A parallel noise-robust algorithm to recover depth information from radial flow fields.

A parallel algorithm operating on the units ('neurons') of an artificial retina is proposed to recover depth information in a visual scene from radial flow fields induced by ego motion along a given axis. The system consists of up to 600 radii with fewer than 65 radially arranged neurons on each radius. Neurons are connected only to their nearest neighbors, and they are excited as soon as a sufficiently strong gray-level change occurs. The time difference of two subsequently activated neurons is then used by the last-excited neuron to compute the depth information. All algorithmic calculations remain strictly local, and information is exchanged only between adjacent active neurons (except for the final read-out). This, in principle, permits parallel implementation. Furthermore, it is demonstrated that the calculation of the object coordinates requires only a single multiplication with a constant, which is dependent on only the retinal position of the active neuron. The initial restriction to local operations makes the algorithm very noise sensitive. In order to solve this problem, a predication mechanism is introduced. After an object coordinate has been determined, the active neuron computes the time when the next neuronal excitation should take place. This estimated time is transferred to the respective next neuron, which will wait for this excitation only within a certain time window. If the excitation fails to arrive within this window, the previously computed object coordinate is regarded as noisy and discarded. We will show that this predictive mechanism relies also on only a (second) single multiplication with another neuron-dependent constant. Thus, computational complexity remains low, and noisy depth coordinates are efficiently eliminated. Thus, the algorithm is very fast and operates in real time on 128 x 128 images even in a serial implementation on a relatively slow computer. The algorithm is tested on scenes of growing complexity, and a detailed error analysis is provided showing that the depth error remains very low in most cases. A comparison to standard flow-field analysis shows that our algorithm outperforms the older method by far. The analysis of the algorithm also shows that it is generally applicable despite its restrictions, because it is fast and accurate enough such that a complete depth percept can be composed from radial flow field segments. Finally, we suggest how to generalize the algorithm, waiving the restriction of radial flow.

Secreted, glycosylated arginase from Xanthoria parietina thallus induces loss of cytoplasmic material from Xanthoria photobionts.

A secreted, glycosylated arginase (lectin) from Xanthoria parietina thallus binds to the cell wall of Xanthoria photobiont when cell wall urease has previously been induced. The uptake of this secreted arginase by the algal cell without cell wall ligand for the lectin increases the concentration of algal putrescine and it is followed by an apparent loss of chlorophyll. However, neither chlorophyllase activity has been detected nor chlorophyllide concentration increases after loading the cells with putrescine. The loss of chlorophyll can be explained by the loss of algal protoplast resulting from the action of a putrescine-activated glucanase and the split of their membrane in an hypoosmotic medium. The loss and split of protoplasts have been shown by light and transmission electron microscopy.

Conformation of [Cd7]-metallothionein-2 from rat liver in aqueous solution determined by nuclear magnetic resonance spectroscopy.

The three-dimensional structure of [Cd7]-metallothionein-2 from rat liver was determined in aqueous solution, using nuclear magnetic resonance spectrometry and distance geometry calculations. The experimental data provided proton-proton distance constraints from measurements of nuclear Overhauser effects, constraints on the geometry of the metal-cysteine clusters determined by heteronuclear correlation spectroscopy, and dihedral angle constraints derived from both coupling constants and nuclear Overhauser effects. The structure calculations were performed with the program DISMAN. As in previous studies with rabbit liver metallothionein-2a, the structure calculations were performed separately for the alpha and beta-domains containing the 4 and 3-metal clusters, respectively, since no interdomain constraints were found. For both domains, the global polypeptide fold, the location of polypeptide secondary structure elements, the architecture of the metal-sulfur cluster and the local chirality of the metal co-ordination are very similar to the solution structure of rabbit metallothionein-2a, but show considerable difference relative to the crystal structure of rat metallothionein-2.

Three-dimensional structure of rabbit liver [Cd7]metallothionein-2a in aqueous solution determined by nuclear magnetic resonance.

In previous work the metal-polypeptide co-ordinative bonds in the major protein species of a reconstituted [113Cd7]metallothionein-2 preparation from rabbit liver in aqueous solution were determined, the secondary polypeptide structure was found to contain several half-turns and 3(10)-helical segments, and a preliminary characterization of the overall polypeptide backbone fold in the beta-domain containing the three-metal cluster, and the alpha-domain containing the four-metal cluster, was obtained. Using a new, more extensive set of nuclear magnetic resonance data these earlier structures were improved by new structure calculations. The new experimental data consist of distance constraints from measurements of nuclear Overhauser effects, and dihedral angle constraints derived from both coupling constants and nuclear Overhauser effects. The structure calculations were performed with the program DISMAN. Since no information on the orientation of the two domains relative to each other could be obtained, the structure calculations were performed separately for the alpha-domain and the beta-domain. The average of the pairwise root-mean-square distances among the 20 structures with the least residual violations of input constraints was 2.9 A for the beta-domain and 1.4 A for the alpha-domain (1 A = 0.1 nm). The overall chirality of the polypeptide fold is right-handed for the beta-domain and left-handed for the alpha-domain. For each of the seven metal ions the local chirality of the co-ordination of the four cysteinyl Sy atoms is clearly defined. The improved structures of both domains show the previously noted differences relative to the recently published crystal structure of metallothionein-2a from rat liver.

