Fig. 3.
B serpins. Serpins are protease inhibitors that inactivate their targets after undergoing an irreversible conformational change. (a and b) Serpins exist in an inactivated form (a) that is regarded as being “stressed.” Cleavage of the 20-residue RSL region, missing in the structure but with the terminal ends shown in red, causes the RSL region to flip over and fit itself into one of the β-sheets (b, inserted RSL strand in red). This exposes the inhibitory region that inactivates the protease. (c–f) Four splice isoforms from different serpin loci mapped onto the structure of serpinB2 (1 × 7). The sections deleted/substituted in the isoforms are shown in purple. In each case, it appears that splicing is likely to cause the structure to fold in a substantially different fashion. Given that the complex structure of the inhibitor is vital to its unique function, it is not clear why so many apparently deleterious isoforms would be necessary.