Molecular charge dominates the inhibition of actomyosin in skinned muscle fibers by SH1 peptides

DNA and actin bind and inhibit interleukin-8 function in cystic fibrosis sputa: in vitro effects of mucolytics

Infection of the cystic fibrosis (CF) airways elicits an exaggerated, interleukin-8 (IL-8) mediated, neutrophil inflammatory response. Necrosing neutrophils release DNA and actin into the airways, increasing the viscoelasticity of airway secretions. Mucolytics aim to improve airway clearance by reducing this viscoelasticity. DNase I reduces the viscoelasticity of CF sputum, and a human recombinant form of this enzyme is widely administered to patients with CF. Gelsolin, which cleaves actin polymers, is also known to reduce CF sputum viscosity in vitro, and it has been proposed as a future mucolytic agent. We have shown that the anionic polymers DNA and actin bind and mask immunologic recognition of the basic peptide IL-8 and prevent this chemokine from binding to neutrophil receptors. Reduction of CF sputum viscosity by DNase I or gelsolin in vitro was demonstrated to increase the proportion of free IL-8 and the IL-8-dependent neutrophil chemotactic activity of sputum supernatants. We hypothesize that an electrostatic interaction between polymer and chemokine may limit the inflammatory potential of the latter, but that this interaction may be weakened by polymer cleavage. The potential risk of increased inflammation via this mechanism suggests a caveat should be attendant on treatment of patients with CF with these mucolytic agents.

Effects on Ca(2+)-activated tension due to a synthetic NH2-terminal actin peptide in single skeletal muscle fibers

Insight into the mechanism of force development in striated muscle will be provided by elucidating the specific regions of the actin molecule that interact with myosin and regulatory subunits of the thin filament during Ca(2+)-activated contraction. There is growing evidence that the acidic NH2-terminal domain of actin 1) may represent an important binding site for myosin and 2) may interact with the inhibitory region of troponin I. The purpose of this study was to determine the effects of a synthetic peptide corresponding to a specific sequence of the NH2-terminal domain of skeletal muscle actin on Ca(2+)-activated tension in chemically skinned single psoas skeletal muscle fibers. This study focused on the highly conserved Lys18-Arg28 amino acid sequence of actin, a region of native actin that is believed to interact with troponin I and myosin. The effects of synthetic actin peptide Lys18-Arg28 on tension development varied, depending on 1) the concentration of Ca2+ in the activating solutions and 2) the peptide concentration. At submaximal concentrations of Ca2+, isometric tension was reversibly potentiated in the presence of 100-500 microM synthetic actin peptide Lys18-Arg28. Importantly, scrambling the sequence of Lys18-Arg28 fully abolished the increase in Ca2+ sensitivity, providing evidence that the observed effects were specific to the sequence of peptide Lys18-Arg28. In contrast, maximum Ca(2+)-activated tension was inhibited by millimolar concentrations of Lys18-Arg28 and the scrambled peptide, indicating that this effect was nonspecific. The effect of peptide Lys18-Arg28 to increase the Ca2+ sensitivity of tension is not known but may be due to an effect of the actin peptide to alter thin filament activation, a possibility consistent with proposed interactions between this domain of actin and the inhibitory region of troponin I.

The specific NH2-terminal sequence Ac-EEED of alpha-smooth muscle actin plays a role in polymerization in vitro and in vivo

The blocking effect of the NH2-terminal decapeptide of alpha-smooth muscle (SM) actin AcEEED-STALVC on the binding of the specific monoclonal antibody anti-alpha SM-1 (Skalli, O., P. Ropraz, A. Trzeviak, G. Benzonana, D. Gillessen, and G. Gabbiani. 1986. J. Cell Biol. 103:2787-2796) was compared with that of synthetic peptides modified by changing the acetyl group or by substituting an amino acid in positions 1 to 5. Using immunofluorescence and immunoblotting techniques, anti-alpha SM-1 binding was abolished by the native peptide and by peptides with a substitution in position 5, indicating that AcEEED is the epitope for anti-alpha SM-1. Incubation of anti-alpha SM-1 (or of its Fab fragment) with arterial SM actin increased polymerization in physiological salt conditions; the antibody binding did not hinder the incorporation of the actin antibody complex into the filaments. This action was not exerted on skeletal muscle actin. After microinjection of the alpha-SM actin NH2-terminal decapeptide or of the epitopic peptide into cultured aortic smooth muscle cells, double immunofluorescence for alpha-SM actin and total actin showed a selective disappearance of alpha-SM actin staining, detectable at approximately 30 min. When a control peptide (e.g. alpha-skeletal [SK] actin NH2-terminal peptide) was microinjected, this was not seen. This effect is compatible with the possibility that the epitopic peptide traps a protein involved in alpha-SM actin polymerization during the dynamic filament turnover in stress fibers. Whatever the mechanism, this is the first evidence that the NH2 terminus of an actin isoform plays a role in the regulation of polymerization in vitro and in vivo.

