysis to analyze short filament segments to reveal local variations in tropomyosin behavior. Segments of Ca2+-free and Ca2+ treated thin filaments were sorted by cross-correlation to low and high Ca2+ models of the thin filament. Most segments from each data set produced reconstructions matching those previously obtained by helical reconstruction, showing low and high Ca2+ tropomyosin positions for low and high Ca2+ filaments. However, approximately 20% of segments from Ca2+-free filaments fitted best to the high Ca2+ model, yielding a corresponding high Ca2+ reconstruction. Conversely, approximately 20% of segments from Ca2+-treated filaments fitted best to the low Ca2+ model and produced a low Ca2+ reconstruction. Hence, tropomyosin position on actin is not fixed in either Ca2+ state. These findings provide direct structural evidence for the equilibration of tropomyosin position in both high and low Ca2+ states, and for the concept that Ca2+ controls the position of this equilibrium. This flexibility in the localization of tropomyosin may provide a means of sterically regulating contraction at low energy cost.

Mini-thin filaments regulated by troponin-tropomyosin

Striated muscle thin filaments contain hundreds of actin monomers and scores of troponins and tropomyosins. To study the cooperative mechanism of thin filaments, "mini-thin filaments" were generated by isolating particles nearly matching the minimal structural repeat of thin filaments: a double helix of actin subunits with each strand approximately seven actins long and spanned by a troponin-tropomyosin complex. One end of the particles was capped by a gelsolin (segment 1-3)-TnT fusion protein (substituting for normal TnT), and the other end was capped by tropomodulin. EM showed that the particles were 46 +/- 9 nm long, with a knob-like mass attributable to gelsolin at one end. Average actin, tropomyosin, and gelsolin-troponin composition indicated one troponin-tropomyosin attached to each strand of the two-stranded actin filament. The minifilaments thus nearly represent single regulatory units of thin filaments. The myosin S1 MgATPase rate stimulated by the minifilaments was Ca2+-sensitive, indicating that single regulatory length particles are sufficient for regulation. Ca2+ bound cooperatively to cardiac TnC in conventional thin filaments but noncooperatively to cardiac TnC in minifilaments in the absence of myosin. This suggests that thin filament Ca2+-binding cooperativity reflects indirect troponin-troponin interactions along the long axis of conventional filaments, which do not occur in minifilaments. Despite noncooperative Ca2+ binding to minifilaments in the absence of myosin, Ca2+ cooperatively activated the myosin S1-particle ATPase rate. Two-stranded single regulatory units therefore may be sufficient for myosin-mediated Ca2+-binding cooperativity. Functional mini-thin filaments are well suited for biochemical and structural analysis of thin-filament regulation.

Modes of Caldesmon Binding to Actin

Smooth muscle caldesmon binds actin and inhibits actomyosin ATPase activity. Phosphorylation of caldesmon by extracellular signal-regulated kinase (ERK) reverses this inhibitory effect and weakens actin binding. To better understand this function, we have examined the phosphorylation-dependent contact sites of caldesmon on actin by low dose electron microscopy and three-dimensional reconstruction of actin filaments decorated with a C-terminal fragment, hH32K, of human caldesmon containing the principal actin-binding domains. Helical reconstruction of negatively stained filaments demonstrated that hH32K is located on the inner portion of actin subdomain 1, traversing its upper surface toward the C-terminal segment of actin, and forms a bridge to the neighboring actin monomer of the adjacent long pitch helical strand by connecting to its subdomain 3. Such lateral binding was supported by cross-linking experiments using a mutant isoform, which was capable of cross-linking actin subunits. Upon ERK phosphorylation, however, the mutant no longer cross-linked actin to polymers. Three-dimensional reconstruction of ERK-phosphorylated hH32K indeed indicated loss of the interstrand connectivity. These results, together with fluorescence quenching data, are consistent with a phosphorylation-dependent conformational change that moves the C-terminal end segment of caldesmon near the phosphorylation site but not the upstream region around Cys(595), away from F-actin, thus neutralizing its inhibitory effect on actomyosin interactions. The binding pattern of hH32K suggests a mechanism by which unphosphorylated, but not ERK-phosphorylated, caldesmon could stabilize actin filaments and resist F-actin severing or depolymerization in both smooth muscle and nonmuscle cells.

