| Research Abstract |
Every protein in the cell turns over with its own half-life, spanning from a few minutes to several weeks. The length of the lifetime of protein is determined by intracellular proteolytic system represented by "calpain". Calpain is a calcium-requiring cysteine protease, which modulate signal transduction system in the cell. It proteolyzes a specific site of other proteins involved in signal transduction system to change their state of activity and/or structure. In skeletal muscle, a muscle-specific calpain called p94 functions in addition to the ubiquitously expressed conventional calpains, μ- and m-calpains. p94 has very short half-life in vitro due to its rapid and exhaustive autolytic activity. In this study, we investigate a behavior of p94 in skeletal muscle to elucidate cellular proteolytic system that modulates various proteins' lifetime. As a result, the followings have been revealed : 1) p94 is stabilized by binding to connetin in vivo. 2) p94 has 4 domain structure, and the specific insertion regions, IS1 and IS2, are involved in the rapid autolysis of p94. If either IS1 or IS2 is deleted, p94 shows stable expression. 3) In skeletal muscle, alternative splicing products of p94 are expressed in a stage-specific manner. Some of them have deletion in IS1 and/or IS2, showing stable proteolytic activity. 4) To our surprise, p94 preferentially cuts calpastatin, which is the endogenous specific inhibitor protein for the conventional calpains. These results suggested p94 in cooperation with the conventional calpains functions key modulator of protein turnover system in skeletal muscle cells.
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