INHIBITION OF INTERLEUKIN-1-BETA CONVERTING-ENZYME BY THE COWPOX VIRUS SERPIN CRMA - AN EXAMPLE OF CROSS-CLASS INHIBITION

We reported previously that human interleukin-1 beta converting enzyme (ICE) is regulated by the CrmA serpin encoded by cowpox virus. We now report the mecha nism and kinetics of this unusual inhibition of a cy steine proteinase by a member of the serpin superfamily previously tho ught to inhibit serine proteinases only, CrmA possesses several charac teristics typical of a number of inhibitory serpins. It is conformatio nally unstable, unfolding around 3 M urea, and stable to denaturation in 8 M urea upon complex formation with ICE. CrmA rapidly inhibits ICE with an association rate constant (k(on)) of 1.7 x 10(7) M(-1) s(-1), forming a tight complex with an equilibrium constant for inhibition ( K-i) of less than 4 x 10(-12) M. These data indicate that CrmA is a po tent inhibitor of ICE, consistent with the dramatic effects of CrmA on modify ing host responses to virus infection. The inhibition of ICE b y CrmA is an example of a ''cross-class'' interaction, in which a serp in inhibits a non-serine proteinase. Since CrmA possesses characterist ics shared by inhibitors of serine proteinases, we presume that ICE, t hough it is a cysteine proteinase, has a substrate binding geometry st rikingly close to that of serine proteinases. We reason that it is the substrate binding geometry, not the catalytic mechanism of a proteina se, that dictates its reactivity with protein inhibitors.