Helical order in tarantula thick filaments requires the "closed" conformation of the myosin head

Myosin heads are helically ordered on the thick filament surface in relaxed muscle. In mammalian and avian filaments this helical arrangement is dependent on temperature and it has been suggested that helical order is related to ATP hydrolysis by the heads. To test this hypothesis, we have used electron microscopy and image analysis to study the ability and temperature dependence of analogs of ATP and ADP.Pi to induce helical order in tarantula thick filaments. ATP or analogs were added to rigor myofibrils or purified thick filaments at 22 degrees C and 4 degrees C and the samples negatively stained. The ADP.Pi analogs ADP.AlF4 and ADP.Vi, and the ATP analogs ADP.BeFx, AMPPNP and ATPgammaNH2, all induced helical order in tarantula thick filaments, independent of temperature. In the absence of nucleotide, or in the presence of ADP or the ATP analog, ATPgammaS, there was no helical ordering. According to crystallographic and tryptophan fluorescence studies, all of these analogs, except ATPgammaS and ADP, induce the "closed" conformation of the myosin head (in which the gamma phosphate pocket is closed). We suggest that helical order requires the closed conformation of the myosin head but is not dependent on the hydrolysis of ATP.

The structure of the vertebrate striated muscle thin filament: a tribute to the contributions of Jean Hanson

Our current understanding of the structure of the thin filaments of muscle and the molecular mechanism by which thin filaments regulate muscle contraction are reviewed and discussed. We focus, in particular, on the crucial role played by Jean Hanson in these studies and on later contributions from those whose work she influenced.

An improved model for prediction of retention times of tryptic peptides in ion pair reversed-phase HPLC: its application to protein peptide mapping by off-line HPLC-MALDI MS

The proposed model is based on the measurement of the retention times of 346 tryptic peptides in the 560- to 4,000-Da mass range, derived from a mixture of 17 protein digests. These peptides were measured in HPLC-MALDI MS runs, with peptide identities confirmed by MS/MS. The model relies on summation of the retention coefficients of the individual amino acids, as in previous approaches, but additional terms are introduced that depend on the retention coefficients for amino acids at the N-terminal of the peptide. In the 17-protein mixture, optimization of two sets of coefficients, along with additional compensation for peptide length and hydrophobicity, yielded a linear dependence of retention time on hydrophobicity, with an R2 value about 0.94. The predictive capability of the model was used to distinguish peptides with close m/z values and for detailed peptide mapping of selected proteins. Its applicability was tested on columns of different sizes, from nano- to narrow-bore, and for direct sample injection, or injection via a pre-column. It can be used for accurate prediction of retention times for tryptic peptides on reversed-phase (300-A pore size) columns of different sizes with a linear water-ACN gradient and with TFA as the ion-pairing modifier.

Drosophila muscle regulation characterized by electron microscopy and three-dimensional reconstruction of thin filament mutants

Wild-type and mutant thin filaments were isolated directly from "myosinless" Drosophila indirect flight muscles to study the structural basis of muscle regulation genetically. Negatively stained filaments showed tropomyosin with periodically arranged troponin complexes in electron micrographs. Three-dimensional helical reconstruction of wild-type filaments indicated that the positions of tropomyosin on actin in the presence and absence of Ca(2+) were indistinguishable from those in vertebrate striated muscle and consistent with a steric mechanism of regulation by troponin-tropomyosin in Drosophila muscles. Thus, the Drosophila model can be used to study steric regulation. Thin filaments from the Drosophila mutant heldup(2), which possesses a single amino acid conversion in troponin I, were similarly analyzed to assess the Drosophila model genetically. The positions of tropomyosin in the mutant filaments, in both the Ca(2+)-free and the Ca(2+)-induced states, were the same, and identical to that of wild-type filaments in the presence of Ca(2+). Thus, cross-bridge cycling would be expected to proceed uninhibited in these fibers, even in relaxing conditions, and this would account for the dramatic hypercontraction characteristic of these mutant muscles. The interaction of mutant troponin I with Drosophila troponin C is discussed, along with functional differences between troponin C from Drosophila and vertebrates.

An open or closed case for the conformation of calponin homology domains on F-actin?

Calponin homology domains link many different proteins to the surface of actin filaments. However, details of the structural interactions involved and the methods used to determine them are controversial. In the case of the actin-binding protein utrophin, for example, several models have been proposed for the binding of utrophin's calponin homology domains to actin. We review and evaluate these models and their supporting data.

