Thayumanasamy Somasundaram

The atomic structure of adeno-associated virus (AAV-2), a vector for human gene therapy

The structure of the adeno-associated virus (AAV-2) has been determined to 3-Å resolution by x-ray crystallography. AAV is being developed as a vector for gene therapy to treat diseases including hemophilia, cancer, and cystic fibrosis. As in the distantly related autonomous parvoviruses, the capsid protein has a β-barrel fold, but long loops between the β-strands share little structural homology with other parvoviruses, leading to unique surface features. Most prominent are groups of threefold-related peaks, each an intimate association of loops from two neighboring subunits. Mutations affecting cell entry and receptor binding are clustered near the positively charged side of each peak, implicating the region in attachment to the cellular receptor, heparan sulfate proteoglycan. Amino acids involved in antibody binding are in the same general vicinity. The structure will guide rational engineering of vector capsids to tailor cellular targeting and to avoid immediate neutralization by an immune system sensitized by prior exposure to AAV.

Induced fit in guanidino kinases - Comparison of substrate-free and transition state analog structures of arginine kinase

Arginine kinase (AK) is a member of the guanidino kinase family that plays an important role in buffering ATP concentration in cells with high and fluctuating energy demands. The AK specifically catalyzes the reversible phosphoryl transfer between ATP and arginine. We have determined the crystal structure of AK from the horseshoe crab (Limulus polyphemus) in its open (substrate‐free) form. The final model has been refined at 2.35 Å with a final R of 22.3% (Rfree = 23.7%). The structure of the open form is compared to the previously determined structure of the transition state analog complex in the closed form. Classically, the protein would be considered two domain, but dynamic domain (DynDom) analysis shows that most of the differences between the two structures can be considered as the motion between four rigid groups of nonsequential residues. ATP binds near a cluster of positively charged residues of a fixed dynamic domain. The other three dynamic domains close the active site with separate hinge rotations relative to the fixed domain. Several residues of key importance for the induced motion are conserved within the phosphagen kinase family, including creatine kinase. Substantial conformational changes are induced in different parts of the enzyme as intimate interactions are formed with both substrates. Thus, although induced fit occurs in a number of phosphoryl transfer enzymes, the conformational changes in phosphagen kinases appear to be more complicated than in prior examples.

Refinement of the arginine kinase transition-state analogue complex at 1.2 Å resolution: mechanistic insights

The three-dimensional crystal structure of an arginine kinase transition-state analogue complex has been refined at 1.2 A resolution, with an overall R factor of 12.3%. The current model provides a unique opportunity to analyze the structure of a bimolecular (phosphagen kinase) enzyme in its transition state. This atomic resolution structure confirms in-line transfer of the phosphoryl group and the catalytic importance of the precise alignment of the substrates. The structure is consistent with a concerted proton transfer that has been proposed for an unrelated kinase. Refinement of anisotropic temperature factors and translation-libration-screw (TLS) analyses led to the identification of four rigid groups and their prevalent modes of motion in the transition state. The relative magnitudes of the mobility of rigid groups are consistent with their proposed roles in catalysis.

An atomic resolution structure for human fibroblast growth factor 1.

A 1.10‐Å atomic resolution X‐ray structure of human fibroblast growth factor 1 (FGF‐1), a member of the β‐trefoil superfold, has been determined. The β‐trefoil is one of 10 fundamental protein superfolds and is the only superfold to exhibit 3‐fold structural symmetry (comprising 3 “trefoil” units). The quality of the diffraction data permits unambiguous assignment of Asn, Gln, and His rotamers, Pro ring pucker, as well as refinement of atomic anisotropic displacement parameters (ADPs). The FGF‐1 structure exhibits numerous core‐packing defects, detectable using a 1.0‐Å probe radius. In addition to contributing to the relatively low thermal stability of FGF‐1, these defects may also permit domain motions within the structure. The availability of refined ADPs allows a translation/libration/screw (TLS) analysis of putative rigid body domains. The TLS analysis shows that β‐strands 6–12 together form a rigid body, and there is a clear demarcation in TLS motions between the adjacent carboxyl‐ and amino‐termini. Although separate from β‐strands 6–12, the individual β‐strands 1–5 do not exhibit correlated motions; thus, this region appears to be comparatively flexible. The heparin‐binding contacts of FGF‐1 are located within β‐strands 6–12; conversely, a significant portion of the receptor‐binding contacts are located within β‐strands 1–5. Thus, the observed rigid body motion in FGF‐1 appears related to the ligand‐binding functionalities. Proteins 2004.

