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Dynamic Protein Structure Paradox: An Integrative Framework for Endpoint-Conditioned Evidentiary Sufficiency in Structure-to-Function Claims.

Accurate coordinates for a represented protein state do not, by themselves, establish activity or any other condition-specific function. This article defines the Dynamic Protein Structure Paradox (DPSP) as the apparent conflict between structural accuracy and functional underdetermination and develops it as an integrative evidentiary assessment framework rather than a new theory or paradigm. The underlying problem has been longstanding, since structural genomics, function annotation, allostery, and disorder research each established that fold does not determine function and that function does not determine fold. DPSP consolidates those results into one endpoint-conditioned rule. Once a measurable endpoint is defined, it assesses four coupled dimensions: relevant-state completeness, context completeness, ensemble or kinetic dependence, and chemical dependence. A rubric rates each dimension as adequate, uncertain, or missing, and a materiality test determines which gaps influence the stated decision. The outcome is one of three mutually exclusive modes of utilization: geometry-led, conditional, or function-measured. The deliverable is a concise evidence statement delineating what the structure supports, which decisive variable remains unmeasured, and what corroboration is necessary. DPSP complements, rather than replaces, existing structural, ensemble, and computational approaches. The framework remains unvalidated, its thresholds are provisional, and the studies necessary to confirm or refute it are specified.

Proteins

Design, synthesis and solution structure of a renin inhibitor. Structural constraints from NOE, and homonuclear and heteronuclear coupling constants combined with distance geometry calculations.

A macrocyclic renin inhibitor was designed using molecular modeling and a model of human renin. The synthesized molecular displayed poor binding affinity. To investigate the reasons for the observed inactivity, the structure of the compound has been studied by NMR spectroscopy and distance geometry. Structural constraints for distance geometry calculations were derived from nuclear Overhauser effects and homonuclear and heteronuclear three bond coupling constants. Homonuclear coupling constants were measured directly from the resolution-enhanced proton spectra and heteronuclear coupling constants were measured from the natural abundance 15N- and 13C-edited TOCSY experiments. One phi angle was determined uniquely by this method and two were reduced to two possible values each. By using a statistical analysis of 400 structures generated with distance geometry, two families of structures were found to be consistent with the NMR data. The solution structures so derived were different from the originally designed structure, including an internal hydrogen bond. This provides a possible explanation for the lack of effectiveness of this compound.

Humans

Periplasmic binding protein structure and function. Refined X-ray structures of the leucine/isoleucine/valine-binding protein and its complex with leucine.

The three-dimensional structure of the native unliganded form of the Leu/Ile/Val-binding protein (Mr = 36,700), an essential component of the high-affinity active transport system for the branched aliphatic amino acids in Escherichia coli, has been determined and further refined to a crystallographic R-factor of 0.17 at 2.4 A resolution. The entire structure consists of 2710 non-hydrogen atoms from the complete sequence of 344 residues and 121 ordered water molecules. Bond lengths and angle distances in the refined model have root-mean-square deviations from ideal values of 0.05 A and 0.10 A, respectively. The overall shape of the protein is a prolate ellipsoid with dimensions of 35 A x 40 A x 70 A. The protein consists of two distinct globular domains linked by three short peptide segments which, though widely separated in the sequence, are proximal in the tertiary structure and form the base of the deep cleft between the two domains. Although each domain is built from polypeptide segments located in both the amino (N) and the carboxy (C) terminal halves, both domains exhibit very similar supersecondary structures, consisting of a central beta-sheet of seven strands flanked on either side by two or three helices. The two domains are far apart from each other, leaving the cleft wide open by about 18 A. The cleft has a depth of about 15 A and a base of about 14 A x 16 A. Refining independently the structure of native Leu/Ile/Val-binding protein crystals soaked in a solution containing L-leucine at 2.8 A resolution (R-factor = 0.15), we have been able to locate and characterize an initial, major portion of the substrate-binding site of the Leu/Ile/Val-binding protein. The binding of the L-leucine substrate does not alter the native crystal structure, and the L-leucine is lodged in a crevice on the wall of the N-domain, which is in the inter-domain cleft. The L-leucine is held in place primarily by hydrogen-bonding of its alpha-ammonium and alpha-carboxylate groups with main-chain peptide units and hydroxyl side-chain groups; there are no salt-linkages. The charges on the leucine zwitterion are stabilized by hydrogen-bond dipoles. The side-chain of the L-leucine substrate lies in a depression lined with non-polar residues, including Leu77, which confers specificity to the site by stacking with the side-chain of the leucine substrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals

Solution structures of two zinc-finger domains from SWI5 obtained using two-dimensional 1H nuclear magnetic resonance spectroscopy. A zinc-finger structure with a third strand of beta-sheet.