Faster force transient kinetics at submaximal Ca2+ activation of skinned psoas fibers from rabbit

The early, rapid phase of tension recovery (phase 2) after a step change in sarcomere length is thought to reflect the force-generating transition of myosin bound to actin. We have measured the relation between the rate of tension redevelopment during phase 2 (r), estimated from the half-time of tension recovery during phase 2 (r = t0.5(-1)), and steady-state force at varying [Ca2+] in single fibers from rabbit psoas. Sarcomere length was monitored continuously by laser diffraction of fiber segments (length approximately 1.6 mm), and sarcomere homogeneity was maintained using periodic length release/restretch cycles at 13-15 degrees C. At lower [Ca2+] and forces, r was elevated relative to that at pCa 4.0 for both releases and stretches (between +/- 8 nm). For releases of -3.4 +/- 0.7 nm.hs-1 at pCa 6.6 (where force was 10-20% of maximum force at pCa 4.0), r was 3.3 +/- 1.0 ms-1 (mean +/- SD; N = 5), whereas the corresponding value of r at pCa 4.0 was 1.0 +/- 0.2 ms-1 for releases of -3.5 +/- 0.5 nm.hs-1 (mean +/- SD; N = 5). For stretches of 1.9 +/- 0.7 nm.hs-1, r was 1.0 +/- 0.3 ms-1 (mean +/- SD; N = 9) at pCa 6.6, whereas r was 0.4 +/- 0.1 ms-1 at pCa 4.0 for stretches of 1.9 +/- 0.5 (mean +/- SD; N = 14). Faster phase 2 transients at submaximal Ca(2+)-activation were not caused by changes in myofilament lattice spacing because 4% Dextran T-500, which minimizes lattice spacing changes, was present in all solutions. The inverse relationship between phase 2 kinetics and force obtained during steady-state activation of skinned fibers appears to be qualitatively similar to observations on intact frog skeletal fibers during the development of tetanic force. The data are consistent with models that incorporate a direct effect of [Ca2+] on phase 2 kinetics of individual cross-bridges or, alternatively, in which phase 2 kinetics depend on cooperative interactions between cross-bridges.

Unloaded shortening of skinned muscle fibers from rabbit activated with and without Ca2+

Unloaded shortening velocity (VUS) was determined by the slack method and measured at both maximal and submaximal levels of activation in glycerinated fibers from rabbit psoas muscle. Graded activation was achieved by two methods. First, [Ca2+] was varied in fibers with endogenous skeletal troponin C (sTnC) and after replacement of endogenous TnC with either purified cardiac troponin C (cTnC) or sTnC. Alternatively, fibers were either partially or fully reconstituted with a modified form of cTnC (aTnC) that enables force generation and shortening in the absence of Ca2+. Uniformity of the distribution of reconstituted TnC across the fiber radius was evaluated using fluorescently labeled sTnC and laser scanning fluorescence confocal microscopy. Fiber shortening was nonlinear under all conditions tested and was characterized by an early rapid phase (VE) followed by a slower late phase (VL). In fibers with endogenous sTnC, both VE and VL varied with [Ca2+], but VE was less affected than VL. Similar results were obtained after extraction of TnC and reconstitution with either sTnC or cTnC, except for a small increase in the apparent activation dependence of VE. Partial activation with aTnC was obtained by fully extracting endogenous sTnC followed by reconstitution with a mixture of aTnC and cTnC (aTnC:cTnC molar ratio 1:8.5). At pCa 9.2, VE and VL were similar to those obtained in fibers reconstituted with sTnC or cTnC at equivalent force levels. In these fibers, which contained aTnC and cTnC, VE and VL increased with isometric force when [Ca2+] was increased from pCa 9.2 to 4.0. Fibers that contained a mixture of a TnC and cTnC were then extracted a second time to selectively remove cTnC. In fibers containing aTnC only, VE and VL were proportional to the resulting submaximal isometric force compared with maximum Ca(2+)-activated control. With aTnC alone, force, VE, and VL were not affected by changes in [Ca2+]. The similarity of activation dependence of VUS whether fibers were activated in a Ca(2+)-sensitive or -insensitive manners implies that VUS is determined by the average level of thin filament activation and that, with sTnC or cTnC, VUS is affected by Ca2+ binding to TnC only.

