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K A Magnus

Publications and source records attributed to K A Magnus.

11 recordsLinked to original sources
PubMed1998

Crystal packing induces a conformational change in profilin-I from Acanthamoeba castellanii.

Profilin-I from Acanthamoeba castellanii is a 13-kDa protein that binds actin and poly-l-proline. The native protein has been crystallized in two different but closely related forms. The second form proved more amenable to three-dimensional structural determination using heavy-atom isomorphous methods to obtain crystallographic phase information. We used the second crystal structure as a test molecule in the molecular replacement procedure to determine the structure of the first crystal form of profilin-I. More residues participate in crystal lattice contacts in the first crystal form than in the second. The two crystal forms differ significantly in the C-terminal helix that interacts with actin and in the loop preceding this helix. Coordinates of some main chain atoms here differ by about 1.0 A, and side chain atoms differ by more than 2.0 A.

Acanthamoeba↗
PubMed1996

Nitrate binding to Limulus polyphemus subunit type II hemocyanin and its functional implications.

The horseshoe crab, Limulus polyphemus, employs hemocyanin as an oxygen carrier in its hemolymph. This hemocyanin displays cooperative oxygen binding and heterotropic allosteric regulation by protons, chloride ions and divalent cations. Here, we report the crystal structure of Limulus polyphemus subunit type II hemocyanin with a nitrate ion bound in the interface of its first and second domains. Interestingly, the nitrate-binding site coincides with the binding site for the allosteric effector chloride. Oxygen-binding data indeed indicate that nitrate, like chloride, reduces the oxygen affinity of this hemocyanin. The observed binding of two distinct anions to a single site suggests that several other anions may also bind at this site. This opens the intriguing possibility that bicarbonate, which is structurally similar to nitrate and closely linked to respiration, can act as an allosteric effector that lowers the oxygen affinity. Such an effect could be another factor in the repertoire of allosteric regulators of this hemocyanin; however, the physiological implications will be a challenge to decipher, since there exists a complex interplay of effects between bicarbonate, chloride, pH and divalent cations.

Animals↗
PubMed1994

X-ray structures of isoforms of the actin-binding protein profilin that differ in their affinity for phosphatidylinositol phosphates.

We determined the structures of Acanthamoeba profilin I and profilin II by x-ray crystallography at resolutions of 2.0 and 2.8 A, respectively. The polypeptide folds and the actin-binding surfaces of the amoeba profilins are very similar to those of bovine and human profilins. The electrostatic potential surfaces of the two Acanthamoeba isoforms differ. Two areas of high positive potential on the surface of profilin II are candidate binding sites for phosphatidylinositol phosphates. The proximity of these sites to the actin binding site provides an explanation for the competition between actin and lipids for binding profilin.

Acanthamoeba↗
PubMed1994

Crystallographic analysis of oxygenated and deoxygenated states of arthropod hemocyanin shows unusual differences.

The X-ray structure of an oxygenated hemocyanin molecule, subunit II of Limulus polyphemus hemocyanin, was determined at 2.4 A resolution and refined to a crystallographic R-factor of 17.1%. The 73-kDa subunit crystallizes with the symmetry of the space group R32 with one subunit per asymmetric unit forming hexamers with 32 point group symmetry. Molecular oxygen is bound to a dinuclear copper center in the protein's second domain, symmetrically between and equidistant from the two copper atoms. The copper-copper distance in oxygenated Limulus hemocyanin is 3.6 +/- 0.2 A, which is surprisingly 1 A less than that seen previously in deoxygenated Limulus polyphemus subunit II hemocyanin (Hazes et al., Protein Sci. 2:597, 1993). Away from the oxygen binding sites, the tertiary and quaternary structures of oxygenated and deoxygenated Limulus subunit II hemocyanins are quite similar. A major difference in tertiary structures is seen, however, when the Limulus structures are compared with deoxygenated Panulirus interruptus hemocyanin (Volbeda, A., Hol, W.G.J.J. Mol. Biol. 209:249, 1989) where the position of domain 1 is rotated by 8 degrees with respect to domains 2 and 3. We postulate this rotation plays an important role in cooperativity and regulation of oxygen affinity in all arthropod hemocyanins.

Allosteric Regulation↗
PubMed1993

Crystal structure of deoxygenated Limulus polyphemus subunit II hemocyanin at 2.18 A resolution: clues for a mechanism for allosteric regulation.

