Velocity of shortening and myosin isozymes in two types of rabbit fast-twitch muscle fibers

The fast-twitch tibialis anterior muscle of the rabbit was stimulated (10 Hz, 8 h/day for 7 wk) to cause complete transformation of the fibers from type IIb to type IIa. The velocity of unloaded shortening of permeabilized single fiber segments dissected from control and chronically stimulated tibialis anterior muscles was measured by the slack test at 20 degrees C. The myosin isozymes in these segments were separated on pyrophosphate-containing polyacrylamide gels. Peptide mapping of the myosin chain was performed on the myosin bands cut from the gels. The velocity of unloaded shortening of the IIb fibers was significantly higher (2.50 +/- 0.09 fiber length/s; n = 6) than that of the IIa fibers (1.33 +/- 0.08 fiber lengths/s; n = 6). The two groups of fibers differed with respect to their alkali light chain complement, as assessed by nondenaturing gel analyses, and with respect to their myosin heavy chain complement, as demonstrated by peptide mapping. Thus two groups of fast-twitch muscle fibers that contain distinguishable myosin isozyme contents differ in their velocities of unloaded shortening by a factor of two.

Single-headed binding of a spin-labeled-HMM-ADP complex to F-actin. Saturation transfer electron paramagnetic resonance and sedimentation studies

The interaction of actin and spin-labeled heavy meromyosin (MSL-HMM) was studied in the presence and absence of adenosine diphosphate or 5'-adenyl-yl-imidodiphosphate (AMPPNP) to determine the contributions of single and double-headed binding. The extent of single-headed binding to actin was deduced from a comparison of the fraction of immobilized heads (fi) with the fraction of bound molecules (fs) determined by saturation-transfer EPR (ST-EPR) and sedimentation, respectively. The ST-EPR measurements depend on the reduced motion of the spin label rigidly bound to the HMM heads upon the interaction of the latter with actin. During titration of acto-MSL-HMM with nucleotide, we measured changes in fi and fs brought about by dissociation of MSL-HMM from actin. On titration with ADP, fs changed very little, remaining above 0.8, while fi decreased to approximately 0.5 at 10mM ADP, a result consistent with extensive single-headed binding of MSL-HMM to actin. On titration with AMPPNP, single-headed binding was not detected; viz., fi and fs decreased in parallel. It was not necessary to postulate a nucleotide induced state of the bound heads, differing in motional properties from that of rigor heads, to account for the results.

The role of carbohydrates in the IgG binding and suppressive activities of shed human Fc receptors

IgG Fc receptor (Fc gamma R) shed from human peripheral mononuclear cells and purified by IgG affinity chromatography bound to ConA, Pisum sativum, Ulex europeus and peanut lectins. The soluble Fc gamma R was not absorbed by wheat germ, soyabean, lentil, Sophora japonica or Helix pomotia lectins. Out of 14 different sugars, none interfered with the interaction between IgG Fc and membrane Fc gamma R. These results show that shed Fc gamma R had glucosyl, mannosyl, fucosyl and galactosyl groups, but that these components were not part of the IgG-binding site(s). The shed FcR had IgG-binding factor (BF) activity; it suppressed secondary in vitro antibody production. In accordance with the binding studies, IgG-BF activity could be adsorbed by ConA and PNA, but not by lentil lectins. The presence of 10 mM galactose or alpha-methylmannoside had no influence on biological activity, suggesting that, although present on human shed Fc gamma R, these sugars do not play a major role in its suppressive activity.

The immunomodulatory effect of human IgG Fc fragments on the oxazolone-specific immune response of high and low responder mice

Intravenous injection of human IgG Fc fragments in mice resulted in the stimulation or inhibition of an oxazolone-specific antibody response depending on the schedule of Fc fragment injection. High and low responder mice for oxazolone were injected with Fc fragments according to two protocols: either on the day of oxazolone priming, or together with the oxazolone boost, and the isotype composition of oxazolone-specific antibodies was analysed by solid phase radioimmunoassay. We found the primary and secondary anti-oxazolone IgM levels increased in all instances, irrespective of the schedule of Fc fragment treatment. In contrast, the oxazolone-specific IgG production was increased only if Fc fragments were injected at the time of antigen priming. Injection of Fc fragments together with a secondary injection of oxazolone resulted in the inhibition of oxazolone-specific IgG production. Both stimulation and inhibition of oxazolone-specific antibodies were more pronounced in the low responder C57BL/6 mice strain.

