A Novel Cysteine Protease Inhibitor of the Egg of Chum Salmon, Containing a Cysteine-rich Thyroglobulin-like Motif

Abstract

ECI, one of three isoforms of cysteine protease inhibitor found in chum salmon eggs was purified to homogeneity, and its complete amino acid sequence was determined. The primary structure of ECI did not resemble those of other cysteine protease inhibitors of the cystatin superfamily but did resemble that of a cysteine-rich motif found as a repetitive structural element in thyroglobulin and several other proteins. The function of the cysteine-rich motif is not yet hypothesis. Two cysteine motif proteins, thyroglobulin and entactin, were tested for papain inhibitory activity and found to have none.

The type‐1 repeats of thyroglobulin regulate thyroglobulin degradation and T3, T4 release in thyrocytes

“…The sequence of this newly isolated small molecule (74 amino acids) is the same as the Tg type-1 consensus sequence [12] except for an additional 10 residue N-terminal segment. ECI was found to display specific inhibitory activities against papain and chum salmon cathepsin B but not against m-calpain, bovine trypsin, or bovine chymotrypsin [13]. Thus, a protein which is extremely similar in its sequence to a repetitive unit of thyroglobulin functions as a specific protease inhibitor.…”

“…The different biological functions performed by these proteins apparently do not directly implicate their Tg type-1 modules [12], except in one case (see below). In a recent report, Yamashita and Konagaya [13] reported the isolation from chum salmon eggs of a 74 residue protein with extremely high sequence similarity with the Tg type-1 module. This protein is a cysteine protease inhibitor.…”

Thyroglobulin Type-1 Domains in Equistatin Inhibit Both Papain-like Cysteine Proteinases and Cathepsin D

“…This domain is exclusively present on the N-terminal section and is repeated 11 times (4). Similar type-1 domains, recognizable by the sequence motif of Cys-Trp-Cys-Val, have been found in many other proteins including saxiphilin (5,6), nidogen (7), insulin-like growth factor-binding proteins (8), pancreatic carcinoma marker proteins (GA-733) (9), testican (10), major histocompatibility complex class II-associated p41 invariant chain (11), chum salmon egg cysteine proteinase inhibitor (ECI) 1 (12), and equistatin (13,14). In these proteins (except ECI and equistatin) the type-1 domain represents only part of the molecule.…”

“…In these proteins (except ECI and equistatin) the type-1 domain represents only part of the molecule. The function of these repetitive sequences is unknown, although there is evidence that the type-1 domain itself can act as an inhibitor of cysteine proteinases, as shown in equistatin, ECI, and p41 fragment (11)(12)(13)(14)(15).Equistatin is a protein isolated from sea anemone Actinia equina. It is a reversible and tightly binding competitive inhibitor of papain-like cysteine proteinases.…”

A fragment of the major histocompatibility complex class II – associated p41 invariant chain inhibits cruzipain, the major cysteine proteinase from Trypanosoma cruzi

“…A novel class of cysteine proteinase inhibitors, distinct from cystatins, which had long been known as the only protein inhibitors of papain-like cysteine proteinases [21], is therefore emerging. The data presented here, together with the two other reports on these inhibitors [15,20] provide the starting point for building a model of their interaction with the target enzymes. The li fragment inhibits cruzipain in a competitive manner, which means that, like cystatins, it binds to the active site of cruzipain (Fig.…”