Contraction of the heart results from interaction of the myosin and actin filaments. Cardiac myosin filaments consist of the molecular motor myosin II, the sarcomeric template protein, titin, and the cardiac modulatory protein, myosin binding protein C (MyBP-C). Inherited hypertrophic cardiomyopathy (HCM) is a disease caused mainly by mutations in these proteins. The structure of cardiac myosin filaments and the alterations caused by HCM mutations are unknown. We have used electron microscopy and image analysis to determine the three-dimensional structure of myosin filaments from wild-type mouse cardiac muscle and from a electron microscopy ͉ MyBP-C ͉ thick filament ͉ three-dimensional reconstruction

Myosin-binding protein C (MyBP-C) is a thick filament protein playing an essential role in muscle contraction, and MyBP-C mutations cause heart and skeletal muscle disease in millions worldwide. Despite its discovery 40 y ago, the mechanism of MyBP-C function remains unknown. In vitro studies suggest that MyBP-C could regulate contraction in a unique way—by bridging thick and thin filaments—but there has been no evidence for this in vivo. Here we use electron tomography of exceptionally well preserved muscle to demonstrate that MyBP-C does indeed bind to actin in intact muscle. This binding implies a physical mechanism for communicating the relative sliding between thick and thin filaments that does not involve myosin and which could modulate the contractile process.

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Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

P-glycoprotein (Pgp) is an efflux pump important in multidrug resistance of cancer cells and in determining drug pharmacokinetics. Pgp is a prototype ATP-binding cassette transporter with two nucleotide-binding domains (NBDs) that bind and hydrolyze ATP. Conformational changes at the NBDs (the Pgp engines) lead to changes across Pgp transmembrane domains that result in substrate translocation. According to current alternating access models (substrate-binding pocket accessible only to one side of the membrane at a time), binding of ATP promotes NBD dimerization, resulting in external accessibility of the drug-binding site (outward-facing, closed NBD conformation), and ATP hydrolysis leads to dissociation of the NBDs with the subsequent return of the accessibility of the binding site to the cytoplasmic side (inward-facing, open NBD conformation). However, previous work has not investigated these events under near-physiological conditions in a lipid bilayer and in the presence of transport substrate. Here, we used luminescence resonance energy transfer (LRET) to measure the distances between the two Pgp NBDs. Pgp was labeled with LRET probes, reconstituted in lipid nanodiscs, and the distance between the NBDs was measured at 37 °C. In the presence of verapamil, a substrate that activates ATP hydrolysis, the NBDs of Pgp reconstituted in nanodiscs were never far apart during the hydrolysis cycle, and we never observed the NBD-NBD distances of tens of Å that have previously been reported. However, we found two main conformations that coexist in a dynamic equilibrium under all conditions studied. Our observations highlight the importance of performing studies of efflux pumps under near-physiological conditions, in a lipid bilayer, at 37 °C, and during substrate-stimulated hydrolysis.

Background: ATP induces dimerization of the nucleotide-binding domains (NBD) of ATP-binding cassette proteins, followed by ATP hydrolysis and dimer dissociation. Results: The rate of dimerization induced by MgATP is faster than previously thought. Conclusion: During the hydrolysis cycle, there is a dynamic equilibrium where neither monomers nor dimers are favored. Significance: Knowledge of the mechanism of hydrolysis by NBDs will help us understand the function of ATP-binding cassette proteins.

ATP-binding cassette exporters use the energy of ATP hydrolysis to transport substrates across membranes by switching between inward-and outward-facing conformations. Essentially all structural studies of these proteins have been performed with the proteins in detergent micelles, locked in specific conformations and/or at low temperature. Here, we used luminescence resonance energy transfer spectroscopy to study the prototypical ATP-binding cassette exporter MsbA reconstituted in nanodiscs at 37°C while it performs ATP hydrolysis. We found major differences when comparing MsbA in these native-like conditions with double electron-electron resonance data and the crystal structure of MsbA in the open inward-facing conformation. The most striking differences include a significantly smaller separation between the nucleotide-binding domains and a larger fraction of molecules with associated nucleotide-binding domains in the nucleotide-free apo state. These studies stress the importance of studying membrane proteins in an environment that approaches physiological conditions. ATP-binding cassette (ABC)2 transporters constitute one of the largest families of membrane proteins and are found in all domains of life (1-3). They can be either importers (most prokaryote ABC transporters) or exporters (most mammal ABC transporters) (1-3). The core structure of ABC proteins consists of two transmembrane domains that form the translocation pathway and two conserved nucleotide-binding domains (NBDs) that bind and hydrolyze ATP (1-3), a process essential for their function. Binding of ATP promotes the formation of an NBD dimer in a head to tail orientation (4, 5). In the resulting structure two ATP molecules are "sandwiched" at the dimer interface, and residues from both NBDs form each of the two nucleotide-binding sites (1, 4, 5). NBD dimerization is essential for ATP hydrolysis and requires ATP binding to both nucleotide-binding sites (6), whereas dissociation of the dimers occurs following hydrolysis at only one of the two sites (7). This ATP-dependent NBDs dimerization/dissociation process is coupled to rearrangements of the transmembrane helices, switching the transporters from an inward-facing conformation (dissociated NBDs) to an outward-facing conformation (dimeric NBDs), with the concomitant translocation of substrate (1, 3).Two of the most studied ABC exporters are the multidrug resistance protein P-glycoprotein (MDR1 and ABCB1) that plays a role in the resistance to chemotherapy of some forms of cancer (3),and its bacterial homolog, the lipid flippase MsbA (8). MsbA is located in the inner membrane of Gram-negative bacteria, where it transports lipid A from the inner to the outer leaflet (9, 10). MsbA has been crystallized in inward-and outward-facing conformations (11) and has also been extensively studied by different spectroscopic techniques (12-18). Several studies point to large motions between the nucleotide-free "open" inward-facing conformation (NBDs separated by tens of Angstroms) and the ATP-bound "closed" outw...

Background:In ATP-binding cassette proteins, ATP binding induces formation of nucleotide-binding domain (NBD) dimers, but the mechanism of nucleotide hydrolysis is unknown. Results: ATP hydrolysis leads to complete separation of NBD dimers, as opposed to dimer opening. Conclusion: NBD dimers dissociate during the hydrolysis cycle. Significance: Elucidation of the molecular mechanism of hydrolysis will help us understand the function of ATP-binding cassette proteins.