Equistatin, a new inhibitor of cysteine proteinases from Actinia equina, is structurally related to thyroglobulin type-1 domain.

It is well known that the activities of the lysosomal cysteine proteinases are tightly regulated by their endogenous inhibitors, cystatins. Here we report a new inhibitor of cysteine proteinases isolated from sea anemone Actinia equina. The inhibitor, equistatin, is an acidic protein with pI 4.7 and molecular weight of 14,129. It binds tightly and rapidly to cathepsin L (ka = 5.7 x 10(7) M-1 s-1, Ki = 0.051 nM) and papain (ka = 1.2 x 10(7) M-1 s-1, Ki = 0.57 nM). The lower affinity for cathepsin B (Ki = 1.4 nM) was shown to be due mainly to a lower second order association rate constant (ka = 0.04 x 10(6) M-1 s-1). The inhibitor is composed of 128 amino acids forming two repeated domains with 48% identity. Neither of the domains shows any sequence homology to cystatins, but they do show a significant homology to thyroglobulin type-1 domains. A highly conserved consensus sequence motif of Cys-Trp-Cys-Val together with conserved Cys, Pro, and Gly residues is present in major histocompatibility complex class II-associated p41 invariant chain, nidogen, insulin-like growth factor proteins, saxiphilin domain a, pancreatic carcinoma marker proteins (GA733), and chum salmon egg cysteine proteinase inhibitor. In each of the domains of the equistatin, the three residues are similarly conserved, and the sequences Val-Trp-Cys-Val and Cys-Trp-Cys-Val are present in domains a and b, respectively. We suggest that equistatin belongs to a new superfamily of protein inhibitors of cysteine proteinases named thyroglobulin type-1 domain inhibitors. This superfamily currently includes equistatin, major histocompatibility complex class II- associated p41 invariant chain fragment, and chum salmon egg cysteine proteinase inhibitor.