Tomato actin and myosin: Contractile proteins from a higher land plant

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.