Sequence-specific 1H-NMR assignments in rat-liver metallothionein-2.

As a basis for the determination of the metal-coordination topology and the three-dimensional fold of the polypeptide chain, sequence-specific assignments were obtained for the 1H nuclear magnetic resonance (NMR) spectrum of metallothionein-2 from rat liver. The 1H spin systems of the 20 metal-bound cysteines were identified from comparison of two metal-homogeneous protein preparations obtained by reconstitution of the apometallothionein-2 with 113Cd2+ and 112Cd2+, respectively. The identification of the spin systems for the remaining amino acid residues and sequential assignments were then obtained with two-dimensional 1H-NMR experiments at 500 MHz. The assignments are complete except for two backbone amide protons, which were not observed, and the side-chain hydrogen atoms beyond beta CH2 for all eight lysines. The chemical shifts are presented at pH 7.0 and 25 degrees C.

Metal co-ordination in rat liver metallothionein-2 prepared with or without reconstitution of the metal clusters, and comparison with rabbit liver metallothionein-2.

Possible origins of the different metal co-ordination topologies in the recently determined structures of rat metallothionein-2 (MT2) in single crystals and rabbit MT2 in solution were investigated. A complete structure determination for rat MT2 in solution by nuclear magnetic resonance (n.m.r.) showed that the differences in the spatial structures cannot be attributed to the different primary structures of the two species. Comparison of [113Cd7]MT2 obtained by reconstitution of the apoprotein in vitro with preparations using a different procedure showed, moreover, that the metal co-ordination observed in solution by n.m.r. is not an artefact of the protein reconstitution. Solutions of high-pressure liquid chromatographically homogeneous biosynthetic preparations of [113Cd, Zn]MT2 were obtained from rat liver following injection of 113Cd into rats in vivo, without further metal exchange after protein isolation. They contain a mixture of several forms of MT2 with different relative metal compositions, giving rise to an increased number of 113Cd resonances. For the components of the four-metal cluster, the major one of these different forms exhibits patterns in the two-dimensional [1H, 113Cd]-correlated spectra that are indistinguishable from those of [113Cd7]MT2, thereby implying identity of cluster coordination and topology. These results are discussed with regard to continued investigations into the differences between the solution structure and crystal structure of MT2.

Spatial structure of rabbit liver metallothionein-2 in solution by NMR.

Sequence-specific assignments of the proton spin systems of (Cd7) rabbit liver metallothionein-2 were obtained with homonuclear two-dimensional nuclear magnetic resonance techniques. On the basis of the NMR data the amino acid sequence had to be modified in six positions, and it was found that the protein consists of 62 rather than 61 residues. Through-bond connectivities between the metals and sequentially assigned cysteine residues were obtained with two-dimensional heteronuclear 1H-113Cd correlated spectroscopy. This information, and distance constraints obtained from nuclear Overhauser enhancement spectra were used as input for distance geometry calculations of the spatial polypeptide fold. The polypeptide chain forms two domains which enclose a three-metal cluster and a four-metal cluster, respectively. The metal coordination within the clusters is markedly different from that of (Cd5, Zn2)-MT-2 from rat liver as determined by X-ray crystallography (Furey, et al., 1986) i.e. in the two structures different metal-cysteine combinations prevail for 5 of the 12 connectivities in the three-metal cluster and for 14 of the 16 connectivities in the four-metal cluster.

Sequence-specific 1H-NMR assignments in rabbit-liver metallothionein-2.

The complete sequence-specific assignment of the 1H nuclear magnetic resonance spectrum of a major subform of rabbit liver metallothionein-2 is presented. The sequential assignment procedures revealed a number of differences with regard to results obtained by earlier partial chemical sequencing of a preparation now known to be microheterogeneous. In particular, the present data indicate a polypeptide chain length of 62 amino acid residues as compared to the occurrence of 61 amino acids in all other known mammalian metallothioneins. In the new sequence, which was also fully confirmed by chemical means, the additional amino acid residue was identified as Ala8' inserted between Ala8 and Ala9 of the standard amino acid numeration. In addition to the predominant protein species all preparations contained a minor component, for which the two-dimensional 1H-nuclear magnetic resonance features are compatible with a chemically different, homologous metallothionein.

Polypeptide fold in the two metal clusters of metallothionein-2 by nuclear magnetic resonance in solution.

The solution conformation of rabbit liver Cd27+-metallothionein-2 was determined by nuclear magnetic resonance (n.m.r.) and distance geometry. The n.m.r. data are based on complete sequence-specific resonance assignments for the polypeptide chain. This letter describes the global arrangement of the polypeptide chain, which forms two distinct domains containing metal clusters of three and four Cd ions, respectively.

Nuclear magnetic resonance identification of "half-turn" and 3(10)-helix secondary structure in rabbit liver metallothionein-2.

Analysis of 1H-1H nuclear Overhauser effects and amide proton-C alpha proton coupling constants in rabbit liver metallothionein-2 resulted in the identification of two segments of 3(10)-helix and numerous secondary structure elements of a novel type, which we call "half-turn". A half-turn can be generated starting from a type II tight turn by rotation of phi 3 from +90 degrees to -90 degrees. Its appearance in metallothionein appears to be a consequence of the constraints on the polypeptide conformation by the large number of metal binding sites.