Skeletal muscle-specific calpain is an intracellular Na+-dependent protease.

Because intracellular [Na(+)] is kept low by Na(+)/K(+)-ATPase, Na(+) dependence is generally considered a property of extracellular enzymes. However, we found that p94/calpain 3, a skeletal-muscle-specific member of the Ca(2+)-activated intracellular "modulator proteases" that is responsible for a limb-girdle muscular dystrophy ("calpainopathy"), underwent Na(+)-dependent, but not Cs(+)-dependent, autolysis in the absence of Ca(2+). Furthermore, Na(+) and Ca(2+) complementarily activated autolysis of p94 at physiological concentrations.

Suppressed disassembly of autolyzing p94/CAPN3 by N2A connectin/titin in a genetic reporter system.

p94/calpain 3 is a skeletal muscle-specific member of the Ca(2+)-regulated cytosolic cysteine protease family, the calpains. Defective p94 protease activity originating from gene mutations causes a muscular dystrophy called calpainopathy, indicating the indispensability of p94 for muscle survival. Because of the existence of the p94-specific regions IS1 and IS2, p94 undergoes very rapid and exhaustive autolysis.

Possible regulation of the conventional calpain system by skeletal muscle-specific calpain, p94/calpain 3.

p94 (also called calpain 3) is the skeletal muscle-specific calpain and is considered to be a "modulator protease" in various cellular processes. Analysis of p94 at the protein level is an urgent issue because the loss of p94 protease activity causes limb-girdle muscular dystrophy type 2A. In this study, we enzymatically characterized one alternatively spliced variant of p94, p94:exons 6(-)15(-)16(-) (p94delta), which lacks two of the p94-specific insertion sequences.