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Page 102<br />

reaction is followed <strong>by</strong> DNA relaxation (unwinding, rotation, etc.), resulting in the site on the target<br />

DNA for the second transfer reaction site now being located close to the active site of the second<br />

monomer [42]. An alternate possibility is that a multimer larger than a dimer is responsible for the<br />

coordinated cutting and strand-transfer reactions. For example, two contacting dimers can be modeled<br />

such that two active sites of the resulting tetramer are located 15–20 Å apart. It is also possible that even<br />

higher order multimers are involved.<br />

There is, as yet, no convincing evidence in support of any one model. The observation that RSV<br />

integrase cuts target DNA with a six-base-pair stagger rather than the five observed for HIV correlates<br />

intriguingly with the apparently longer distance (~ 38 Å vs. 35 Å) between active sites in the RSV<br />

dimer. However, understanding the coordinated cutting and joining reaction awaits three-dimensional<br />

information on the arrangement of monomers within an integrase multimer binding to DNA.<br />

E. Three-Dimensional <strong>Structure</strong>s of Other Domains of HIV-1 Integrase<br />

Three-dimensional structural information has not yet been obtained for a full-length integrase protein. In<br />

its absence, attempts have been made to determine the structure of the smaller domains consisting of the<br />

separately expressed N- and C-termini that flank the core whose structure is now known.<br />

The Amino Terminus of Integrase<br />

While the N-terminus of HIV-1 integrase, consisting of residues 1 to 55, has been separately expressed,<br />

purified, and biophysically characterized [31], structural data has not yet been obtained. This protein<br />

domain binds metal ions such as Zn 2+, Co 2+, and Cd 2+ stoichiometrically, and is monomeric at low<br />

protein concentrations. Dramatic changes in helix content (from 14% to 32%) are observed in the<br />

circular dichroism (CD) spectrum upon addition of metal. Analysis of CD spectral features led<br />

researchers to conclude that it is highly probably that integrase contains a zinc finger that folds in much<br />

the same way as the TFIIIA-like DNA binding proteins, with two His residues located on an α helix and<br />

two cysteines part of a β sheet [31]. However, confirmation of such a model awaits structure<br />

determination <strong>by</strong> x-ray crystallography or NMR spectroscopy.<br />

The Carboxy Terminus of Integrase<br />

When the C-terminal domain is expressed as a separate polypeptide, IN 213–288 can be purified from the<br />

initial soluble fraction from cell lysates [33]. This small protein fragment, therefore, was an attractive<br />

target for structure determination.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_102.html [4/5/2004 4:51:59 PM]

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