The conformation of cross-linked actin.S-1 in the presence and absence of ATP

Electron microscopy studies have shown that the structure of the complex of myosin subfragment 1 (S-1) cross-linked to actin with 1-ethyl-3-[3-(dimethyl-amino) propyl] carbodiimide is very different in the presence and absence of ATP (Craig, R., Greene, L. E., and Eisenberg, E. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 3247-3251). More recent studies have found that the structure of the cross-linked complex between S-1 modified extensively with N-ethylmaleimide (NEM.S-1) and actin resembles that of the rigor complex both in the presence and absence of ATP, whereas the structure of the cross-linked complex between S-1 modified with N',N'-p-phenylenedimaleimide (pPDM.S-1) and actin resembles that of the cross-linked actin.S-1 complex in the presence of ATP. In the present study, we have obtained biochemical evidence supporting these results. The conformation of the different cross-linked actin.S-1 complexes was determined by studying their effect on the troponin-tropomyosin-actin complex (regulated actin). The basis of this probe for conformation is that S-1.ATP, which is in the weak-binding conformation, interacts very differently with regulated actin than S-1 or S-1.ADP, which are in the strong-binding conformation. We find that both in the presence and absence of ATP, cross-linked NEM.S-1 appears to be in the strong-binding conformation, whereas cross-linked pPDM.S-1 appears to be shifted toward the weak-binding conformation. In contrast, cross-linked unmodified S-1 appears to be in the strong-binding conformation in the presence of ADP and the weak-binding conformation in the presence of ATP. Therefore, in agreement with electron microscopy studies, the cross-linked actin.S-1 complex appears to be able to alternate between the weak-binding and strong-binding conformation during the cross-bridge cycle.     

Head group and chain length dependence of phospholipid self-assembly studied by spin-label electron spin resonance

The critical micelle concentrations (cmc's) of a variety of spin-labeled phospholipids, 1-acyl-2-[4-(4,4-dimethyloxazolidine-N-oxyl)valeryl]-sn-glycero-3-pho sph o derivatives, have been determined by electron spin resonance (ESR) spectroscopy. The narrow, three-line ESR spectra of the rapidly tumbling monomers are clearly distinguished from the spin-spin broadened spectra of the micellar aggregates, allowing a direct determination of the concentrations of the two species. The influence of both the hydrocarbon chain length and the polar head group on the energetics of self-assembly has been studied. For phosphatidylcholine, 1n [cmc] decreases linearly with the length of the sn-1 chain. The gradient of this linear dependence corresponds to a free energy of transfer of the monomer from the aqueous phase to the micelle of delta Gtr = -1.1RT per CH2 group. The cmc's of the 1-lauroyl derivatives of both phosphatidylcholine and phosphatidylglycerol have relatively shallow, biphasic temperature dependences with a minimum at approximately 20 degrees C. Both of these properties are characteristic of the hydrophobic effect, with the free energy of transfer being slightly less than that for the solubility of n-hydrocarbons in water, corresponding to the reduced configurational entropy of the lipid chains in the micellar state. The cmc's of the 1-lauroyl derivatives of the phospholipids in 0.15 M NaCl, for their various charge states, are as follows: phosphatidic acid(2-), 0.77 mM; phosphatidic acid(1-), 0.13 mM; phosphatidylserine(1-), 0.24 mM; phosphatidylglycerol(1-), 0.17 mM; phosphatidylcholine, 0.10 mM; phosphatidylethanolamine, 0.05 mM.(ABSTRACT TRUNCATED AT 250 WORDS)     

Folding and assembly of oligomeric proteins in Escherichia coli.

High levels of expression of oligomeric proteins in heterologous systems are frequently associated with misfolding and accumulation of the polypeptides in inclusion bodies. This reflects aspects of the folding and assembly pathways of oligomeric proteins, which generally proceed from either folding intermediates or native-like metastable species that are not in their final conformation. Methods for optimizing the yield of correctly assembled oligomers are discussed.     

