Extractability of actin and actin-like protein from myosin-removed myofibrils of skeletal muscle.

Extractability of 42,000 dalton components (42 K component) from myosin-removed myofibrils (called I-Z-I segments in this paper) under low-salt conditions was compared at pH 6.5 and pH 8.0. Kinetic and isoelectric focusing analysis of the 42 K components revealed that the 42 K component extracted at pH 8.0 was actin, which depolymerized from the end of the I-filaments of the I-Z-I segments. On the other hand, isoelectric focusing of the 42 K component extracted at pH 6.5 indicated two protein bands. The relative mobility of the minor band was virtually identical to that of actin. While the isoelectric point of the major band was more acidic than that of skeletal muscle actin. Therefore, it is concluded that there are at least two kinds of 42,000 dalton components in the myosin-removed myofibrils.