Structure-function relations of biotin derivatives of apamin

Apamin was biotinylated at various residues so as to produce an apamin derivative that was suitable for labelling apamin sensitive K+ channels. We labelled the sole histidine residue (His-18) of apamin with diazobenzoyl biocytin (DBB) and Lys-4 with NHS-biotin and NHS-XX-biotin. We found that at least two labelled species were produced by DBB. Proton NMR spectroscopy revealed that in addition to labelling the His-18, DBB labelled the Gln-16 and Gln-17 of apamin. Both NHS-biotin and NHS-XX-biotin appeared to specifically label Lys-4. To test the potency of these apamin derivatives, we developed an assay using apamin reversal of the adrenaline induced relaxation of the mouse ascending colon. The biological activity of the His-18 derivative was 46-fold less than that of native apamin. Biotinylation of Lys-4 with NHS-biotin reduced the activity by only 6-fold. The inclusion of a 14-carbon spacer between the Lys-4 and the biotin resulted in a derivative with only a 4-fold reduction in potency.

FTIR spectroscopy study of PTHrP(1-34) involved in humoral hypercalcaemia of malignancy

The components of secondary structure of the biologically-active N-terminal domain of human parathyroid-hormone-related protein (residues 1-34) and several truncated species were examined using Fourier transform infrared (FTIR) spectroscopy. The major structural features include a segment of alpha-helix within the N-terminal segment probably extending from Glu-4 to Lys-11 with three beta-turns localized to the segments Gly-12 to Ile-15, Gln-16 to Arg-20 and His-25 to Ala-29. Some beta-sheet was detected in the full-length peptide, but not in any of the C-terminal truncated samples. These structural features were studied in the smaller peptides for the purpose of localization of the various components and with a view to describing the region likely to form the bulk of the receptor binding site.

NMR solution structure of the [Ala26]parathyroid-hormone-related protein(1-34) expressed in humoral hypercalcemia of malignancy

The structure of the biologically active N-terminal domain of human parathyroid-hormone-related protein (residues 1-34) containing an Ala substituted for a His in position 26 was studied by two-dimensional proton NMR spectroscopy. Unambiguous NMR assignments of all backbone and side-chain hydrogens were made with the aid of totally correlated spectroscopy experiments, which provided through-bond 1H-1H connectivities, and NOE spectroscopy, which provided through-space and sequential backbone connectivities. The NMR data were utilized in distance-geometry algorithms to generate a family of structures. The major structural features include two segments of alpha-helix extending from Glu4 to Lys13 and from Leu27 to Thr33, with two turns from Gln16 to Arg19 and Phe22 to His25. A salt-bridge appears likely between Arg20 and Glu30 which may be critical for holding the receptor-binding domain together.

Actin dynamics studied by solid-state NMR spectroscopy

Solid-state nuclear magnetic resonance spectroscopy was used to study the motion of 2H and 19F probes attached to the skeletal muscle actin residues Cys-10, Lys-61 and Cys-374. The probe resonances were observed in dried and hydrated G-actin, F-actin and F-actin-myosin subfragment-1 complexes. Restricted motion was exhibited by 19F probes attached to Cys-10 and Cys-374 on actin. The dynamics of probes attached to dry cysteine powder or F-actin were very similar and the binding of myosin had little effect indicating that the local probe environment imposes the major influence on motion in the solid state. Correlation times determined for the solid state probes indicated that they were undergoing some rapid internal motion in both G-actin and F-actin such as domain twisting. The probe size influenced the motion in G-actin and appeared to sense monomer rotation but not in F-actin where segmental mobility and intramonomer co-ordination appeared to dominate.