This paper describes the detailed three-dimensional structures of two zinc-finger domains from the yeast transcription factor SWI5, calculated using the results of the n.m.r. experiments described in the accompanying paper. The structure of finger 2 is essentially similar to those previously obtained by others for isolated, synthetic single zinc-finger domains in solution, and for the three zinc-finger peptide Zif268 in its crystalline complex with DNA. The N-terminal half of the sequence forms a two-stranded, irregular beta-sheet containing both of the metal-binding cysteine residues, while the remainder of the structure forms a helix. Approximately the first half of this helix is alpha-helical, whereas the C-terminal portion, including the two metal-binding histidine residues, is 3(10) helical. Four invariant hydrophobic residues form a core to the structure. In contrast to all previously described structures of zinc-finger domains, finger 1 has an additional strand in the beta-sheet, formed by residues N-terminal to the formal start of the finger motif. This additional strand plays a role in stabilising the folded form of finger 1, since a two-finger peptide lacking the N-terminal residues showed folded structure in finger 2 but not in finger 1.

Amino Acid Sequence

Progress in multidimensional NMR investigations of peptide and protein 3-D structures in solution. From structure to functional aspects.

2-D and 3-D NMR techniques were used to investigate the conformations in solution of several peptides and proteins for which crystalline structures are not available yet. Insect defensin A is a small (40 aa) antibiotic protein exhibiting a characteristic 'loop-helix-beta-sheet' structure. A striking analogy was found with charybdotoxin, a scorpion toxin in which a CSH (cysteine stabilized alpha-helix) motif is also present. Wheat phospholipid transfer protein (PLTP) (90 aa) has a 3-D structure resulting from the packing of four helices and of a C-terminal less well-defined fragment. Preliminary results show that PLTP forms a complex with lyso-PC and that such an interaction results in a conformational change affecting principally the C-terminal half of the protein. A last example is given with surfactin, a lipopeptide biosurfactant from bacterial origin. Its protonated form shows a very compact structure in which the two acidic residues located on the top of a 'horse saddle' topology face each other, whereas the ionized form could adopt a more extended conformation. A common property of these compounds is their capacity to interact with lipids. The present structural data open the way for a further establishment of structure-activity relationships.

Amino Acid Sequence

Evidence from cassette mutagenesis for a structure-function motif in a protein of unknown structure.

The three-dimensional structure of most enzymes is unknown; however, many enzymes may have structural motifs similar to those in the known structures of functionally related enzymes. Evidence is presented that an enzyme of unknown structure [Ile-transfer RNA (tRNA) synthetase] may share a functionally important structural motif with an enzyme of related function (Tyr-tRNA synthetase). This approach involves (i) identifying segments of Ile-tRNA synthetase that have been unusually conserved during evolution, (ii) predicting the function of one such segment by assuming a structural relation between Ile-tRNA synthetase and Tyr-tRNA synthetase, and (iii) testing the predicted function by mutagenesis and subsequent biochemical analysis. Random mutations were introduced by cassette mutagenesis into a ten-amino-acid segment of Ile-tRNA synthetase that was predicted to be involved in the formation of the binding site for isoleucine. Few amino acid substitutions appear to be tolerated in this region. However, one substitution (independently isolated twice) increased the Michaelis constant Km for isoleucine in the adenylate synthesis reaction by greater than 6000-fold, but had little effect on the Km for adenosine triphosphate, the apparent Km for tRNA, or the rate constant kcat.

Amino Acyl-tRNA Synthetases

Alternative structures for alternating poly(dA-dT) tracts: the structure of the B-DNA decamer C-G-A-T-A-T-A-T-C-G.

