Calpain processing of brain microtubules from the Atlantic cod, Gadus morhua

Microtubules isolated from Atlantic cod (Gadus morhua) brains retained assembly competence and ultraculture, although treatment with rabbit calpain resulted in loss of MAPs. In addition, spirals and aberrant structures formed when calpain I was activated post assembly. No such effect was seen with calpain II. Soluble fractions from cod brain were found to contain proteolytic activity that could be blocked by exogenously added calpastatin. Calpain was also isolated from cod muscle tissue with 10 times less yield, compared to rabbit lung. On the basis of Ca(2+)-requirements for activation in the mM range, electrophoretic mobility, antigenicity and hydrophobicity, we conclude that the proteolytic activity was attributable to calpain II. There was no difference in effects of rabbit and cod calpain II on cod microtubule proteins, indicating that calpain is a conserved protein. Our results suggest that calpains might be involved in the Ca(2+)-dependent irreversible regulation of cod brain microtubules.