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Figure 3<br />

Stereo view of IFN-β crystal structure [2].<br />

Page 444<br />

The structure of IFN-τ was also examined <strong>by</strong> CD [10]. Analysis of the IFN-τ spectra predicts that the<br />

secondary structural elements derived from CD spectra indicate approximately 70% α-helix. The<br />

remainder of the molecule is either predicted to be random or a combination of β sheet and turn. Since it<br />

is known that algorithms that predict secondary structures from CD spectra are most accurate at<br />

identifying α helices, we are confident that IFN-τ is mainly α helical. The CD spectra for the synthetic<br />

peptides of IFN-τ were also obtained. The peptides IFN-τ(1–37), IFN-τ(62–92), IFN-τ(119–150), and<br />

IFN-τ(139–172) all show the presence of α helix, while IFN-τ(34–64) and IFN-τ(90–122) are mainly<br />

random. The presence of an α helix in the peptides supports the CD analysis of the intact protein and<br />

also roughly indicates the location of helical segments.<br />

The secondary structure of IFN-τ, including the location of the α helices and loop region, was then<br />

predicted using a neural network-<strong>based</strong> computer program called PHD that relies on sequence<br />

alignments of all proteins related to the target sequence [25,26]. When this prediction is correlated with<br />

the CD data, peptides that possess considerable α helicity are predicted to contain entire helical<br />

segments, and conversely, peptides with little helicity are predicted to be within loop regions.<br />

A model of the 3-D structure of IFN-τ was constructed using a distance geometry-<strong>based</strong> homology<br />

modeling method with mouse IFN-β acting as a template. The distance constraints were generated<br />

between residues within IFN-τ that are homologous to residues of IFN-β. Dihedral-angle restraints of α<br />

helices were generated from the secondary-structure prediction of IFN-τ. No constraints were applied to<br />

the 13-residue carboxy tail of IFN-τ, which is absent in IFN-β, since it is likely to be flexible in a<br />

manner similar to other proteins such as IFN-γ. Additional distance constraints were added from putative<br />

disul-<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_444.html (1 of 2) [4/9/2004 12:11:24 AM]

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