The type 1 domain of thyroglobulin is a protein module (Thyr-1) that occurs in a variety of secreted and membrane proteins. Several examples of Thyr-1 modules have been previously identified as inhibitors of the papain family of cysteine proteinases. Saxiphilin is a neurotoxin-binding protein from bullfrog and a homolog of transferrin with a pair of such Thyr-1 modules located in the N-lobe. Saxiphilin is now characterized as a potent inhibitor of three cysteine proteinases as follows: papain,

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... uman cathepsin B, and cathepsin L. The stoichiometry of enzyme inhibition reveals that both Thyr-1 domains of saxiphilin inhibit papain (apparent K i ‫؍‬ 1.72 nM), but only one of these domains inhibits cathepsin B (K i ‫؍‬ 1.67 nM) and cathepsin L (K i ‫؍‬ 0.02 nM). Physical association of saxiphilin and papain blocked from turnover at the active-site cysteine residue can be detected by cross-linking with glutaraldehyde. The rate of association of saxiphilin and cathepsin B is strongly pH-dependent with an optimum at pH 5.2, reflecting control by at least two H ؉ -titratable groups. These results further demonstrate that various Thyr-1 domains are selective inhibitors of cysteine proteinases with utility in the study of protein interactions and degradation.