The X-ray crystal structure of the decamer C-G-A-T-A-T-A-T-C-G has been solved with two contrasting cations, Ca2+ and Mg2+. Crystals with calcium are space group P2(1)2(1)2(1), cell dimensions a = 38.76 A, b = 40.06 A, and c = 33.73 A, and diffract to 1.7-A resolution. Crystals with magnesium have the same space group, cell dimensions a = 38.69 A, b = 39.56 A, and c = 33.64 A, and diffract to 2.0 A. Their structures were solved independently by molecular replacement, beginning with idealized Arnott B-DNA geometry. The calcium structure refined to R = 17.8% for the 3683 reflections greater than 2 sigma, with 404 DNA atoms, 95 solvent peaks, and 1 Ca(H2O)7(2+) ion. The magnesium structure refined to R = 16.5% for the 1852 reflections greater than 2 sigma, with 404 DNA atoms, 62 solvent peaks, and 1 Mg(H2O)6(2+) ion. The two structures are virtually identical and are isostructural with C-G-A-T-C-G-A-T-C-G [Grzeskowiak et al. (1991) J. Biol. Chem. 266, 8861-8883] and C-G-A-T-T-A-A-T-C-G [Quintana et al. (1992) J. Mol. Biol. 225, 375-395]. Comparison of C-G-A-T-A-T-A-T-C-G with C-G-C-A-T-A-T-A-T-G-C-G [Yoon et al. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 6332-6336] shows that the expected alternation of twist angles is found in the central A-T-A-T-A-T region of the decamer (A-T small, T-A large), but the minor groove remains wide at the center, rather than narrow. Minor groove narrowing is produced, in these two structures, not by overwinding of the helix, but by an increase in base pair propeller. This analysis confirms the concept that poly(dA-dT).poly(dA-dT) is polymorphous, with different local conformations possible in different local environments.

Base Sequence

3D structure of bovine pancreatic ribonuclease A in aqueous solution: an approach to tertiary structure determination from a small basis of 1H NMR NOE correlations.

A method is proposed to generate initial structures in cases where the distance geometry method may fail, such as when the set of 1H NMR NOE-based distance constraints is small in relation to the size of the protein. The method introduces an initial correlation between the phi and psi backbone angles (based on empirical observations) which is relaxed in later stages of the calculation. The obtained initial structures are refined by well-established methods of energy minimization and restrained molecular dynamics. The method is applied to determine the solution structure of Ribonuclease A (124 residues) from a NOE basis consisting of 467 NOE cross-correlations (97 intra-residue, 206 sequential, 23 medium-range and 141 long-range) obtained at 360 MHz. The global shape and backbone overall fold of the eight final refined structures are close to those shown by the crystal structure. A meaningful difference in the positioning of the catalytically important His119 side chain in the solution and crystal structures has been detected.

Amino Acid Sequence

The secondary structure of human 28S rRNA: the structure and evolution of a mosaic rRNA gene.

We have determined the secondary structure of the human 28S rRNA molecule based on comparative analysis of available eukaryotic cytoplasmic and prokaryotic large-rRNA gene sequences. Examination of large-rRNA sequences of both distantly and closely related species has enabled us to derive a structure that accounts both for highly conserved sequence tracts and for previously unanalyzed variable-sequence tracts that account for the evolutionary differences in size among the large rRNAs. Human 28S rRNA is composed of two different types of sequence tracts: conserved and variable. They differ in composition, degree of conservation, and evolution. The conserved regions demonstrate a striking constancy of size and sequence. We have confirmed that the conserved regions of large-rRNA molecules are capable of forming structures that are superimposable on one another. The variable regions contain the sequences responsible for the 83% increase in size of the human large-rRNA molecule over that of Escherichia coli. Their locations in the gene are maintained during evolution. They are G + C rich and largely nonhomologous, contain simple repetitive sequences, appear to evolve by frequent recombinational events, and are capable of forming large, stable hairpins. The secondary-structure model presented here is in close agreement with existing prokaryotic 23S rRNA secondary-structure models. The introduction of this model helps resolve differences between previously proposed prokaryotic and eukaryotic large-rRNA secondary-structure models.

Animals

Structural properties of a phosphatidylcholine-cholesterol system as studied by small-angle neutron scattering: ripple structure and phase diagram.

Small-angle neutron scattering has been used to study structural features of lamellar bilayer membranes of dimyristoylphosphatidylcholine (DMPC) and DMPC mixed with various amount of cholesterol. The studies were recorded at a fixed hydration level of 17% 2H2O, i.e. just below saturation. Bragg reflections gives information on the ripple structure and on the bilayer periodicity. The crystalline Lc phase, which was stabilized after long time storage at low temperature, exhibits major small angle scattering when cholesterol is mixed into the membrane. The intermediate P beta' gel-phase, which is characteristic by the rippled structure, is dramatically stabilized by the introduction of cholesterol. The ripple structure depends significantly both on the cholesterol content and on the temperature. At high temperatures, T greater than 15 degrees C, the inverse ripple periodicity varies basically linearly with cholesterol content, and approach zero (i.e. periodicity goes to infinite) at 20 mol% cholesterol, approximately. At lower temperatures the correlation is more complex. The data indicate additional phase boundaries below 2 mol% and at approx. 8 mol%. Secondary rippled structures are observed in the low temperature L beta'-phase for cholesterol content below approx. 8 mol%. The data gives detailed insight into the phosphatidylcholine cholesterol phase diagram, which is discussed on the basis of a simple model in which the cholesterol complexes are fixed to the defect stripes of the rippled structure.