Cross-linking myosin subfragment 1 Cys-697 and Cys-707 modifies ATP and actin binding site interactions

Skeletal muscle myosin is an enzyme that interacts allosterically with MgATP and actin to transduce the chemical energy from ATP hydrolysis into work. By modifying myosin structure, one can change this allosteric interaction and gain insight into its mechanism. Chemical cross-linking with N,N'-p-phenylenedimaleimide (pPDM) of Cys-697 to Cys-707 of the myosin-ADP complex eliminates activity and produces a species that resembles myosin with ATP bound (Burke et al., 1976). Nucleotide-free pPDM-modified myosin subfragment 1 (S1) was prepared, and its structural and allosteric properties were investigated by comparing the nucleotide and actin interactions of S1 to those of pPDM-S1. The structural properties of the nucleotide-free pPDM-S1 are different from those of S1 in several respects. pPDM-S1 intrinsic tryptophan fluorescence intensity is reduced 28%, indicating a large increase of an internal quenching reaction (the fluorescence intensity of the related vanadate complex of S1, S1-MgADP-Vi, is reduced by a similar degree). Tryptophan fluorescence anisotropy increases from 0.168 for S1 to 0.192 for pPDM-S1, indicating that the unquenched tryptophan population in pPDM-S1 has reduced local freedom of motion. The actin affinity of pPDM-S1 is over 6,000-fold lower than that of S1, and the absolute value of the product of the net effective electric charges at the acto-S1 interface is reduced from 8.1 esu2 for S1 to 1.6 esu2 for pPDM-S1. In spite of these changes, the structural response of pPDM-S1 to nucleotide and the allosteric communication between its ATP and actin sites remain intact. Compared to pPDM-S1, the fluorescence intensity of pPDM-S1 *MgADP is increased 50%(compared to 8 and 31% increases, respectively, for MgADP and MgATP binding to S1). Compared to acto-pPDM-S1, the absolute value of the product of the net effective electric charge at the actin binding interface of acto-pPDM-S1 *MgADP increases 7.3 esu2 (compared to a 0.9 esu2 decrease and an 11.0 esu2 increase, respectively, for MgADP and MgATP binding to acto-Sl).The interaction free energy for the ligands MgADP and actin, is -2.0 kcal/mol for pPDM-S1, compared to -1.2 kcal/mol for unmodified S1.

Antibody and peptide probes of interactions between the SH1-SH2 region of myosin subfragment 1 and actin's N-terminus

The negatively charged residues in the N-terminus of actin and the 697-707 region on myosin subfragment 1 (S-1), containing the reactive cysteines SH1 and SH2, are known to be important for actin-activated myosin ATPase activity. The relationship between these two sites was first examined by monitoring the rates of SH1 and SH2 modification with N-ethylmaleimide in the presence of actin and, secondly, by testing for direct binding of SH1 peptides to the N-terminal segment on actin. While actin alone protected SH1 from N-ethylmaleimide modification, this effect was abolished by an antibody against the seven N-terminal amino acids on actin, F(ab)(1-7), and was greatly reduced when the charge of acidic residues at actin's N-terminus was altered by carbodiimide coupling of ethylenediamine. Neither F(ab)(1-7) nor ethylenediamine treatment reversed the effect of F-actin on SH2 reactivity in SH1-modified S-1. These results show a communication between the SH1 region on S-1 and actin's N-terminus in the acto-S-1 complex. To test whether such a communication involves the binding of the SH1 site on S-1 to the N-terminal segment of actin, the SH1 peptide IRICRKG-NH2(4+) was used. Cosedimentation experiments revealed the binding of three to six peptides per actin monomer. Peptide binding to actin was affected slightly, if at all, by F(ab)(1-7). The antibody also did not change the polymerization of G-actin by the peptides. The peptides caused a small reduction in the binding of S-1 to actin and did not change the binding of F(ab)(1-7).(ABSTRACT TRUNCATED AT 250 WORDS)