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Page 96<br />

region in the HIV-1 structure is the uninterpretable stretch from residues 140 to 153, showing that<br />

beyond the three-dimensional similarity, the molecules also share a similar disorder pattern despite their<br />

different crystal packing interactions. It is clear that in the apoenzyme (metal-free) form of the HIV-1<br />

integrase core, disorder is present in parts of the active site. However, in the holo form, structural<br />

stability must be necessary to form a metal-binding site.<br />

Position of the Third Essential Carboxylate<br />

Does the structure of ASV integrase give us a hint about the likely conformation of the polypeptide<br />

chain around Glu152 in the holoenzyme form of the HIV-1 integrase core? The answer is probably yes,<br />

considering the overall similarity of the structures. The first residue after the disordered part in the HIV-<br />

1 integrase core is Met154, which is also the first residue in helix C. The corresponding helix in the<br />

ASV core is longer, running between Gln153 and Gly175. The residue analogous to Glu152 is Glu157<br />

in the ASV integrase core structure, located a half-turn upstream from Ala159, which corresponds to<br />

Met154 in the HIV-1 integrase structure. It is plausible to assume, therefore, that the polypeptide chain<br />

in the holoenzyme form of HIV-1 integrase would also be in a helical conformation around Glu152, and<br />

its location would be very close to the one that can be inferred from the location of Glu157 in the ASV<br />

integrase core. Secondary structure prediction also supports this assumption, assigning α-helical<br />

structure around Glu152. Why does this helical turn show significant disorder in the HIV-1 integrase<br />

structure? The answer might be found in the amino acid sequence: Pro145 is a highly conserved residue<br />

among retroviral integrases, the only exception being ASV integrase where it is substituted with a Ser.<br />

Since the main chain nitrogen of a proline is not capable of participating in hydrogen bonding, it is very<br />

rarely found in helices. It is likely that if the polypeptide chain around Glu152 were helical in the<br />

holoenzyme form of the HIV-1 integrase core, then this helix would start after Pro145. There is no such<br />

restriction in the ASV integrase core, and it is possible that this is why helix C is longer in ASV than in<br />

HIV. This may also explain the disorder around Glu152 in the HIV-1 integrase core, since it is closer to<br />

the end of the helix and more susceptible to disordering effects. A longer helix and therefore a more<br />

ordered active site in the apo form may be a unique feature of the ASV integrase core.<br />

B. Similarity to Other Polynucleotidyl Transferases<br />

Overall Protein Folds<br />

The catalytic core domain of HIV-1 integrase has a topologically identical fold with the RNase H<br />

domain of HIV-1 reverse transcriptase [37], the RuvC Holli-<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_96.html [4/5/2004 4:55:39 PM]

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