This paper describes the initial isolation of actin‐ and myosin‐like proteins from the cytoplasm of the endocarp tissue cells of the fruit of the tomato, Lycopersicon esculentum. Low ionic strength buffers extracted the 42,000 molecular weight tomato actin in the depolymerized form. Tomato actin can be polymerized in 0.1 M KCl, 2 mM MgCl2 to form 6 nm diameter filaments resembling rabbit skeletal muscle F‐actin in their ultrastructure and pattern of decoration with rabbit myosin subfragment‐1 (S‐1). Tomato F‐actin activates the low ionic strength Mg2+ ATPase of rabbit S‐1 up to ten‐fold. High ionic strength extracts of tomato yield a myosinlike enzyme whose ATPase activity in 0.5 M KCl is maximal in the presence of K+‐EDTA and is repressed in the presence of Mg2+. The column‐purified enzyme forms a complex with rabbit F‐actin, which can be dissociated by Mg2+ ATP. The low ionic strength Mg2+ ATPase of tomato myosin can be activated ten‐fold by rabbit actin and up to nineteen‐fold by tomato actin. No activation of the tomato myosin by rabbit F‐actin occurs in the absence of free calcium ions.
- S‐1 decoration of actin
- actin activation of myosin
- calcium sensitivity of actomyosin interaction
- higher land plant contractile system
- polymerization of actin