Inhibition of human immunodeficiency virus type 1 infectivity by the gp41 core: role of a conserved hydrophobic cavity in membrane fusion.

The gp41 envelope protein of human immunodeficiency virus type 1 (HIV-1) contains an alpha-helical core structure responsible for mediating membrane fusion during viral entry. Recent studies suggest that a conserved hydrophobic cavity in the coiled coil of this core plays a distinctive structural role in maintaining the fusogenic conformation of the gp41 molecule. Here we investigated the importance of this cavity in determining the structure and biological activity of the gp41 core by using the N34(L6)C28 model. The high-resolution crystal structures of N34(L6)C28 of two HIV-1 gp41 fusion-defective mutants reveal that each mutant sequence is accommodated in the six-helix bundle structure by forming the cavity with different sets of atoms. Remarkably, the mutant N34(L6)C28 cores are highly effective inhibitors of HIV-1 infection, with 5- to 16-fold greater activity than the wild-type molecule. The enhanced inhibitory activity by fusion-defective mutations correlates with local structural perturbations close to the cavity that destabilize the six-helix bundle. Taken together, these results indicate that the conserved hydrophobic coiled-coil cavity in the gp41 core is critical for HIV-1 entry and its inhibition and provides a potential antiviral drug target.

Direct observation of protein solvation and discrete disorder with experimental crystallographic phases.

A complete and accurate set of experimental crystallographic phases to a resolution of 1.8 angstroms was obtained for a 230-residue dimeric fragment of rat mannose-binding protein A with the use of multiwavelength anomalous dispersion (MAD) phasing. An accurate image of the crystal structure could thus be obtained without resort to phases calculated from a model. Partially reduced disulfide bonds, local disorder, and differences in the mobility of chemically equivalent molecules are apparent in the experimental electron density map. A solvation layer is visible that includes well-ordered sites of hydration around polar and charged protein atoms, as well as diffuse, partially disordered solvent shells around exposed hydrophobic groups. Because the experimental phases and the resulting electron density map are free from the influence of a model, they provide a stringent test of theoretical models of macromolecular solvation, motion, and conformational heterogeneity.

A database study of nonbonded intramolecular sulfur-nucleophile contacts.

A search of the Cambridge Structural Database (1991, version 4.5) was performed to investigate nonbonded intramolecular 1,4 S...O close contacts of the kind seen in the thiazole nucleoside tiazofurin and other classes of compounds. The search yielded 362 structures with 1,4 S...O connectivity. S...O distances in 70% of these structures were less than the sum of the sulfur and oxygen van der Waals radii. Findings indicate that 1,4 S...O close contacts are common and so probably result from intramolecular interactions rather than from external crystal packing forces. A structure containing a sulfur atom in a conjugated ring system is more likely to exhibit 1,4 S...O close contacts than a structure containing a sulfur atom in an unconjugated and/or acyclic environment. Sulfur-nitrogen contacts were also investigated and were found to show similar properties. These results are consistent with findings from previous quantum-chemical-based studies performed on model fragments [Burling & Goldstein (1992). J. Am. Chem. Soc. 114, 2313-2320].

Structures of the 4-cyano and 4-methylamidate analogs of tiazofurin.

4-Cyanotiazofurin [2-(beta-D-ribofuranosyl)thiazole-4-carbonitrile, (1)], C9H10N2O4S, M(r) = 242.3, monoclinic, P2(1), a = 7.329(1), b = 8.295 (1), c = 8.697(1) A, beta = 90.90 (1) degree, V = 528.7(1) A3, Z = 2, Dx = 1.52 g cm-3, Cu K alpha, lambda = 1.54178 A, mu = 27.2 cm-1, F(000) = 252, T = 293 K, R = 0.0487 for all 1171 unique reflections. 4-Methylamidatetiazofurin [methyl 2-(beta-D-ribofuranosyl)thiazole-4-carboximidate, (2)], C10H14N2O5S, M(r) = 274.3, orthorhombic, P2(1)2(1)2(1), a = 8.596(1), b = 11.060(1), c = 26.064(1) A, V = 2478.1(2) A3, Z = 8, Dx = 1.47 g cm-3, Cu K alpha, lambda = 1.54178 A, mu = 24.5 cm-1, F(000) = 1152, T = 293 K, R = 0.0374 for all 2902 unique reflections. Compound (2) crystallizes with two crystallographic unique structures in the asymmetric unit [(2a) and (2b)]. All three structures show a close contact between the thiazole sulfur and the pentose oxygen O(1'). S...O(1') distances are 2.936 (3) A in (1), 2.773 (2) A in (2a) and 2.878 (2) A in (2b), resulting from C-glycosidic torsion angles of 34.5 (4), 15.6 (3) and 27.2 (3) degrees respectively. This interesting feature is conserved in the crystal structures of other thiazole nucleosides [Burling & Goldstein (1992).

Structures of the 2',3'-dideoxy and 2',3'-didehydro-2',3'-dideoxy analogs of tiazofurin.

2',3'-Dideoxytiazofurin [2-(2',3'-dideoxy-beta-D-glycero-pentafuranosyl)thiazole-4- carboxamide]hemihydrate (1), C9H12N2O3S.1/2H2O, Mr = 237.3, monoclinic, C2, a = 23.141 (2), b = 5.912 (1), c = 8.252 (1) A, beta = 90.71 (1) degrees, V = 1128.9 (4) A3, Z = 4, Dx = 1.396 g cm-3, Cu K alpha, lambda = 1.54178 A, mu = 25.0 cm-1, F(000) = 500, T = 295 K, R = 0.0319 for all 1316 unique reflections. 2',3'-Didehydro-2',3'-dideoxytiazofurin [2-(2',3'-dideoxy-beta-D-glyceropent-2-enofuranosyl)thiazole -4-carboxamide] (2), C9H10N2O3S, Mr = 226.3, orthorhombic, P2(1)2(1)2(1), a = 22.172 (2), b = 8.019 (1), c = 5.991 (1) A, V = 1065.2 (4) A3, Z = 4, Dx = 1.411 g cm-3, Cu K alpha, lambda = 1.54178 A, mu = 25.9 cm-1, F(000) = 472, T = 295 K, R = 0.0344 for all 957 unique reflections. Both structures show a close contact between the thiazole S and the pentose O(1') atoms. S...O(1') distances are 2.834 (2) A in (1) and 2.835 (1) A in (2), resulting from C-glycosidic torsion angles of 14.1 (2) and 5.2 (3) degrees respectively. This unusual feature is conserved in the crystal structures of other thiazole nucleosides [Goldstein, Mao & Marquez (1988). J. Med. Chem. 31, 1026-1031].