CH2 and CH3 domain deleted IgG1 paraproteins inhibit differently Fc receptor mediated binding and cytolysis

Domain deleted paraproteins are suitable tools to study the interaction between IgG domains and Fc receptor (FcR) binding sites. The effect of the C gamma 2 or C gamma 3 domain deleted paraproteins was compared on antibody dependent cellular cytotoxicity (ADCC) and on FcR mediated rosette formation. The C gamma 2 domain deleted paraprotein (TIM) had no significant effect on lymphocyte or monocyte mediated ADCC, while the C gamma 3 domain deleted paraprotein (SIZ) inhibited both types of cytotoxicity. FcR dependent rosette formation was also inhibited by SIZ but TIM was ineffective. The data further confirm our previous results suggesting a significant role of C gamma 2 domain in the transfer of killing signal in ADCC and that of C gamma 3 domain in the high affinity binding to lymphocyte FcR.

Saturation transfer electron paramagnetic resonance study of the mobility of myosin heads in myofibrils under conditions of partial dissociation

The rotational motion of rigidly spin-labeled myosin heads of glycerinated myofibrils as reflected in saturation-transfer EPR spectra behaves to a first approximation as though the heads consist of two populations with different rotational motions. An immobilized fraction has a correlation time (tau 2) of approximately 0.5 ms, comparable to that of spin-labeled subfragment-1 (S1) bound to thin filaments, while a mobile fraction has a tau 2 of 10 microseconds, comparable to that of the heads of purified myosin filaments. The effects of nonhydrolyzable ATP analogues, potassium pyrophosphate (PPi), or adenylyl imidodiphosphate, Ca2+, temperature, or ionic strength on the spectra can be analyzed in terms of the fraction of myosin heads immobilized by attachment to thin filaments, without requiring changes in the motion of either attached or detached heads.

Calcium binding to the low affinity sites in troponin C induces conformational changes in the high affinity domain. A possible route of information transfer in activation of muscle contraction

Residues 89-100 of troponin C (C89-100) and 96-116 of troponin I (I96-116) interact with each other in the troponin complex (Dalgarno, D.C., Grand, R.J.A., Levine, B.A. Moir, A., J.G., Scott, G.M.M., and Perry, S.V. (1982) FEBS Lett. 150, 54-58) and are necessary for the Ca2+ sensitivity of actomyosin ATPase (Syska, H., Wilkinson, J.M., Grand, R.J.A., and Perry, S.V. (1976) Biochem. J. 153, 375-387 and Grabarek, Z., Drabikowski, W., Leavis, P.C., Rosenfeld, S.S., and Gergely, J. (1981) J. Biol. Chem. 256, 13121-13127). We have studied Ca2+-induced changes in the region C89-100 by monitoring the fluorescence of troponin C (TnC) labeled at Cys-98 with 5-(iodoacetamidoethyl)aminonaphthalene-1-sulfonic acid. Equilibrium titration of the labeled TnC with Ca2+ indicates that the probe is sensitive to binding to both classes of sites in free TnC as well as in its complex with TnI. When Mg2 X TnC is mixed with Ca2+ in a stopped flow apparatus, there is a rapid fluorescence increase related to Ca2+ binding to the unoccupied sites I and II followed by a slower increase (k = 9.9 s-1) that represents Mg2+-Ca2+ exchange at sites III and IV. In the TnC X TnI complex, the fast phase is much larger and the Mg2+-Ca2+ exchange at sites III and IV results in a small decrease rather than an increase in the fluorescence of the probe. The possibility is discussed that the fast change in the environment of Cys-98 upon Ca2+ binding to sites I and II may be instrumental in triggering activation of the thin filament by facilitating a contact between C89-100 and I96-116.