Heterogeneity of Z-band structure within a single muscle sarcomere: implications for sarcomere assembly

The vertebrate striated muscle Z-band connects actin filaments of opposite polarity from adjacent sarcomeres and allows tension to be transmitted along a myofibril during contraction. Z-bands in different muscles have a modular structure formed by layers of alpha-actinin molecules cross-linking actin filaments. Successive layers occur at 19 nm intervals and have 90 degrees rotations between them. 3D reconstruction from electron micrographs show a two-layer "simple" Z-band in fish body fast muscle, a three-layer Z-band in fish fin fast muscle, and a six-layer Z-band in mammalian slow muscle. Related to the number of these layers, longitudinal sections of the Z-band show a number of zigzag connections between the oppositely oriented actin filaments. The number of layers also determines the axial width of the Z-band, which is a useful indicator of fibre type; fast fibres have narrow (approximately 30-50 nm) Z-bands; slow and cardiac fibres have wide (approximately 100-140 nm) Z-bands. Here, we report the first observation of two different Z-band widths within a single sarcomere. By comparison with previous studies, the narrower Z-band comprises three layers. Since the increase in width of the wider Z-band is about 19 nm, we conclude that it comprises four layers. This finding is consistent with a Z-band assembly model involving molecular control mechanisms that can add additional layers of 19 nm periodicity. These multiple Z-band structures suggest that different isoforms of nebulin and titin with a variable number of Z-repeats could be present within a single sarcomere.

Implementation of an Algorithm for Modeling Disulfide Bond Patterns Using Mass Spectrometry

The paper describes the implementation of a software system based on the Fenyö disulfide bond assignment algorithm. The system allows an investigator to enter data derived from mass spectrum peak assignments, a target protein sequence and other experimental conditions. The output of the system is the set of disulfide bonding pattern models that are consistent with the experimental evidence. The software and code are available through a public web site, which also has a functioning, publicly accessible version of the disulfide bond modeler. This implementation was tested as part of a project to check homology-based assignments disulfide bonding patterns of human integrins.

An atomic model for actin binding by the CH domains and spectrin-repeat modules of utrophin and dystrophin

Utrophin and dystrophin link cytoskeletal F-actin filaments to the plasmalemma. Genetic strategies to replace defective dystrophin with utrophin in individuals with muscular dystrophy requires full characterization of these proteins. Both contain homologous N-terminal actin-binding motifs composed of a pair of calponin-homology (CH) domains (CH1 and CH2) that are connected by spectrin-repeat modules to C-terminal membrane-binding sequences. Here, electron microscopy and 3D reconstruction of F-actin decorated with utrophin and dystrophin actin-binding constructs were performed using Utr261 (utrophin's CH domain pair), Utr416 (utrophin's CH domains and first spectrin-repeat) and Dys246 (dystrophin's CH domain pair). The lozenge-like utrophin CH domain densities localized to the upper surface of actin subdomain 1 and extended azimuthally over subdomain 2 toward subdomains 3 and 4. The cylinder-shaped spectrin-repeat was located at the end of the CH domain pair and was aligned longitudinally along the cleft between inner and outer actin domains, where tropomyosin is present when on thin filaments. The connection between the spectrin-repeat module and the CH domains defined the orientation of CH1 and CH2 on actin. Resolution of utrophin's CH domains and spectrin-repeats permitted docking of crystal structures into respective EM densities, leading to an atomic model where both CH and spectrin-domains bind actin. The CH domain-actin interaction for dystrophin was found to be more complex than for utrophin. Binding assays showed that Utr261 and Utr416 interacted with F-actin as monomers, whereas Dys246 appeared to associate as a dimer, consistent with a bilobed Dys246 structure observed on F-actin in electron microscope reconstructions. One of the lobes was similar in shape, position and orientation to the monomeric CH domains of Utr261, while the other lobe apparently represented a second set of CH domains in the dimeric Dys246. The extensive contact made by dystrophin on actin may be used in vivo to help muscles dissipate mechanical stress from the contractile apparatus to the extracellular matrix.

