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Page 92<br />

The role of the N-terminus of integrase, residues 1 to 50, is still unclear. Within this region are four<br />

strictly conserved amino acids: two His and two Cys residues. In HIV-1 integrase, the spacing is His-X 3-<br />

His-X 23-Cys-X 2-Cys. This cluster of His and Cys residues is reminiscent of a zinc-binding motif, and it<br />

has been demonstrated that the full-length protein binds Zn 2+ [22,30], and that the separately expressed<br />

domain consisting only of residues 1 to 55 also binds Zn 2+ stoichiometrically [31]. However, it has not<br />

been shown that either the structural integrity or the enzymatic activities of integrase require Zn 2+. While<br />

truncation of residues from the N-terminus of HIV integrase results in loss of 3' processing and strand<br />

transfer activities [22,25], in the case of RSV integrase, the N-terminal region can be replaced <strong>by</strong><br />

unrelated sequences, and the enzyme is still capable of all three in vitro activities [32].<br />

B. Biophysical Properties of Full-Length Recombinant HIV-1 Integrase<br />

It has been known for some time that recombinant HIV-1 integrase is a particularly poorly behaved<br />

protein in solution. Its solubility in most usual buffers is limited to approximately 1 mg/mL, and even<br />

then only in the presence of high concentrations of NaCl. At ~1 mg/mL, HIV-1 integrase slowly<br />

precipitates out of solution, revealing one of its characteristic features, a tendency towards aggregation.<br />

These properties of the protein are not unreasonable, since in its viral environment integrase is probably<br />

never required to be a soluble protein. To maintain the integrity of preintegration complexes, it may<br />

even be advantageous for the protein to have the properties of being rather insoluble and sticking to<br />

itself, nucleic acid, and perhaps other proteins.<br />

C. Properties of Truncated Versions of HIV-1 Integrase<br />

It has been our approach to protein structure determination <strong>by</strong> x-ray crystallography that it is imperative<br />

to begin with well-characterized and well-behaved protein. In particular, it is important that the protein<br />

be reasonably soluble and monodisperse in solution. Unfortunately, as discussed above, recombinant<br />

HIV-1 integrase satisfies neither of these conditions. One approach we and others have taken to<br />

circumvent these problems has been to examine truncated versions of HIV-1 integrase to determine if<br />

removal of amino acids from either terminus or both affects solubility and aggregation properties.<br />

Although we observed that two proteins we constructed, IN 213–288 and IN 50–288, were more soluble than<br />

the full-length HIV-1 integrase, IN 1–288 [33, and unpublished observations], our first target protein for<br />

crystallization efforts was the core domain of HIV-1 integrase consisting of residues 50 to 212, IN 50–212.<br />

We reasoned that this protein domain was likely to be compact and<br />

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