19F NMR study of the myosin and tropomyosin binding sites on actin

Actin was labeled with pentafluorophenyl isothiocyanate at Lys-61. The label was sufficiently small not to affect the rate or extent of actin polymerization unlike the much larger fluorescein 5-isothiocyanate which completely inhibits actin polymerization [Burtnick, L. D. (1984) Biochim. Biophys. Acta 791, 57-62]. Furthermore, the label resonances in the 376.3-MHz 19F NMR spectrum were unaffected by actin polymerization. However, the binding of the relaxing protein tropomyosin resulted in the fluorinated Lys-61 resonances broadening out beyond detection due to a substantial increase in the effective correlation time of the label. Similarly, the binding of myosin subfragment 1 to F-actin resulted in the dramatic broadening of the labeled Lys-61 resonances. Thus, Lys-61 on actin appears to be closely associated with the binding sites for both tropomyosin and myosin, suggesting that both these proteins can compete for the same site on actin. The other region of actin known to be involved in myosin binding, Cys-10, was found to be more remote from the actin-actin interfaces than Lys-61. Labels on Cys-10 exhibited substantially greater mobility than fluorescein 5-isothiocyanate attached to Lys-61 which appeared to be held down on the surface of the actin monomer. This may sterically hinder the actin-actin interaction about 1 nm from the tropomyosin/myosin binding site.

NMR study of a 34-residue N-terminal fragment of the parathyroid-hormone-related protein secreted during humoral hypercalcemia of malignancy

The proton resonances of the biologically active peptide parathyroid-hormone-related protein (residues 1-34) were assigned using one-dimensional spin-decoupling techniques, two-dimensional correlated spectroscopy and by comparing the spectra of the peptides 1-20, 1-25, 1-29, 7-34 and 15-34. The conformation of 1-34 was determined using one- and two-dimensional nuclear Overhauser enhancement spectroscopy in the rotating frame. Amide proton temperature coefficients, vicinal coupling constants and circular dichroic spectra helped reveal a surprisingly compact structure with residues 3-9 forming alpha-helix, type-I beta-turns between residues 10-13 and 16-19 and several interactions between the N-terminal residues and the C-terminal residues. Of these latter, the strongest appeared to be between Asp-10 and Phe-22. One peptide surface in the deduced model presents multiple positive charges, while the opposite surface has a hydrophobic character possibly functioning to exclude water from the binding interface and enhancing the binding constant.

Spatial relationship between the nucleotide-binding site, Lys-61 and Cys-374 in actin and a conformational change induced by myosin subfragment-1 binding

The spatial relationship between Lys-61, the nucleotide binding site and Cys-374 was studied. Lys-61 was labelled with fluorescein-5-isothiocyanate as a resonance energy acceptor, the nucleotide-binding site was labelled with the fluorescent ATP analogues epsilon ATP or formycin-A 5'-triphosphate (FTP) and Cys-374 was labelled with 5-(2-[(iodoacetyl)amino]ethyl)aminonaphthalene-1-sulfonic acid (1,5-IAEDANS) as a resonance energy donor. The distances between the nucleotide binding site and Lys-61 or between Lys-61 and Cys-374 were calculated to be 3.5 +/- 0.3 nm and 4.60 +/- 0.03 nm, respectively. (The assumption has been made in calculating these distances that the energy donor and acceptor rotate rapidly relative to the fluorescence lifetime.) On the other hand, when doubly-labelled actin with 1,5-IAEDANS at Cys-374 and FITC at Lys-61 was polymerized in the presence of a twofold molar excess of phalloidin [Miki, M. (1987) Eur. J. Biochem. 164, 229-235], the fluorescence of 1,5-IAEDANS bound to actin was quenched significantly. This could be attributed to inter-monomer energy transfer. The inter-monomer distance between FITC attached to Lys-61 in a monomer and 1,5-IAEDANS attached to Cys-374 in its nearest-neighbour monomer in an F-actin filament was calculated to be 3.34 +/- 0.06 nm, assuming that the likely change in the intra-monomer distance does not change during polymerization by more than 0.4 nm. One possible spatial relationship between Lys-61, Cys-374 and the nucleotide binding site in an F-actin filament is proposed. The effect of myosin subfragment-1 (S1) binding on the energy transfer efficiency was studied. The fluorescence intensity of AEDANS-FITC-actin decreased by 30% upon interaction with S1. The fluorescence intensity of AEDANS-FITC-actin polymer in the presence of phalloidin increased by 21% upon interaction with S1. The addition of ATP led to the fluorescence intensity returning to the initial level. Assuming that the change of fluorescence intensity can be attributed to conformational change in the actin molecule induced by S1 binding, the intra-monomer distance was reduced by 0.4 nm and the inter-monomer distance was increased by 0.2 nm.

