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Page 402<br />

crystallography and heteronuclear NMR spectroscopy [40–49]. In addition to these, the structure of IL-<br />

1β converting enzyme has been determined [50]. The molecule is an oligomeric cysteine protease<br />

consisting of 10- and 20-kDa subunits. To date proIL-1β is its only physiological substrate. As<br />

processing is essential for IL-1β extracellular transport and activity, inhibitors of ICE could provide a<br />

method to block the production of the active form of IL-1β. Although the elucidation of these structures<br />

have been important in determining various differences among the individual proteins, perhaps a more<br />

important result would be the elucidation of the structures of the two interleukin-1 receptor molecules,<br />

either individually or in complex with their substrates. This information could provide a means to draw<br />

together the present structural and biochemical knowledge into a coherent picture of the agonistic<br />

activity of IL-1α and IL-1β as opposed to the antagonist effect of IL-1Ra. The structures should also<br />

provide a foundation upon which structure-<strong>based</strong> drug design could proceed. To date no crystals or<br />

structural reports for the receptors have been published.<br />

A. The β-Trefoil Fold<br />

Structurally IL-1 exhibits a unique fold, known as a β-trefoil fold. Each IL-1 molecule show the<br />

characteristic β-trefoil fold and small deviations of the back-bone despite the relatively low sequence<br />

identity (Table 2). Figure 2 shows the structural alignment of the IL-1 molecules. The β-trefoil fold was<br />

first observed in Kunitz-type soybean trypsin inhibitor [51]. The fold has since been identified numerous<br />

times in the Kunitz family of protease inhibitors, the interleukin-1 system molecules, and the acidic and<br />

basic fibroblast growth factors [52]. Although these proteins have diverse biological function and low<br />

sequence identity to each other, all appear to have a common structural core. Each one, however, binds<br />

to its specific receptor with high affinity. The present known examples of proteins with such a fold are<br />

composed of between 125 and 170 amino acids. The overall fold itself is an antiparallel β barrel<br />

consisting of six two-stranded hairpins. Three of these form a barrel structure (strands denoted β1, β4,<br />

β5, β8, β9, and β12) while the other three are in a triangular array that caps the barrel. The arrangement<br />

of these moieties is such as to give the molecule a pseudo-three-fold axis [52]. Each fragment<br />

contributes one pair of antiparallel beta strands to the barrel and one pair to the cap of the barrel, thus<br />

forming a so called “open” and “closed” end to the structure [42].<br />

B. Three-Dimensional <strong>Structure</strong> of IL-1β<br />

The structure of IL-1β was the first of the IL-1 family to be solved and has been solved independently<br />

five times: four times <strong>by</strong> x-ray crystallography [40–<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_402.html [4/5/2004 5:44:18 PM]

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