On co-existence,foraging strategy and the biogeography of weasels and stoats (Mustela nivalis and M. erminea) in Britain

Summary Mustela nivalis and M. erminea, two sympatric species of weasels of superficially similar appearance and habits, have different breeding and foraging strategies associated with the difference in their body size. M. nivalis is more efficient in exploiting small rodent prey, and can breed rapidly to take immediate advantage of rodent peaks, but is vulnerable to local extinction during rodent declines. M. erminea has more generalized food habits, and is the larger and probably the dominant species, but is limited by delayed implantation to producing only one litter a year. M. nivalis is therefore superior in exploitation competition, and erminea in interference competition. We offer the hypothesis that the co-existence of the two species is permitted by a balance of these competitive advantages determined, at a given time or place, by the heterogeneity of the environment and the distribution of the prey fauna. We use this hypothesis to explain cases where co-existence has either broken down or is not recorded (the results of simultaneous introductions to New Zealand and Terschelling Island, and of myxomatosis in Britain, and the distribution of nivalis and Erminea on the offshore islands of Britain). We argue that the diversity and size distribution of the prey fauna of an island (which are both related to its area and isolation) are important in deciding the species and size of mustelids surviving there; for example, we suggest that nivalis was present in Ireland in immediate post-glacial times but became extinct with the lemmings.     

Visualization of Drug-Nucleic Acid Interactions at Atomic Resolution. IX. Structures of Two N,N-Dimethylproflavine: 5-Iodocytidylyl (3′-5′) Guanosine Crystalline Complexes

Abstract

This paper describes two complexes containing N,N-dimethylproflavine and the dinucleoside monophosphate, 5-iodocytidylyl(3′-5′)guanosine (iodoCpG). The first complex is triclinic, space group PI, with unit cell dimensions a = 11.78 Å, b = 14.55 Å, c = 15.50 Å, a = 89.2°, β = 86.2°, γ = 96.4°. The second complex is monoclinic, space group P2₁, with a = 14.20 Å, b = 19.00 Å, c = 20.73 Å, β = 103.6°. Both structures have been solved to atomic resolution and refined by Fourier and least squares methods. The first structure has been refined anisotropically to a residual of 0.09 on 5,025 observed reflections using block diagonal least squares, while the second structure has been refined isotropically to a residual of 0.13 on 2,888 reflections with full matrix least squares. The asymmetric unit in both structures contains two dimethylproflavine molecules and two iodoCpG molecules; the first structure has 16 water molecules (a total of 134 non-hydrogen atoms), while the second structure has 18 water molecules (a total of 136 non-hydrogen atoms). Both structures demonstrate intercalation of dimethylproflavine between base-paired iodoCpG dimers. In addition, dimethylproflavine molecules stack on either side of the intercalated duplex, being related by a unit cell translation along b and a axes, respectively.

The basic structural feature of the sugar-phosphate chains accompanying dimethylproflavine intercalation in both structures is the mixed sugar puckering pattern: C3′ endo (3′-5′) C2′ endo. This same structural information is again demonstrated in the accompanying paper, which describes a complex containing dimethylproflavine with deoxyribo-CpG.

Similar information has already appeared for other “simple” intercalators such as ethidium, acridine orange, ellipticine, 9-aminoacridine, N-methyl-tetramethylphenanthrolinium and terpyridine platinum. “Complex” intercalators, however, such as proflavine and daunomycin, have given different structural information in model studies. We discuss the possible reasons for these differences in this paper and in the accompanying paper.     

Active, inactive and in vitro synthesized forms of polyphenoloxidase during leaf development

Polyphenoloxidase (PPO; EC 1.14.18.1) activity decreased 8-fold from young to mature Vicia faba L. Moensch (cv. Long pod) leaves. The K_m for catechol remained relatively constant from young to mature leaves. Electrophoretic separation and analysis showed that only one active form was present in extracts from various leaf sizes. The amount of this form appeared to decrease with leaf size/age. In extracts which had not tanned, electroblotting and immunostaining indicated that one enzyme form was present with a molecular mass of 45 kDa. Two leaf categories contained greater amounts of this immunological cross-reacting PPO than other leaf categories. When extracts were allowed to darken, immunoblotting detected three enzyme forms with molecular masses of 45, 59 and 63 kDa. The latter two immunological crossracting species had no detectable enzyme activity. Poly-A⁺ mRNA was isolated from six leaf sizes and translated in vitro. A product corresponding to PPO was present in all leaf categories. Greater amounts of this translation product were observed in medium-sized leaves than in very young or mature leaves. These results suggest that: (1) enzymatic assays for PPO are not reliable indicators of the total amount of PPO protein present in developing leaves, (2) immunoblotting can detect inactive enzyme forms, (3) only one active form of the enzyme is present at all developmental stages, and (4) mRNA corresponding to PPO is present at all developmental stages but appears to be more abundant in certain leaf sizes/ages.