Modulation of the interaction between the two halves of troponin C by the other troponin subunits

The interactions between troponin subunits have been studied by intrinsic fluorescence and electron spin resonance (ESR) spectroscopy. The tryptophan fluorescence of troponin T (TnT) and troponin I (TnI) when complexed with troponin C (TnC) undergoes a Ca2+-dependent transition. The midpoints of such spectral changes occur at pCa approximately equal to 6, suggesting that the conformational change of TnT and TnI is induced by Ca2+ binding to the low-affinity sites of TnC. When TnC is labelled at Cys-98 with a maleimide spin probe (MSL), the spin signal is sensitive to Ca2+ binding to both the high and the low-affinity sites of TnC in the presence of either or both of the other two troponin subunits. Since Cys-98 is located in the vicinity of one of the high-affinity sites, these results are indicative of a long-range interaction between the two halves of the TnC molecule. Our earlier kinetic studies [Wang, C.-L. A., Leavis, P. C. & Gergely, J. (1983) J. Biol. Chem. 258, 9175-9177] have shown such interactions in TnC alone. Since the ESR spectral change associated with metal binding to the low-affinity sites is only observed when MSL-TnC is complexed with TnT and/or TnI, this long-range interaction within TnC appears to be mediated through the other troponin subunits.

The influence of new thymopoietin derivatives on the immune response of inbred mice

The immunomodulatory activities of new synthetic thymopoietin derivates TP4 (Arg-Lys-Asp-Val) and TP3 (Arg-Lys-Asp) have been compared to those of TP5 (Arg-Lys-Asp-Val-Tyr) which exhibits most of the biological activity of the native hormone and probably represents the active site. Both TP4 and TP3 are shown to exert similar immunomodulatory activities to TP5 affecting both humoral and cellular responses. Primary and secondary antibody responses of high responder mice were enhanced whilst the intensity of DTH reactivity was decreased. The effect on humoral immunity was particularly apparent following administration of TP4 or TP3 to mice undergoing primary antibody responses following immunization with sub-optimal doses of antigen or suppression by CY treatment. Administration of peptide(s) elicited DTH responses in mice previously shown to exhibit genetically determined unresponsiveness: in these animals antibody responses were not modulated. The data may be interpreted that the tetra- and tri-peptide representing the N-terminal sequence of TP5 possess immunomodulatory activity which is in many aspects similar to that of TP5. The experimental systems and protocols employed are shown to be appropriate for investigating the effect(s) of potential immunomodulators on humoral and cellular immunity.

Isotype distribution and fine specificity of the antibody response of inbred mouse strains to four compounds belonging to a new group of synthetic branched polypeptides

A new group of synthetic branched polypeptides was developed to initiate a systematic study of the relationships between the chemical structure (charge, size, primary structure, configuration and conformation), the carrier potential and the antigenic properties of these biodegradable and biocompatible macromolecules. This model system has two main advantages over the previously used ones: (i) the side chains grafted to the poly(L-lysine) backbone are composed of about three DL-Ala and a single chain-terminating amino acid with different absolute configuration and/or identity, and (ii) the conformation of these polypeptides is characterized in solution. The size, charge and inside area of the four molecules selected for this study were identical; however, the identity, the absolute configuration of the chain-terminating amino acids (D-Leu, Leu, Phe or D-Phe) and, in consequence, the conformation of the macromolecules were different. The qualitative and quantitative features of the antibody response induced by the four polypeptides were characterized in inbred mouse strains by IgM and IgG type antibody levels, as well as by isotype distribution and fine specificity of antibodies produced during the primary and memory response. The intensity of the memory response and the characteristics of subclass distribution were dependent on the conformation of the branched polypeptides. These molecules carry at least two types of antigenic determinants. One is ordered to the tetrapeptide side chain, the expression of which proved to be inversely correlated with the backbone-originated helix content of the molecules. The other antigenic determinant corresponds to the common inside area of the polypeptides which is less conformation-dependent and therefore common to all four polypeptides.

Non-covalently bound C3 enhances lysis of rabbit erythrocytes through the alternative pathway

Rabbit red blood cells (RaRBC, 3 x 10(7)/ml PBS) were incubated with different amounts of purified human C3 at 37 degrees for 30 min and washed twice in PBS. Different amounts of normal human serum containing 2 mM Mg2+ and 5 mM EGTA were added to the C3-treated and control RaRBC. The extent of lysis was measured after a further incubation at 37 degrees for 40 min. Enhanced lysis was observed with C3-treated RaRBC as compared to control cells. The enhancing effect was dependent on the dose of C3 used for the treatment of RaRBC. Investigation the kinetics of lysis, the lag phase was observed to be significantly shorter with the C3-treated than with the control RaRBC. No enhancement was found when RaRBC were pretreated with preformed C3b fragment. KSCN-treated C3 (C3b-like C3), however, had a lysis-enhancing effect. These results suggest that noncovalently bound C3 molecules may have a role in the initiation and/or maintenance of the alternative pathway activation on activator cells.

A possible self-regulating mechanism mediated by C3b-acceptor-bound C3b generated by stimulated macrophages

Macrophages have been shown to produce C3 and to bear Fc receptors (FcR), and besides the various C3 receptors, they possess C3b acceptors (C3bA) as well as surface proteases capable of cleaving C3. Using the immune adherence method, we demonstrated that the amount of covalently fixed (i.e., C3bA-bound) C3b is markedly increased upon cell stimulation by phorbol myristate acetate or aggregated IgG, even in the absence of C3. The enhancement of nascent C3b (C3bx) binding to C3bA on these cells could be reversed by inhibiting the process at different stages, using either cycloheximide, phenyl-methyl-sulphonyl-fluoride, salycil hydroxamic acid, or methylamine. On the basis of our present results and earlier results, we propose a self-regulatory mechanism by which activated, C3-producing macrophages cleave C3 by their surface proteases. C3bx generated in this way fixes covalently to C3bA of the producer cells, resulting in the inhibition of FcR on these cells.

Mapping the functional topography of Fc gamma with monoclonal antibodies: localization of epitopes interacting with the binding sites of Fc receptor on human K cells

A panel of monoclonal antibodies (mAb) specific for the C gamma 2, C gamma 3 or inter C gamma 2/C gamma 3 domain epitopes was tested for inhibition of antibody-dependent cellular cytotoxicity (ADCC) specific for anti-D IgG-coated erythrocytes. Significant inhibition of ADCC was demonstrable for some antibodies having specificity for C gamma 2 or C gamma 3 domain epitopes, while others gave no inhibition. Fab fragments of a representative C gamma 2-specific antibody (A55) and C gamma 3-specific antibody (x3a8) retained their inhibitory capacity in lymphocyte-mediated ADCC, but only A55 Fab inhibited monocyte-mediated lysis. Furthermore, the Fab portion of A55 completely abolished the complement-dependent enhancement of ADCC mediated by concanavalin A-stimulated cells, while x3a8 Fab had no effect in this system. On the other hand, x3a8 Fab inhibited the binding of anti-D IgG-sensitized erythrocytes to lymphocytes while A55 Fab did not influence this latter interaction. The results suggest that C gamma 2 domain-FcR interaction is essential for the triggering of lytic process both in lymphocyte and in monocyte-mediated ADCC, while C gamma 3 domain has no role in the latter but is responsible for the appropriate contact between effector lymphocytes and target cells. A site in the region of Lys274 appears to be critical for triggering of both lymphocyte and monocyte-mediated ADCC.

Solution conformation of the C-terminal domain of skeletal troponin C. Cation, trifluoperazine and troponin I binding effects

Proton magnetic resonance spectroscopy has been used to study the cation (Mg2+, Ca2+)-dependent conformational states of the C-terminal domain of rabbit skeletal troponin C under a variety of solution conditions. Nuclear Overhauser data and paramagnetic probe observations provide definition of the configuration of this region of troponin C. Comparative study of homologous proteins identify common features of the tertiary structure relevant to the cation binding reaction. Complex formation with troponin I and the drug trifluoperazine is observed to adjust the solution conformation of the C-terminal domain of troponin C. The interactive conformational response to cation coordination and the binding of the drug and troponin I are discussed.

Complement-dependent cellular cytotoxicity: lymphoblastoid lines that activate complement component 3 (C3) and express C3 receptors have increased sensitivity to lymphocyte-mediated lysis in the presence of fresh human serum

Lymphocyte-mediated lysis of cells of the Raji, Daudi, Jijoye, and Bjab lines was elevated when fresh human serum was added to the assay. A higher proportion of effector-target conjugates was observed in the presence of human serum. In similar experiments lysis of 1301, Rael, and P3HR-1 cells was unaltered. All cell lines activated the alternative pathway of complement but they varied in the expression of receptors for complement component 3 (C3) and in the ability to fix the C3 cleavage products on their membrane. The enhancement of lysis in the presence of human serum occurred only with those cells that bound C3. This characteristic was correlated to the expression of C3 receptors. Analysis of the nature of the deposited C3 was performed with Raji cells. Raji cells exposed to human serum bound C3b as indicated by the immunoadherence test. The C3b was further processed to C3bi, because the immunoadherence declined with time and conjugate formation increased with Daudi cells, which carry the C3 receptors CR2 and CR3. This suggests that in the lytic assay lymphocytes with C3bi receptors are recruited in the presence of human serum. We assume that the bridge of C3 molecules between targets and effectors increases the avidity of their interaction.

Troponin-C-mediated calcium-sensitive changes in the conformation of troponin I detected by pyrene excimer fluorescence

Troponin I (TnI) from rabbit white skeletal muscle was labeled at cysteines 48 and 64 with the fluorescent reagent N-(1-pyrene)maleimide. The fluorescence spectra of pyrene-labeled TnI (pyr-TnI) exhibit peaks characteristic of pyrene in its monomeric form and an additional peak resulting from formation of excited dimers (excimers), indicating that the labeled cysteines are close together. Formation of a pyr-TnI-TnC complex in the absence of Ca2+ has little effect on the spectrum, but when Ca2+ is bound to the low-affinity sites of TnC there is a substantial decrease in excimer and a corresponding increase in monomer fluorescence. The involvement of the low-affinity sites in the Ca2+-induced effect is consistent with the fact that Mg2+ has no effect on pyrene fluorescence. On rapid mixing of the pyr-TnI-TnC complex with Ca2+ in a stopped-flow apparatus, most of the excimer decrease is complete within the instrumental dead time, indicating a rate constant k greater than 350 s-1, which is comparable to that of the conformational change in TnC resulting from Ca2+ binding to the low-affinity sites. Rapid mixing of the Mg2-TnC-pyr-TnI complex with Ca2+ yields similar results, suggesting that the type of metal ion present at the high-affinity sites has little, if any, effect on the probe. It has been suggested previously that Cys 48 and 64 are located in a TnT-binding region of TnI (Chong P.C.S. and Hodges, R.S. (1982) J. Biol. Chem. 255, 3757). Our results suggest that a Ca2+-induced structural change in the TnI-binding region of TnC could be transmitted to TnT by affecting the TnT-binding region of TnI as part of the chain of events in the regulation of muscle contraction.

Functional cooperation of C3b-acceptors, Fc gamma-receptors and cell-surface proteases on macrophages

Macrophages are FcR-positive cells, synthetize complement components and express proteolytic enzymes on their surface. In this paper a functional cooperation of C3b acceptor (C3bA) sites, which bind covalently nascent C3b molecules via their metastable binding site, IgG FcRs and cell surface proteases are described and the possible importance of this cooperation in regulation of immune response is discussed. It was found that isolated monocytes did not express C3bA in contrast to cultured macrophages which showed immune adherence positivity. Stimulation of macrophages resulted in enhanced expression of C3bA. C3 synthetized by macrophages was shown to be cleaved by cellular proteases which resulted in the binding of nascent C3b to C3bA. C3bA-nascent C3b interaction inhibited FcR-dependent effector functions, such as immune complex phagocytosis and antibody-dependent cellular cytotoxicity.

The influence of Igh-1 genes on the class and subclass distribution of oxazolone-specific antibodies

Previous studies have demonstrated that the level of the oxazolone-specific antibody response induced by contact sensitization is under the control of H-2 and Igh-1-linked genes. The aim of the present study was to clarify the role of H-2 and Igh-1 genes in the regulation of antibody affinity and isotype composition of oxazolone-specific antibodies. Analysis of the antibody response to oxazolone has revealed different ratios of IgG2a and IgG2b antibodies in mice carrying the Igh-1b allele and in strains carrying alleles a, c, and e. The characteristic ratio of IgG2a and IgG2b isotypes persisted during the whole period of the primary and secondary antibody response of CBA and CBA-Igb Igh-C congenic mice. The Igh-1-linked genes influenced the isotype distribution and not the affinity of oxazolone-specific antibodies induced by contact sensitization.