The promise of hematopoietic stem cell transplantation for autoimmune diseases

Hematopoietic stem cell transplantation (HSCT) is being increasingly utilized for the treatment of a whole spectrum of severe autoimmune diseases refractory to conventional therapy. Although allogeneic HSCT has been followed by durable complete remission in a restricted number of patients with coincidental disease, the autologous procedure is generally preferred because of its lesser toxicity. Most autoimmune diseases are the consequence of a multistep process, mainly originating from the interplay of genetic, environmental, and hormonal factors. It has been postulated that if immunosuppressive regimens can eliminate or effectively reduce the level of autoreactive T and B cells, then regeneration of de novo immunity even in the autologous setting may bypass the initial breakdown of self-tolerance and ensure prolonged disease remission. As mentioned in a recent review of this field, protocol design including conditioning regimen, patient selection, stem cell source and final outcome are likely to be disease-specific. The following is a summary of the 2002 International Bone Marrow Transplantation Registry/American Society of Blood and Bone Marrow Transplantation (IBMTR/ASBMT) satellite symposium in Orlando, Florida on 24 February 2002 on 'Expanding the Promise of Hematopoietic Stem Cell Transplantation in Autoimmune Diseases'.

Ca2+ causes release of myosin heads from the thick filament surface on the milliseconds time scale

We have used electron microscopy to study the structural changes induced when myosin filaments are activated by Ca2+. Negative staining reveals that when Ca2+ binds to the heads of relaxed Ca2+ -regulated myosin filaments, the helically ordered myosin heads become disordered and project further from the filament surface. Cryo-electron microscopy of unstained, frozen-hydrated specimens supports this finding, and shows that disordering is reversible on removal of Ca2+. The structural change is thus a result of Ca2+ binding alone and not an artifact of staining. Comparison of the two techniques suggests that negative staining preserves the structure induced by Ca2+ -binding. We therefore used a time-resolved negative staining technique to determine the time scale of the structural change. Full disordering was observed within 30 ms of Ca2+ addition, and had started to occur within 10 ms, showing that the change occurs on the physiological time scale. Comparison with studies of single heavy meromyosin molecules suggests that an increased mobility of myosin heads induced by Ca2+ binding underlies the changes in filament structure that we observe. We conclude that the loosening of the array of myosin heads that occurs on activation is real and physiological; it may function to make activated myosin heads freer to contact actin filaments during muscle contraction.

Capturing time-resolved changes in molecular structure by negative staining

Imaging structural intermediates of biological processes is a key step in understanding biological function. Because intermediates are commonly short-lived, lasting only milliseconds, the main methods used to capture them have been conventional imaging of analog or inhibited states, having extended lifetimes, or rapid (millisecond timescale) freezing of intermediates with subsequent observation by cryo-EM. We have developed a simpler method that fixes structure on the millisecond timescale. The procedure consists of briefly (milliseconds) exposing the macromolecular structure of interest on an EM grid to conditions that initiate the structural change, then immediately fixing with uranyl acetate or tannic acid. Specimens are then observed by negative staining. The key finding that validates this approach is our demonstration that uranyl acetate, and in some cases tannic acid, fixes protein molecular structure on the millisecond timescale. This is demonstrated by our observation that exposure of actin and myosin filaments to these fixatives for as little as 10 ms is sufficient to fully preserve them against changes that normally induce rapid and major alteration in their molecular structure. Fixation appears to stabilize both ionic and hydrophobic bonds. This approach should be of general utility for studying transient molecular changes in many systems.

The troponin tail domain promotes a conformational state of the thin filament that suppresses myosin activity

In cardiac and skeletal muscles tropomyosin binds to the actin outer domain in the absence of Ca(2+), and in this position tropomyosin inhibits muscle contraction by interfering sterically with myosin-actin binding. The globular domain of troponin is believed to produce this B-state of the thin filament (Lehman, W., Hatch, V., Korman, V. L., Rosol, M., Thomas, L. T., Maytum, R., Geeves, M. A., Van Eyk, J. E., Tobacman, L. S., and Craig, R. (2000) J. Mol. Biol. 302, 593-606) via troponin I-actin interactions that constrain the tropomyosin. The present study shows that the B-state can be promoted independently by the elongated tail region of troponin (the NH(2) terminus (TnT-(1-153)) of cardiac troponin T). In the absence of the troponin globular domain, TnT-(1-153) markedly inhibited both myosin S1-actin-tropomyosin MgATPase activity and (at low S1 concentrations) myosin S1-ADP binding to the thin filament. Similarly, TnT-(1-153) increased the concentration of heavy meromyosin required to support in vitro sliding of thin filaments. Electron microscopy and three-dimensional reconstruction of thin filaments containing TnT-(1-153) and either cardiac or skeletal muscle tropomyosin showed that tropomyosin was in the B-state in the complete absence of troponin I. All of these results indicate that portions of the troponin tail domain, and not only troponin I, contribute to the positioning of tropomyosin on the actin outer domain, thereby inhibiting muscle contraction in the absence of Ca(2+).

