Complex of human neutrophil elastase with 1/2SLPI.

SLPI (secretory leukocyte protease inhibitor) is a 107-residue non-glycosylated protease inhibitor, which inhibits a wide range of serine proteases, trypsin, chymotrypsin, neutrophil elastase, chymase and cathepsin G. X-ray crystallographic analyses have shown that SLPI comprises two separate domains of similar architecture [Grütter, Fendrich, Huber & Bode (1988), EMBO J. 7, 345-351] and the C-terminal domain interacts with bovine alpha-chymotrypsin. In order to understand SLPI's multiple functions against various serine proteases, the complex HNE (human neutrophil elastase) has been co-crystallized with 1/2SLPI (recombinant C-terminal domain of SLPI; Arg58-Ala107), which has a biological activity similar to full SLPI. The 1/2SLPI and HNE complex structure was solved at 1.7 A resolution, and compared with the interaction mechanism of elafin, which is a specific inhibitor of elastase. It was found that P1 Leu72i and six hydrogen bonds between the main chains in the primary contact region have sufficient ability to inhibit HNE and PPE (porcine pancreatic elastase), and P5 Tyr68i is important in increasing the selectivity of 1/2SLPI against HNE. The mechanisms of the functions of SLPI are relatively unknown, but the current study could help understand the selectivity of SLPI against HNE and PPE.

Solution structure of the DFF-C domain of DFF45/ICAD. A structural basis for the regulation of apoptotic DNA fragmentation.

DFF45/ICAD has dual functions in the final stage of apoptosis, by acting as both a folding chaperone and a DNase inhibitor of DFF40/CAD. Here, we present the solution structure of the C-terminal domain of DFF45, which is essential for its chaperone-like activity. The structure of this domain (DFF-C) consists of four alpha helices, which are folded in a novel helix-packing arrangement. The 3D structure reveals a large cluster of negatively charged residues on the molecular surface of DFF-C. This observation suggests that charge complementation plays an important role in the interaction of DFF-C with the positively charged catalytic domain of DFF40, and thus for the chaperone activity of DFF45. The structure of DFF-C also provides a rationale for the loss of the chaperone activity in DFF35, a short isoform of DFF45. Indeed, in DFF35, the amino acid sequence is truncated in the middle of the second alpha helix constituting the structure of DFF-C, and thus both the hydrophobic core and the cluster of negative charges are disrupted.