Cholesterol

Similarities and differences between human cyclophilin A and other beta-barrel structures. Structural refinement at 1.63 A resolution.

The structure of the unligated recombinant human cyclophilin A (CyP A) has been refined to an R-factor of 0.18 at 1.63 A resolution. The root-mean-squared deviations of the refined structure are 0.013 A and 2.50 degrees from ideal geometries of bond length and bond angle, respectively. Eight antiparallel beta-strands of CyP A form a right-handed beta-barrel. The structure of CyP A is compared with other members in the antiparallel eight-stranded beta-barrel family and with the parallel eight-stranded alpha/beta barrels. Although all known eight-stranded barrels are right-handed, the tilted angle of the strands against the barrel axis varies from 45 degrees for retinol binding protein and 49 degrees for CyP A to 70 degrees for superoxide dismutase. As a result, the beta-barrel of CyP A is not completely superimposable with other members of beta-barrels. The structure of CyP A has a unique topology, distinct from other members in the beta-barrel family. In addition, CyP A is a closed beta-barrel so that neither the immunosuppressive drug cyclosporin A (CsA) nor the proline-containing substrate can bind to the hydrophobic core of the CyP A barrel, while the hydrophobic core of most other barrels is open for ligation. These observations probably indicate that CyP A is neither functionally nor evolutionally related to other beta-barrel structures. Details of interactions between solvent molecules and the active site residues of CyP A are illustrated. A water-co-operated mechanism, where the cis<-->trans isomerization might possibly consist of (1) transition of the prolyl bond and (2) release of N or C-terminal residues of substrate from CyP, is addressed. The refined structure reveals no disulfide bridges in CyP A. Cys115 is near the CsA site, but unlikely to be directly involved in CsA binding because of steric hindrance from Thr119 and Leu122. This geometry probably rules out any mechanisms involving a tetrahedral intermediate formed between cysteine and substrate during cis<-->trans isomerization.

Amino Acid Isomerases

Structural and functional features of Drosophila chorion proteins s36 and s38 from analysis of primary structure and infrared spectroscopy.

Amino acid composition, Fourier transform analysis and secondary structure prediction methods strongly support a tripartite structure for Drosophila chorion proteins s36 and s38. Each protein consists of a central domain and two flanking 'arms'. The central domain contains tandemly repetitive peptides, which apparently generate a secondary structure of beta-sheet strands alternating with beta-turns, most probably, forming a twisted beta-pleated sheet or beta-barrel. The central domains of s36 and s38 share similarities, but they are recognizably different. The flanking 'arms', with different primary and secondary structure features, presumably serve protein-specific functions. The possible roles of the protein domains for the establishment of higher order structure in Drosophila chorion and the possible function of the molecules are discussed. The predicted secondary structure of Drosophila chorion proteins s36 and s38 is supported by experimental information obtained from Fourier transform infrared spectroscopic studies of Drosophila chorions.

Amino Acid Sequence

Secondary structural analysis of two recombinant murine proteins, interleukins 1 alpha and 1 beta: is infrared spectroscopy sufficient to assign structure?

The secondary structure for two murine recombinant proteins, interleukins 1 alpha and 1 beta (rmIL-1 alpha and -1 beta), has been analyzed by Fourier transform infrared (IR) spectroscopy and then compared to results obtained by X-ray diffraction, circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopy. The IR results obtained here for rmIL-1 alpha and -1 beta suggested that their secondary structures consisted predominantly of beta-sheets or strands. However, the analysis also revealed a significant absorption band near 1656 cm-1, which is typically assigned to alpha-helical or random structures. When these same murine polypeptides were analyzed by CD, no evidence of alpha-helical structures was observed. Further, published X-ray diffraction and NMR studies characterizing the human forms of IL-1 alpha and -1 beta indicate the absence of alpha-helices and that the human proteins are composed mainly of beta-strands (i.e., greater than 55%), with approximately 24% of the amino acids involved in large loops connecting the strands. The murine IL-1 proteins, when compared to their respective human counterparts, each show greater than 80% sequence homology. Given this fact, the CD analyses, and the result that this IR band amounted to 21% of the overall integrated area, the absorption peak at 1656 cm-1 was attributed to the presence of large loops rather than to alpha-helical or random structures. Such a structural assignment appears reasonable and is totally consistent with the established existence of large loops in the human forms as well as in other proteins found to fold similarly (viz., human bFGF).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals