Cross-bridge binding to actin and force generation in skinned fibers of the rabbit psoas muscle in the presence of antibody fragments against the N-terminus of actin

Response of equatorial x-ray reflections and stiffness to altered sarcomere length and myofilament lattice spacing in relaxed skinned cardiac muscle

Low angle x-ray diffraction measurements of myofilament lattice spacing (D(1,0)) and equatorial reflection intensity ratio (I(1,1)/I(1,0)) were made in relaxed skinned cardiac trabeculae from rats. We tested the hypothesis that the degree of weak cross-bridge (Xbr) binding, which has been shown to be obligatory for force generation in skeletal muscle, is modulated by changes in lattice spacing in skinned cardiac muscle. Altered weak Xbr binding was detected both by changes in I(1,1)/I(1,0) and by measurements of chord stiffness (chord K). Both measurements showed that, similar to skeletal muscle, the probability of weak Xbr binding at 170-mM ionic strength was significantly enhanced by lowering temperature to 5 degrees C. The effects of lattice spacing on weak Xbr binding were therefore determined under these conditions. Changes in D(1,0), I(1,1)/I(1,0), and chord K by osmotic compression with dextran T500 were determined at sarcomere lengths (SL) of 2.0 and 2.35 micro m. At each SL increasing [dextran] caused D(1,0) to decrease and both I(1,1)/I(1,0) and chord K to increase, indicating increased weak Xbr binding. The results suggest that in intact cardiac muscle increasing SL and decreasing lattice spacing could lead to increased force by increasing the probability of initial weak Xbr binding.

Structural transition at actin's N-terminus in the actomyosin cross-bridge cycle

Force and motion generation by actomyosin involves the cyclic formation and transition between weakly and strongly bound complexes of these proteins. Actin's N-terminus is believed to play a greater role in the formation of the weakly bound actomyosin states than in the formation of the strongly bound actomyosin states. It has been the goal of this project to determine whether the interaction of actin's N-terminus with myosin changes upon transition between these two states. To this end, a yeast actin mutant, Cys-1, was constructed by the insertion of a cysteine residue at actin's N-terminus and replacement of the C-terminal cysteine with alanine. The N-terminal cysteine was labeled stoichiometrically with pyrene maleimide, and the properties of the modified mutant actin were examined prior to spectroscopic measurements. Among these properties, actin polymerization, strong S1 binding, and the activation of S1 ATPase by pyrenyl-Cys-1 actin were not significantly different from those of wild-type yeast actin, while small changes were observed in the weak S1 binding and the in vitro motility of actin filaments. Fluorescence changes upon binding of S1 to pyrenyl-Cys-1 actin were measured for the strongly (with or without ADP) and weakly (with ATP and ATPgammaS) bound acto-S1 states. The fluorescence increased in each case, but the increase was greater (by about 75%) in the presence of MgATP and MgATPgammaS than in the rigor state. This demonstrates a transition at the S1 contact with actin's N-terminus between the weakly and strongly bound states, and implies either a closer proximity of the pyrene probe on Cys-1 to structural elements on S1 (most likely the loop of residues 626-647) or greater S1-induced changes at the N-terminus of actin in the weakly bound acto-S1 states.

Effect of ATP analogues on the actin-myosin interface

The interaction between skeletal myosin subfragment 1 (S1) and filamentous actin was examined at various intermediate states of the actomyosin ATPase cycle by chemical cross-linking experiments. Reaction of the actin-S1 complex with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide and N-hydroxysuccinimide generated products with molecular masses of 165 and 175 kDa, in which S1 loops of residues 626-647 and 567-578 were cross-linked independently to the N-terminal segment of residues 1-12 of one actin monomer, and of 265 kDa, in which the two loops were bound to the N termini of two adjacent monomers. In strong-binding complexes, i.e., without nucleotide or with ADP, S1 was sequentially cross-linked to one and then to two actin monomers. In the weak-binding complexes, two types of cross-linking pattern were observed. First, during steady-state hydrolysis of ATP or ATPgammaS at 20 degreesC, the cross-linking reaction gave rise to a small amount of unknown 200 kDa product. Second, in the presence of AMPPNP, ADP.BeFx, ADP.AlF4-, or ADP.VO43- or with S1 internally cross-linked by N,N'-p-phenylenedimaleimide, only the 265 kDa product was obtained. The presence of 200 mM salt inhibited cross-linking reactions in both weak- and strong-binding states, while it dissociated only weak-binding complexes. These results indicate that, in the weak-binding state populated with the ADP.Pi analogues, skeletal S1 interacts predominantly and with an apparent equal affinity with the N termini of two adjacent actin monomers, while these ionic contacts are much less significant in stabilizing the rigor actin-S1 complexes. They also suggest that the electrostatic actin-S1 interface is not influenced by the type of ADP.Pi analogue bound to the active site.

Electrophoretic separation and quantitation of cardiac myosin heavy chain isoforms in eight mammalian species

A protocol for sample preparation and gel electrophoresis is described that reliably results in the separation of the alpha- and beta-isoforms of cardiac myosin heavy chain (MHC-alpha and MHC-beta) in eight mammalian species. The protocol is based on a simple, nongradient denaturing gel. The magnitude of separation of MHC-alpha and MHC-beta achieved with this protocol is sufficient for quantitative determination of the relative amounts of these two isoforms in mouse, rat, guinea pig, rabbit, canine, pig, baboon, and human myocardial samples. The sensitivity of the protocol is sufficient for the detection of MHC isoforms in samples at least as small as 1 microgram. The glycerol concentration in the separating gel is an important factor for successfully separating MHC-alpha and MHC-beta in myocardial samples from different species. The effect of sample load on MHC-alpha and MHC-beta band resolution is illustrated. The results also indicate that inclusion of a homogenization step during sample preparation increases the amount of MHC detected on the gel for cardiac samples to a much greater extent than for skeletal muscle samples. Although the protocol described in this study is excellent for analyzing cardiac samples, it should be noted that the same protocol is not optimal for separating MHC isoforms expressed in skeletal muscle, as is illustrated.

Perturbations of functional interactions with myosin induce long-range allosteric and cooperative structural changes in actin

The role of the rotational dynamics of actin filaments in their interaction with myosin was studied by comparing the effect of myosin subfragment 1 (S1) with two other structural perturbations, which have substantial inhibitory effects on activation of myosin ATPase and in vitro motility of F-actin: (1) binding of the antibody fragment Fab(1-7) against the first seven N-terminal residues and (2) copolymerization with monomers treated with the zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC), referred to as EDC-actin. The rotational motion of actin was measured by time-resolved phosphorescence anisotropy (TPA) of erythrosin iodoacetamide (ErIA) attached to Cys 374 on actin. The binding of S1 in a rigor complex (no nucleotide) induced intramonomer (allosteric) and intermonomer (cooperative) structural changes that increased the residual anisotropy of labeled F-actin, indicating a conformational change in the region of the C terminus. Similar allosteric and cooperative changes were induced by binding of Fab(1-7) and by copolymerization of the ErIA-labeled actin monomers with EDC-actin. This suggests that the functional perturbations transform actin to a form resembling the rigor actomyosin complex. The correlation of the perturbation-induced changes in TPA of actin with the functional effects suggests that the actomyosin interaction can be inhibited by stabilization of actin in one of its structural intermediates.

Mutational analysis of the role of the N terminus of actin in actomyosin interactions. Comparison with other mutant actins and implications for the cross-bridge cycle

Yeast actin mutants with acidic residues at the N terminus either neutralized (DNEQ) or deleted (delta-DSE) were used to assess the role of N-terminal acidic residues in the interactions of actin with myosin in the contractile cycle. Cosedimentation experiments revealed an approximately 3-fold decrease in the binding constant for DNEQ and delta-DSE actins to myosin subfragment-1 (S1) relative to that of wild type actin both in the presence of MgATP and in the absence of nucleotides (strong binding). DNEQ and delta-DSE actins protected S1 from tryptic digestion as well as the wild type and rabbit actins. The activation of S1 ATPase by DNEQ and delta-DSE actins (up to 50 microM) was very low but increased greatly after cross-linking these mutant actins to S1 by dimethyl suberimidate. Thus, the increased dissociation of mutant actins from S1 in the presence of ATP is the main cause for the low acto-S1 ATPase activities. At low-ionic strength conditions and in the presence of methylcellulose, the DNEQ and delta-DSE actins moved in the in vitro motility assays at a mean velocity similar to that of wild type actin (3.0 microns/s). Yet, the sliding velocity of the N-terminal and D24A/D25A and E99A/E100A mutant actins decreased relative to that of the wild type at all levels of external load introduced into the assay and at low densities of heavy meromyosin (HMM) on the cover slip. This indicates a lower relative force generation with the mutant actins. In contrast, the force generated under the same conditions with the 4Ac mutant actin (with four acidic charges at the N terminus) was higher than with wild type actin. At higher-ionic strength conditions (I = 150 mM), the sliding of the DNEQ and delta-DSE as well as that of the D24A/D25A and E99A/E100A actins ceased even in the presence of methylcellulose, while I341A actin (deficient in strong binding to myosin) still moved. These results indicate the importance of electrostatic actomyosin interactions under physiological salt conditions and show functionally distinct roles for the different myosin binding sites on actin.