Mass determination of native smooth muscle myosin filaments by scanning transmission electron microscopy

The thick filaments of vertebrate smooth muscle have a fundamentally different arrangement of myosin molecules from the bipolar, helical organization present in striated muscle filaments. This side-polar, non-helical structure is probably critical to the ability of smooth muscles to shorten by large amounts; however, details of myosin organization beyond this general description are unknown. The non-helical arrangement of myosin precludes the use of helical reconstruction methods for structural determination, and a tomographic approach is required. As a first step towards this goal we have determined the number of myosin molecules present at each 14.5 nm repeat in native smooth muscle myosin filaments by scanning transmission electron microscopy. The mass-per-length of myosin filaments was 159 kDa/nm, corresponding to 4.38(+/-0.11) (mean+/-s.e.m.) myosin molecules at each 14.5 nm level. The mass of thin filaments in the preparation (intrinsic control) was 21 kDa/nm, consistent with current models of smooth muscle thin filament structure, and the mass of tobacco mosaic virus (mass standard) was within 5% of the known value. We conclude that native smooth muscle myosin filaments contain four myosin molecules at each 14.5 nm level, two on each side of the side-polar structure.

Mechanism of phosphorylation of the regulatory light chain of myosin from tarantula striated muscle

Contraction is modulated in many striated muscles by Ca2+-calmodulin dependent phosphorylation of the myosin regulatory light chain (RLC) by myosin light chain kinase. We have investigated the biochemical mechanism of RLC phosphorylation in tarantula muscle to better understand the basis of myosin-linked regulation. In an earlier study it was concluded that the RLC occurred as two species, both of which could be phosphorylated, potentiating contraction. Here we present evidence that only a single species exists, and that this can be phosphorylated at one or two sites. In relaxed muscle we find evidence for a substantial level of basal phosphorylation at the first site. This is augmented on activation, followed by partial phosphorylation of the second site. We find in addition that Ca2+ has a dual effect on light chain phosphorylation, depending on its concentration. At low concentration (relaxing conditions) only basal phosphorylation is observed, while at higher concentrations (activating conditions) RLC phosphorylation is stimulated. At still higher Ca2+ concentrations we find partial inhibition of RLC phosphorylation, suggesting an additional mechanism by which the muscle cell can fine tune contractile activity by controlling the level of free Ca2+.

Purification of native myosin filaments from muscle

Analysis of the structure and function of native thick (myosin-containing) filaments of muscle has been hampered in the past by the difficulty of obtaining a pure preparation. We have developed a simple method for purifying native myosin filaments from muscle filament suspensions. The method involves severing thin (actin-containing) filaments into short segments using a Ca(2+)-insensitive fragment of gelsolin, followed by differential centrifugation to purify the thick filaments. By gel electrophoresis, the purified thick filaments show myosin heavy and light chains together with nonmyosin thick filament components. Contamination with actin is below 3.5%. Electron microscopy demonstrates intact thick filaments, with helical cross-bridge order preserved, and essentially complete removal of thin filaments. The method has been developed for striated muscles but can also be used in a modified form to remove contaminating thin filaments from native smooth muscle myofibrils. Such preparations should be useful for thick filament structural and biochemical studies.

Rational design of diflunisal analogues with reduced affinity for human serum albumin

Many lead compounds bind to serum albumin and exhibit markedly reduced efficacy in vivo as compared to their potency in vitro. To aid in the design of compounds with reduced albumin binding, we performed nuclear magnetic resonance (NMR) structural and binding studies on the complex between domain III of human serum albumin (HSA-III) and diflunisal, a cyclooxygenase inhibitor with antiinflammatory activity. The structural studies indicate that the aromatic rings of diflunisal are involved in extensive and specific interactions with hydrophobic residues that comprise the binding pocket in subdomain IIIA. The carboxylic acid of diflunisal forms electrostatic interactions with the protein similar to those observed in the X-ray structure of HSA complexed to myristic acid. In addition to the structural studies, NMR-derived binding constants were obtained for diflunisal and closely related analogues to develop a structure-affinity relationship for binding to subdomain IIIA. On the basis of the structural and binding data, compounds were synthesized that exhibit more than a 100-fold reduction in binding to domain III of HSA, and nearly a 10-fold reduction in affinity for full length albumin. Significantly, several of these compounds maintain activity against cyclooxygenase-2. These results suggest a rational strategy for designing out albumin binding in potential drug molecules by using structure-based design in conjunction with NMR-based screening.