The nerve growth factor protein (NGF) favors polymerization of brain actin and induces its organization to form paracrystalline structures that activate myosin ATPase (ATP phosphohydrolase, EC 3.6.1.3) to an extent greater than actin alone. Binding studies show that the initial 1:1 stoichiometry of NGF-G-actin complexes decreases to 1:7-10 when polymerization is ended and paracrystalline structures are formed. The ratio becomes even lower when heavy meromyosin is added in the absence of ATP, suggesting that heavy meromyosin displaces NGF bound to actin microfilaments. This conclusion is supported by the finding that when heavy meromyosin is added to NGF-microfilament complexes, under conditions for "decorating" microfilaments, the usual paracrystalline structure of the complexes disappears. The NGFmediated organization of actin and activation of myosin ATPase is visualized as a self-regulatory and self-propagating mechanism, because progressive displacement of the growth factor induced by heavy meromyosin binding to F actin as ATP consumption proceeds renders an increasingly higher amount of NGF free for new interactions. These findings are discussed in the light of the mechanism of action of NGF in the target cells. The earliest ultrastructural change accompanying neurite growth induced by nerve growth factor (NGF) (1) in the target cells is the appearance and organization of microtubules and other filamentous proteins (2). These contractile elements play an essential role in fiber growth and elongation, as shown by the finding that inhibitors of their assembly and/or function, such as colchicine or cytochalasin B, block neurite growth or induce fiber retraction when added to preformed neurites (3).It has been shown that NGF interacts in vitro with tubulin (4) and actin (5) precursor elements of microtubules and microfilaments (MFs) and changes some of their most important physicochemical properties (6-9). These findings led us to hypothesize a relationship between these in vitro effects and in vivo NGF induction of fiber growth, postulating a direct action of this growth factor on some functional properties of filamentous proteins (5,8).This paper describes the properties of the NGF-actin complexes, shows how they activate myosin ATPase (ATP phosphohydrolase, EC 3.6.1.3), and discusses how these findings could fit with the hypothesis that NGF-mediated neutrite growth may involve a direct interaction with the contractile network formed by microtubules, MFs, and their precursor elements.The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.METHODS AND MATERIALS Purification of NGF, Brain Actin, Muscle Myosin, and Heavy Meromyosin. The 2.5S NGF was purified from mouse salivary gland according to the procedure described by Bocchini and Angeletti (10). The slight ATPase activity still contaminating the growth factor after the first CM-cellulo...