The crystal structure of Limulus polyphemus subunit type II hemocyanin in the deoxygenated state has been determined to a resolution of 2.18 A. Phase information for this first structure of a cheliceratan hemocyanin was obtained by molecular replacement using the crustacean hemocyanin structure of Panulirus interruptus. The most striking observation in the Limulus structure is the unexpectedly large distance of 4.6 A between both copper ions in the oxygen-binding site. Each copper has approximate trigonal planar coordination by three histidine N epsilon atoms. No bridging ligand between the copper ions could be detected. Other important new discoveries are (1) the presence of a cis-peptide bond between Glu 309 and Ser 310, with the carbonyl oxygen of the peptide plane hydrogen bonded to the N delta atom of the copper B ligand His 324; (2) localization of a chloride-binding site in the interface between the first and second domain; (3) localization of a putative calcium-binding site in the third domain. Furthermore, comparison of Limulus versus Panulirus hemocyanin revealed considerable tertiary and quaternary rigid body movements, although the overall folds are similar. Within the subunit, the first domain is rotated by about 7.5 degrees with respect to the other two domains, whereas within the hexamer the major movement is a 3.1 degrees rotation of the trimers with respect to each other. The rigid body rotation of the first domain suggests a structural mechanism for the allosteric regulation by chloride ions and probably causes the cooperative transition of the hexamer between low and high oxygen affinity states. In this postulated mechanism, the fully conserved Phe49 is the key residue that couples conformational changes of the dinuclear copper site into movements of the first domain.

Allosteric Regulation↗
PubMed1991

Hexamers of subunit II from Limulus hemocyanin (a 48-mer) have the same quaternary structure as whole Panulirus hemocyanin molecules.

Hemocyanins are copper-containing proteins that transport oxygen in a variety of invertebrates. Considerable evidence has accumulated that arthropodan hemocyanins are multimers of a fundamental hexameric unit. X-Ray crystallographic structure determination has revealed that the hemocyanin molecule from the spiny lobster Panulirus interruptus is a single hexamer having 32 point group symmetry. Using crystals of subunit II, one of 8 polypeptide types comprising the octahexameric hemocyanin of the horseshoe crab Limulus polyphemus, and the molecular replacement method for crystallographic phase determination we show that subunit II forms assemblies with the same hexameric quaternary structure as the whole Panulirus hemocyanin molecule. Observation of the same hexameric motif in two widely separated species provides strong additional evidence that this quaternary structural unit is a universal building block of arthropodan hemocyanins.

Amino Acid Sequence↗
PubMed1988

Crystallization of actophorin, an actin filament-severing protein from Acanthamoeba.

Actophorin is an actin monomer-binding and actin filament-severing protein from Acanthamoeba castellanii. It crystallizes out of polyethylene glycol in a form suitable for high resolution x-ray analysis. The crystals are orthorhombic and have the symmetry of the space group P2(1)2(1)2(1) with lattice constants a = 39.8 +/- 0.5, b = 47.3 +/- 0.6, and c = 69.9 +/- 1.6 A. They diffract to a resolution of at least 2.8 A, and the asymmetric unit contains one actophorin monomer of Mr 15,000.

Acanthamoeba↗
PubMed1986

Crystallization of Acanthamoeba profilin-I.

Profilin-I, a protein that inhibits actin polymerization in Acanthamoeba castellanii, has been crystallized in a form suitable for high resolution x-ray analysis. The crystals have the symmetry of the space group C2 with lattice constants a = 110.4 +/- 0.2, b = 31.7 +/- 0.1, c = 33.5 +/- 0.1 A, beta = 112.2 degrees. They diffract to at least 2.0-A resolution. The asymmetric unit contains one 12,800-dalton monomer of profilin-I.

Amoeba↗
PubMed1983

Histones H3 and H2a are homologous to the lambda repressor and cro proteins in 22 residue segments implicated in DNA binding.

The histones H3 and H2a from calf thymus are homologous to the repressor and cro repressor proteins of bacteriophage lambda in a 22-residue segment that has been implicated by mutational and model-building studies in DNA binding. In the lambda proteins this segment is folded into a helix-turn-helix unit of supersecondary structure, and we propose that the homologous regions in the histones possess the same fold. Homology was quantified with a unified procedure based on criteria of identity of key residues, primary structural homology and similarity of secondary structural potential. It has previously been shown that a set of other prokaryotic DNA-binding proteins have primary structural homology with the two lambda proteins. Homologies detected between the histones H4 and H2b and members of this set suggest that these histones also contain the putative DNA-binding fold.

Amino Acid Sequence↗
PubMed1978

Location of amino acid residues in human deoxy hemoglobin.

A table has been compiled of the spatial disposition of the amino acid residues in the human deoxy hemoglobin tetramer. The table also indicates regions of possible contact between residues in each subunit and possible contacts between subunits.

Amino Acids↗