Fluorescence resonance energy transfer between sites in G-actin. The spatial relationship between Cys-10, Tyr-69, Cys-374, the high-affinity metal and the nucleotide

Intramonomer fluorescence resonance energy transfer spectroscopy was employed to investigate the spatial relationship between labels attached to the residues Cys-10, Tyr-69, Cys-374, the high-affinity metal binding site and the nucleotide binding site in G-actin. The separation between the fluorescence donor 5-(dimethylamino)naphthalene-1-sulphonyl (Dns) chloride (dansyl chloride) used to label Tyr-69 and the acceptor 4-dimethylaminophenylazophenyl-4'-maleimide (DABM) used to label Cys-374 was found to be 3.6 nm. The distance separating Dns on Tyr-69 from DABM on Cys-10 was found to be 2.7 nm. The distance separating the acceptor DABM bound to Cys-374 from the fluorescence donor formycin A 5'-triphosphate (FTP) occupying the nucleotide binding site was determined to be 3.0 nm. A slightly larger separation was determined between the FTP site and DABM attached to Cys-10. In this case a value of 3.2 nm was obtained. The distance separating Dns on Tyr-69 from Co2+ in the high-affinity metal binding site was determined to be 1.1 nm. Finally, the separation of FTP, now acting as donor, from the Dns molecule attached to Tyr-69 and acting as the acceptor was determined to be 2.1 nm. The likely relationship between these label sites on actin is represented by a model which is used to assist in the determination of the actin structure, with particular reference to the environment of the metal and nucleotide binding sites.

Conformation of the ATP binding peptide in actin revealed by proton NMR spectroscopy

The actin peptide 106-124 exists in a completely conserved region of the sequence and binds strongly to both ATP and tripolyphosphate. Binding particularly affects residues 116 and 118 and generally affects the two segments 115-118 and 121-124 [Barden, J. A., & Kemp, B. E. (1987) Biochemistry 26, 1471-1478]. One-dimensional nuclear Overhauser enhancement difference spectroscopy was used to detect molecular interactions between both adjacent and nonadjacent residues. The N-terminal segment 106-112 was found to be largely extended. A sharp bend was detected between Pro-112 and Lys-113. The triphosphate moiety binds to the strongly hydrophilic central segment of the peptide. Evidence was obtained for a reverse turn involving residues 121-124. Amide proton temperature coefficients and coupling constants provide evidence for a type I beta-turn. A model of the ATP binding site is proposed together with its relationship to other parts of the actin structure and to the phalloidin binding site.

