Comparative studies on proteasomes (multicatalytic proteinases) isolated from spermatozoa and eggs of sea urchins

Characterization of proteolytic activities during intestinal regeneration of the sea cucumber, Holothuria glaberrima

Proteolysis carried out by different proteases control cellular processes during development and regeneration. Here we investigated the function of the proteasome and other proteases in the process of intestinal regeneration using as a model the sea cucumber Holothuria glaberrima. This echinoderm possesses the ability to regenerate its viscera after a process of evisceration. Enzymatic activity assays showed that intestinal extracts at different stages of regeneration possessed chymotrypsin-like activity. This activity was inhibited by i) MG132, a reversible inhibitor of chymotrypsin and peptidylglutamyl peptidase hydrolase (PGPH) activities of the proteasome, ii) E64d, a permeable inhibitor of cysteine proteases and iii) TPCK, a serine chymotrypsin inhibitor, but not by epoxomicin, an irreversible and potent inhibitor of all enzymatic activities of the proteasome. To elucidate the role which these proteases might play during intestinal regeneration, we carried out in vivo experiments injecting MG132, E64d and TPCK into regenerating animals. The results showed effects on the size of the regenerating intestine, cell proliferation and collagen degradation. These findings suggest that proteolysis by several proteases is important in the regulation of intestinal regeneration in H. glaberrima.

Sperm proteasomes are responsible for the acrosome reaction and sperm penetration of the vitelline envelope during fertilization of the sea urchin Pseudocentrotus depressus

The roles of sperm proteasomes in fertilization were investigated in the sea urchin Pseudocentrotus depressus. Two proteasome inhibitors, MG-132 and MG-115, inhibited fertilization at 100 microM, whereas chymostatin and leupeptin showed no inhibition. Among three proteasome substrates, Z-Leu-Leu-Glu-MCA showed the strongest inhibition toward fertilization. MG-132 inhibited the egg-jelly-induced, but not ionomycin-induced, acrosome reaction. In addition, MG-132, but not E-64-d, inhibited fertilization of dejellied eggs by acrosome-reacted sperm. MG-132 showed no significant inhibition toward the binding of reacted sperm to the vitelline layer. Proteasomes were detected by Western blotting in the acrosomal contents, which are partially released upon exocytosis. We also found that the inhibition pattern of the caspase-like activity of the proteasome in the acrosomal contents by chymostatin and proteasome inhibitors coincided well with their inhibitory abilities toward fertilization. Furthermore, the vitelline layer of unfertilized eggs appears to be ubiquitinated as revealed by immunocytochemistry and Western blotting. Extracellular ATP, required for the degradation of ubiquitinated proteins by the proteasome, was also necessary for fertilization. These results indicate that the sperm proteasome plays a key role not only in the acrosome reaction but also in sperm penetration through the vitelline envelope, most probably as a lysin, during sea urchin fertilization.

[Structures and functions of the 26S proteasome Rpn10 family]

The ubiquitin-dependent proteolytic pathway is thought to be one of the vital systems for cellular regulations, including control of the cell cycle, differentiation and apoptosis. In this pathway, poly-ubiquitinated proteins are selectively degraded by the 26S proteasome, a multisubunit proteolytic machinery. Recognition of the poly-ubiquitin chain by the 26S proteasome should be a key step leading to the selective degradation of target proteins, and the Rpn10 subunit of the 26S proteasome has been shown to preferentially bind the poly-ubiquitin chain in vitro. We previously reported that the mouse Rpn10 mRNA family is generated from a single gene by developmentally regulated, alternative splicing. To determine whether such alternative splicing mechanisms occur in organisms other than the mouse, we searched for Rpn10 isoforms in various species. Here we summarize the gene organization of the Rpn10 in lower species and provide evidence that the competence for generating all distinct forms of Rpn10 alternative splicing has expanded through evolution. Some of the Rpn10 family genes were found to be expressed in distinct developmental stages, suggesting that they have distinct functions during embryogenesis. For example, Rpn10c and Rpn10e were exclusively expressed at specific developmental stages and in specific tissues, while Rpn10a was expressed constitutively. Our experimental results indicate that the respective Rpn10 proteins possess distinct roles in the progression of development. Furthermore, some of the Rpn10 variants specifically interacted with important developmental regulators.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Participation of 650-kDa protease (20 S proteasome) in starfish oocyte maturation

A protease involved in oocyte maturation of a starfish, Asterina pectinifera, was explored. Trypsin-like and chymotrypsin-like activities of the 650-kDa protease in oocyte extract were revealed to increase more than twice under the influence of 1-methyladenine before germinal vesicle breakdown (GVBD) during maturation. The inhibitory potencies of leupeptin and its five analogs against the chymotrypsin-like activity, but not the trypsin-like activity, of this protease was well in accord with those against GVBD (Takagi Sawada et al. (1989). Dev. Biol. 133, 609-612). These results indicate that the chymotrypsin-like activity of the 650-kDa protease (most probably 20 S proteasome) plays a key role in starfish oocyte maturation.