Cleavage of actin by thrombin.

Under certain conditions actin can be split by thrombin. Actin prepared in the presence of excess Ca(++) was found to be resistant to thrombin. However, if actin was purified without added Ca(++), both G- and F- actin underwent thrombic digestion, although a considerable proportion of actin molecules remained intact. Similar results were obtained with actin (in 50% sucrose) devoid of nucleotide and divalent cations but retaining its native characteristic. The removal of tightly bound Ca(++) from actin by EDTA accelerated the thrombic splitting and made the complete fragmentation of G-actin possible. Thrombin first cleaves actin into two pieces and subsequently one of them, fragment K (molecular weight 37,000 on sodium dodecyl sulfate-polyacrylamide gel), splits further, resulting in fragments L (molecular weight 27,000) and M (molecular weight about 10,000).