Interaction of phalloidin with chemically modified actin

Modification of Tyr-69 with tetranitromethane impairs the polymerizability of actin in accordance with the previous report [Lehrer, S. S. and Elzinga, M. (1972) Fed. Proc. 31, 502]. Phalloidin induces this chemically modified actin to form the same characteristic helical thread-like structure as normal F-actin. The filaments bind myosin heads and activate the myosin ATPase activity as effectively as normal F-actin. When a dansyl group is introduced at the same point [Chantler, P. D. and Gratzer, W. B. (1975) Eur. J. Biochem. 60, 67-72], phalloidin still induces the polymerization. The filaments bind myosin heads and activate the myosin ATPase activity. These results indicate that Tyr-69 is not directly involved in either an actin-actin binding site or the myosin binding site on actin. Moreover, the results suggest that phalloidin binds to actin monomer in the presence of salt and its binding induces a conformational change in actin which is essential for polymerization, or that actin monomer fluctuates between in unpolymerizable and polymerizable form while phalloidin binds to actin only in the polymerizable form and its binding locks the conformation which causes the irreversible polymerization of actin. Modification of Tyr-53 with 5-diazonium-(1H)tetrazole blocks actin polymerization [Bender, N., Fasold, H., Kenmoku, A., Middelhoff, G. and Volk, K. E. (1976) Eur. J. Biochem. 64, 215-218]. Phalloidin is unable to induce the polymerization of this modified actin nor does it bind to it. Phalloidin does not induce the polymerization of the trypsin-digested actin core. These results indicate that the site at which phalloidin binds is involved in polymerization and the probable conformational change involved in polymerization may be modulated through this site.

Fluorescence resonance energy transfer measurements of distances in actin and myosin. A critical evaluation

The contractile proteins actin and myosin are of considerable biological interest. They are essential for muscle contraction and in eukaryotic cells they play a crucial role in most contractile phenomena. Over the years since the first fluorescence resonance energy transfer (FRET) paper appeared, an extensive body of literature has accumulated on this technique using actin, myosin and the actomyosin complex. These papers are reviewed with several aims in mind: we assess the reliability and consistency of intra- and inter-molecular distances measured between the fluorescent probes attached to specific sites on these proteins; we determine whether the measurements can be assembled into an internally consistent model which can be fitted to the known dimensions of the actomyosin complex; several of the FRET distances are consistent with the available structural data from crystallographic and electron microscopic dimensions; the modelled FRET distances suggest that the assumed value of the orientation factor (k2 = 2/3) is reasonable; we conclude that the model has a predictive value, i.e. it suggests that a small number of the published dimensions may be incorrect and predicts the magnitude of a larger number of measurements which have not yet been reported; and finally (vi) we discuss the contribution of FRET determinations to the current debate on the molecular mechanism of contraction.

NMR of a synthetic peptide spanning the triphosphate binding site of adenosine 5'-triphosphate in actin

The amino acid residues 114-118 in actin were found to be implicated strongly in the binding of nucleotide, and as would be expected for such an important binding site, they are located in a completely conserved region of the actin sequence. A 19-residue peptide with the actin sequence 106-124 was synthesized in order to span the putative triphosphate binding site. Proton NMR spectra of the actin peptide 114-118 in the presence and absence of ATP indicated that Arg-116 and Lys-118 are particularly involved in binding ATP. A strong binding of ATP to the peptide 106-124 also was measured. Tripolyphosphate bound to the peptide 106-124 somewhat more weakly than ATP. Binding involved residues 115-118 and 121-124, indicating the presence of a reverse turn between these segments. Proton resonances were assigned by using two-dimensional double quantum correlated spectroscopy, one-dimensional spin decoupling techniques, one-dimensional nuclear Overhauser enhancement difference spectroscopy, and pH titration. The alpha CH resonances of Ala-3 and Asn-6 are markedly shifted downfield with respect to values in small unstructured peptides due to their close proximity to the side chains of Pro-4 and Pro-7, respectively. Several other resonances display chemical shifts which are indicative of a structured environment. Assignment of the amide proton resonances in H2O and measurements of the coupling constant 3JHNCH and the chemical shifts of the amide protons reveal that much of the synthetic peptide, particularly the backbone, exhibits a highly structured environment and represents a good model for the triphosphate binding site in actin.