Structure determination of adeno-associated virus 2: Three complete virus particles per asymmetric unit

The atomic structure of adeno-associated virus 2 (AAV-2) has been determined to 3.0 A resolution. AAV-2 crystallized in space group P1, with unit-cell parameters a = 249.7, b = 249.7, c = 644.8 A, alpha = 90.0, beta = 101.2, gamma = 120.0 degrees. The crystals contained three full virus particles in the asymmetric unit, allowing 180-fold non-crystallographic symmetry averaging. The particle orientations were determined using the self-rotation function and found to have similar but resolvably different orientations. Approximate alignment of icosahedral and interparticle threefold screw symmetry led to a native Patterson that was interpretable in terms of approximate particle positions. Accurate positions required a Patterson correlation search that was constrained to be consistent with non-crystallographic threefold projection symmetry evident in the diffraction intensities. Initial phases to 15.0 A resolution were calculated by molecular replacement using the known structure of a distantly related homolog (23% sequence identity). Real-space averaging was performed and phases were extended from 15.0 to 3.0 A. An atomic model was fitted and refined using a simulated-annealing real-space procedure.

The Structure of Lombricine Kinase IMPLICATIONS FOR PHOSPHAGEN KINASE CONFORMATIONAL CHANGES

Lombricine kinase is a member of the phosphagen kinase family and a homolog of creatine and arginine kinases, enzymes responsible for buffering cellular ATP levels. Structures of lombricine kinase from the marine worm Urechis caupo were determined by x-ray crystallography. One form was crystallized as a nucleotide complex, and the other was substrate-free. The two structures are similar to each other and more similar to the substrate-free forms of homologs than to the substrate-bound forms of the other phosphagen kinases. Active site specificity loop 309–317, which is disordered in substrate-free structures of homologs and is known from the NMR of arginine kinase to be inherently dynamic, is resolved in both lombricine kinase structures, providing an improved basis for understanding the loop dynamics. Phosphagen kinases undergo a segmented closing on substrate binding, but the lombricine kinase ADP complex is in the open form more typical of substrate-free homologs. Through a comparison with prior complexes of intermediate structure, a correlation was revealed between the overall enzyme conformation and the substrate interactions of His178. Comparative modeling provides a rationale for the more relaxed specificity of these kinases, of which the natural substrates are among the largest of the phosphagen substrates.

Photoacoustic investigation of phase transitions in solids

The use of the photoacoustic effect in the investigation of first- and second-order phase transitions has been examined. Changes in the amplitude of the photoacoustic signal across the phase transition are compared with changes in thermal properties such as specific heat and thermal diffusivity. The systems studied include NaNO2, TlNO3, CsNO3, NH4NO3, BaTiO3, CoO, Cu2HgI4, VO2 and V3O5. The current photoacoustic studies are discussed in the light of the theoretical models available.

A novel technique for enhancing photoacoustic signals from solids

Enhancement of the photoacoustic signal from condensed materials by several folds is achieved by the introduction of a liquid with high vapor pressure in the photoacoustic cell. The enhancement is especially marked for low absorption coefficients and high chopping frequencies. Typically the enhancement is two to nine times in the presence of diethyl ether at 293 K. A linear relationship is observed between the enhancement and the vapor pressure of the liquid.

Conversion of type I 4:6 to 3:5 β‐turn types in human acidic fibroblast growth factor: Effects upon structure, stability, folding, and mitogenic function

Human acidic fibroblast growth factor (FGF‐1) is a member of the β‐trefoil superfold, a protein architecture that exhibits a characteristic threefold axis of structural symmetry. FGF‐1 contains 11 β‐turns, the majority being type I 3:5; however, a type I 4:6 turn is also found at three symmetry‐related locations. The relative uniqueness of the type I 4:6 turn in the FGF‐1 structure suggests it may play a key role in the stability, folding, or function of the protein. To test this hypothesis a series of deletion mutations were constructed, the aim of which was to convert existing type I 4:6 turns at two locations into type I 3:5 turns. The results show it is possible to successfully substitute the type I 4:6 turn by a type I 3:5 turn with minimal impact upon protein stability or folding. Thus, these different turn structures, even though they differ in length, exhibit similar energetic properties. Additional sequence swapping mutations within the introduced type I 3:5 turns suggests that the turn sequence primarily affects stability but not turn structure (which appears dictated primarily by the local environment). Although the results suggest that a stable, foldable β‐trefoil protein may be designed utilizing a single turn type (type I 3:5), a type I 4:6 turn at turn 1 of FGF‐1 appears essential for efficient mitogenic function. Proteins 2006.

De-icing: recovery of diffraction intensities in the presence of ice rings.

Macromolecular structures are routinely determined at cryotemperatures using samples flash-cooled in the presence of cryoprotectants. However, sometimes the best diffraction is obtained under conditions where ice formation is not completely ablated, with the result that characteristic ice rings are superimposed on the macromolecular diffraction. In data processing, the reflections that are most affected by the ice rings are usually excluded. Here, an alternative approach of subtracting the ice diffraction is tested. High completeness can be retained with little adverse effect upon the quality of the integrated data. This offers an alternate strategy when high levels of cryoprotectant lead to loss of crystal quality.