Calcium-dependent proteolysis of several polypeptides from rat brain and synaptosomal cytosol was observed including proteolysis of polypeptides of M r 340 000 and 300 000. These latter polypeptides comigrated with high-M r microtubule-associated proteins of microtubule preparations from brain or synaptosomal cytosol. Calcium influx into intact synaptosomes due to depolarisation with high potassium or veratridine or treatment with the ionophore A23187 did not result in Ca2+-dependent proteolysis of any polypeptides. This may be due to the low calcium sensitivity of the protease since no proteolysis of the M r 340 000 and 300 000 polypeptides was seen in synaptosomal cytosal at < 10 μM free Ca2+.