Localization of the phalloidin and nucleotide-binding sites on actin

Phalloidin was found to block nucleotide exchange in F-actin, without interfering with nucleotide hydrolysis. This inhibition of nucleotide exchange occurs under conditions in which monomers are able to exchange. The distance separating a fluorescent chromophore attached to phalloidin from the nucleotide on actin was determined using fluorescence resonance energy-transfer spectroscopy. They are separated by less than 1.0 nm. Added confirmation of the close proximity of phalloidin to nucleotide was obtained by extracting a small peptide-ATP complex from an actin digest. The peptide comprises residues 114-118, which are from the same region as the residues that others have shown to crosslink to phalloidin [Vandekerckhove et al. (1985) EMBO J. 4, 2815-2818]. The results suggest that phalloidin has two major effects. It traps actin monomers in a conformation which appears to be distinct from G-actin and it stabilizes the structure of F-actin, an event accompanied by the trapping of ADP.

Extraction of myosin light chains and actin from bovine cardiac muscle acetone powder

A joint preparation of the two myosin light chains and actin from bovine cardiac muscle acetone powder is described. There is a significant improvement in the ease of purification, while the yield of the myosin light chains equals the best yields obtained from the use of established techniques. The actin yield greatly exceeds that obtained in an earlier published report.

Fluorescence resonance energy transfer between the nucleotide binding site and Cys-10 in G-actin and F-actin

Intramonomer fluorescence resonance energy transfer between the donor epsilon-ATP bound to the nucleotide site and the acceptor N-(4-dimethylamino-3,5-dinitrophenyl)maleimide (DDPM) or 4-dimethylaminophenyl-azophenyl-4'-maleimide bound to Cys-10 in G-actin was measured. The donor-acceptor distance was calculated to be about 40 A. The intermonomer energy transfer in F-actin occurring between epsilon-ADP and DABMI was also measured. The radial coordinate of Cys-10 was calculated to be 25 A based on the helical symmetry of F-actin and the recently calculated radial coordinate of the nucleotide binding site in F-actin i.e. 25 A (Miki, M., Hambly, B. and dos Remedios, C.G. (1986) Biochim. Biophys. Acta 871, 137-141). (The assumption has been made in calculating these distances that the energy donor and acceptor rotate rapidly relative to the fluorescence lifetime.) Corresponding distances separating the donor nucleotide in one monomer from acceptors on Cys-10 in the first and second nearest neighbours in F-actin are 39-40 A and 41-43 A.

The distance separating Cys-10 from the high-affinity metal binding site in actin

The fluorescence resonance energy transfer between 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulphonic acid (1,5-IAEDANS) attached to Cys-10 residue and Co2+ bound to the high affinity metal site was measured. The resonance energy transfer efficiency was 8 +/- 2%, which corresponds to a distance of 2.1 nm using the absorption spectrum of Co-EDTA and 3.0 nm using the more relevant absorption spectrum of Co2+ bound to carboxypeptidase as a model spectrum of Co2+ bound to actin, respectively.

Fluorescence energy transfer between Cys-10 residues in F-actin filaments

Fluorescence energy transfer was measured between Cys-10 residues in an F-actin filament using 5-[2-((iodoacetyl)amino)-ethyl]aminonaphthalene-1-sulphonic acid (1,5-IAEDANS) as a fluorescence energy donor and 4-dimethylaminophenylazophenyl-4'-maleimide (DABMI) as the acceptor. Both labels were covalently attached to Cys-10 residues in an F-actin filament. Taking the helical structure of the F-actin filament into consideration, the radial coordinate of Cys-10 was calculated to be 23 A. This corresponds to a distance between adjacent sites along the long pitch helix of 56.1 A and along the genetic helix of 53.3 A.

The distances separating Tyr-69 from the high-affinity nucleotide and metal binding sites in actin

The nucleotide binding site in actin was occupied with the fluorescent analogue formycin A 5' triphosphate which acted as a fluorescent donor for the acceptor chromophore dansyl chloride attached to Tyr-69. The distance separating the two chromophores was calculated to be 2.1 nm from the fluorescence energy transfer measurements. Similar measurements were made of the distances separating dansyl chloride, acting as donor, on Tyr-69 from Co2+ occupying the metal binding site. A distance of 2.1 nm was similarly obtained.

