<P> In vitro actin polymerization, or nucleation, starts with the self - association of three G - actin monomers to form a trimer . ATP - bound actin then itself binds the barbed end, and the ATP is subsequently hydrolyzed . ATP hydrolysis occurs with a half time of about 2 seconds, while the half time for the dissociation of the inorganic phosphate is about 6 minutes . This autocatalyzed event reduces the binding strength between neighboring subunits, and thus generally destabilizes the filament . In vivo actin polymerization is catalyzed by a class of filament end - tracking molecular motors known as actoclampins . Recent evidence suggests that the rate of ATP hydrolysis and the rate of monomer incorporation are strongly coupled . </P> <P> Subsequently, ADP - actin dissociates slowly from the pointed end, a process significantly accelerated by the actin - binding protein, cofilin . ADP bound cofilin severs ADP - rich regions nearest the (−) - ends . Upon release, the free actin monomer slowly dissociates from ADP, which in turn rapidly binds to the free ATP diffusing in the cytosol, thereby forming the ATP - actin monomeric units needed for further barbed - end filament elongation . This rapid turnover is important for the cell's movement . End - capping proteins such as CapZ prevent the addition or loss of monomers at the filament end where actin turnover is unfavorable, such as in the muscle apparatus . </P> <P> Actin polymerization together with capping proteins were recently used to control the 3 - dimensional growth of protein filament so as to perform 3D topologies useful in technology and the making of electrical interconnect . Electrical conductivity is obtained by metallisation of the protein 3D structure . </P> <P> As a result of ATP hydrolysis, filaments elongate approximately 10 times faster at their barbed ends than their pointed ends . At steady - state, the polymerization rate at the barbed end matches the depolymerization rate at the pointed end, and microfilaments are said to be treadmilling . Treadmilling results in elongation in the barbed end and shortening in the pointed - end, so that the filament in total moves . Since both processes are energetically favorable, this means force is generated, the energy ultimately coming from ATP . </P>

Microtubules and microfilaments are made from proteins that are shaped like