Kinetic studies show that Ca2+ and Tb3+ have different binding preferences toward the four Ca2+-binding sites of calmodulin

The stepwise addition of Tb3+ to calmodulin yields a large tyrosine-sensitized Tb3+ luminescence enhancement as the third and fourth ions bind to the protein [Wang, C.-L. A., Aquaron, R. R., Leavis, P. C., & Gergely, J. (1982) Eur. J. Biochem. 124, 7-12]. Since the only tyrosine residues in calmodulin are located within binding sites III and IV, these results suggest that Tb3+ binds first to sites I and II. Recent NMR studies have provided evidence that Ca2+, on the other hand, binds preferentially to sites III and IV. Kinetic studies using a stopped-flow apparatus also show that the preferential binding of Ca2+ and lanthanide ions is different. Upon rapid mixing of 2Ca-calmodulin with two Tb3+ ions, there was a small and rapid tyrosine fluorescence change, but no Tb3+ luminescence was observed, indicating that Tb3+ binds to sites I and II but not sites III and IV. When two Tb3+ ions are mixed with 2Dy-calmodulin, Tb3+ luminescence rises rapidly as Tb3+ binds to the empty sites III and IV, followed by a more gradual decrease (k = 0.4 s-1 as the ions redistribute themselves over the four sites. These results indicate that (i) both Tb3+ and Dy3+ prefer binding to sites I and II of calmodulin and (ii) the binding of Tb3+ to calmodulin is not impeded by the presence of two Ca2+ ions initially bound to the protein. Thus, the Ca2+ and lanthanide ions must exhibit opposite preferences for the four sites of calmodulin: sites III and IV are the high-affinity sites for Ca2+, whereas Tb3+ and Dy3+ prefer sites I and II.

The nature of the trifluoperazine binding sites on calmodulin and troponin-C

We have employed 1H-nuclear magnetic resonance spectroscopy to study the interaction of the drug trifluoperazine with calmodulin and troponin-C. Distinct trifluoperazine-binding sites exist in the N- and C-terminal halves of both proteins. Each site consists of a group of hydrophobic side-chains brought into proximity by the Ca2+-dependent juxtaposition of two alpha-helical segments of the protein, each, in turn, belonging to a different Ca2+-binding site in the protein half. The trifluoperazine-induced inhibition of the biological activating ability of calmodulin appears to result from conformational restrictions conferred upon the protein by the bound drug.

Effect of nucleotides, divalent cations and temperature on the tryptic susceptibility of myosin subfragment 1

The kinetics of tryptic breakdown of the heavy chain of chymotryptic myosin subfragment 1 (S1) according to the following scheme (where the numbers represent approximate masses in kDa) are altered at 21 degrees C by divalent (Formula: see text) cations (Me2+) and by ATP, ADP, adenosine 5'-[beta, gamma-imino]triphosphate or PPi, with or without Me2+. ATP or its analogs slow step 2 and accelerate steps 3 and 4, while Me2+ accelerates step 2. ATP and its analogs decrease the amount of a transient 27-kDa peptide [Hozumi, T. & Muhlrad, A. (1981) Biochemistry 20, 2945-2950]. We have found direct evidence for the suggestion in this reference that the 27-kDa peptide is not an obligatory precursor of the 25-kDa fragment and that ATP or ADP suppresses the formation of the larger N-terminal fragment rather than accelerates its breakdown. Cross-linking of sulfhydryl groups located in the 20-kDa fragment leads to trapping of MgADP in the N-terminal 25-kDa peptide [Wells, J.A. & Yount, R.G. (1980) Biochemistry 19, 1711-1717]; this process affects the tryptic fragmentation of S1 similarly to, but less effectively than, nucleotides. Acts-S1 formation prevents the effect of ATP on fragmentation. At 37 degrees C S1 loses ATPase activity; tryptic digestion proceeds more rapidly and the 50-kDa and 25-kDa fragments are degraded to small peptides. Nucleotides protect against the effects of higher temperature by producing conformational changes not only in the 27-kDa N-terminal portion (containing the putative nucleotide binding site) of the heavy chain of S1 but also in the 50-kDa peptide.