Selective labelling of Cys-10 on actin

Conditions are described for the selective modification of Cys-10 on actin achieved following the blocking of the more reactive Cys-374. Labelling of Cys-10 did not affect the formation of actin filaments. This residue should be capable of serving as a site for fluorescence donors or acceptors and thus will be a useful locus to probe F-actin structure.

Fluorescence energy transfer between Tyr 69 and Cys 374 in actin

Modification of Tyr 69 with the chromophore dansyl chloride was found to completely block exchange of the bound ATP on actin. No significant conformational change was detected in the actin after labelling thus indicating that the dansyl chloride is close to and probably sterically blocks the ATP site. The distance separating dansyl chloride attached to Tyr 69 and IAEDANS attached to Cys 374 on actin was found using fluorescence energy transfer to be 3.9 nm. This result is consistent with the known distance between the ATP site and Cys 374.

Conformational changes in actin resulting from Ca2+/Mg2+ exchange as detected by proton NMR spectroscopy

Skeletal muscle actin can be maintained in a monomeric form in very low ionic strength solutions as well as in high concentrations (0.6 M) of MgCl2 or CaCl2. 400-MHz 1H-NMR spectra revealed characteristic changes which show that the conformation of actin alters by exchanging Ca2+ for Mg2+ in the single high-affinity cation binding site. When all low-affinity cation binding sites are filled (in the presence of high concentrations of Ca2+ or Mg2+), the spectra show that actin conformation differs from that in low-ionic-strength buffer. A comparison of actin in 0.6 M CaCl2 and 0.6 M MgCl2 revealed that the environment of only a small number of protons is affected by the exchange. A new proposal for the essential steps involved in actin polymerization is presented.

The environment of the high-affinity cation binding site on actin and the separation between cation and ATP sites as revealed by proton NMR and fluorescence spectroscopy

The lanthanide ions Lu3+ (diamagnetic) and Gd3+ (paramagnetic broadening probe) were used to displace Ca2+ from the high-affinity cation binding site on G-actin. The effects of these higher-affinity ions on the proton nuclear magnetic resonance spectrum of actin were recorded. The aliphatic proton envelope in the Gd-actin sample exhibited a complex array of changes due to the proximity of Gd to several aliphatic residues. No such changes were observed in the diamagnetic Lu-actin control spectrum. By contrast, the aromatic proton envelope remained largely unaffected in both Gd-actin and Lu-actin samples. However, the adenosine moiety on the actin-bound ATP became increasingly mobilized without the triphosphate chain being released from the ATP binding site. Maximum adenosine mobilization occurred with approximately 1 mol of lanthanide ion bound per mol of actin. The absence of changes in the aromatic proton envelope suggests that the high-affinity cation binding site is in a region well removed from the adenosine moiety of bound ATP as well as any aromatic side-chains. The separation of the ATP and cation sites was further explored using the fluorescent ATP analogues FTP and epsilon-ATP. Tb3+ bound to the high-affinity cation site was found to be separated by 16 A from the FTP chromophore bound to the nucleotide binding site on actin. Since this distance is greater than can be accommodated on a model of the Tb-ATP complex, we conclude that the sites are physically separate. This conclusion was further reinforced by experiments involving the quenching of epsilon-ATP fluorescence by Mn2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin tube formation: effects of variations in commonly used solvent conditions

Crystalline tubular aggregates of actin spontaneously assemble in the presence of certain of the lanthanide ions. These tubes are now known to contain a high degree of structural order and it has been suggested that they may be sensitive to small changes in the primary sequence. However, there have been no detailed studies of the effects of solution conditions associated with their formation. In this report we systematically examine the effects of lanthanide ion concentration, ionic radius, adenosine nucleotide concentration, divalent cation concentration, pH, KCl concentration and incubation time. The stringent control of these parameters leads to a high degree of predictability of the structural parameters of the tubes and will thus be of use in identifying actin isozymes.