ATP binding to a protease-resistant core of actin.

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Abstract
Actin [from rabbit muscle] can be cleaved by trypsin or chymotrypsin into a large, autonomous fragment with .apprx. 80% of the mass of the undegraded polypeptide. The protease-resistant cores obtained with either enzyme are very similar. Although the fragment does not bind Ca ions and fails to polymerize to the filamentous form of actin or to stimulate myosin ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity, it retains the full capacity to bind ATP. This observation suggests that it represents an independent functional unit. Cleavage of globular actin with either trypsin or chymotrypsin occurs with half-times of 3 min, while that of filamentous actin proceeds with reaction half-times of 20 min for trypsin and nearly 2 h for chymotrypsin. Denaturation and renaturation of the trypsin-resistant core shows that .apprx. 20% of the molecules refold to functional forms which indicates that the fragment can be considered as an independent unit of folding as well.