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The crystal structure of human platelet factor 4 (PF4) has been solved to a resolution of 2.4 A by molecular replacement and refined to an R-factor of 24.1%. The structure consists of four polypeptide chains which form a tetrameric unit. N-terminal residues, previously defined as a random coil or extended loop region, form antiparallel beta-sheet-like structures that form noncovalent associations between dimers. These antiparallel beta-sheet-like structures are positioned lateral to the beta-bilayer motif and stabilize the tetrameric unit. A positively charged ring of lysine and arginine side chains encircles the PF4 tetramer sphere, presenting multiple potential sites and orientations for heparin binding. The electrostatic interactions of multiply charged amino acid side chains and hydrogen bonding interactions at the AB/CD dimer interface serve to stabilize the tetrameric structure further.

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The X-ray crystal structure of the pure-spermine form of the left-handed Z-DNA duplex [d(CGCGCG)]2 has been determined at a temperature of -110 degrees C. Whereas the previously described room temperature structure of the pure-spermine form showed only the presence of a single "interhelix" spermine molecule, mediating contacts between neighboring duplexes (Egli et al., 1991), a second "intrahelix" spermine molecule as well as two hydrated sodium ions were found in the structure determined at low temperature. This second spermine molecule binds primarily within the minor groove of two hexamer duplexes that are stacked in an end-to-end fashion in the crystal lattice. Thus, the intrahelix spermine molecule interacts with a single infinite helix. The spine of hydration observed in other structures of Z-DNA hexamers is partially replaced and partially displaced by the intrahelix spermine molecule. In Z-DNA, phosphate groups are relatively closely spaced across the minor groove compared to the right-handed double-helical conformation of B-DNA. The intrahelix spermine molecule decreases cross-groove electrostatic repulsion within the Z-DNA helix, thereby increasing its relative stability. This structure may therefore provide an explanation for the role of spermine as a very effective inducer of the conformational B-DNA to Z-DNA transition with alternating dG-dC sequences in solution.