<P> Microfilament formation by treadmilling has been found to be atypical in stereocilia . In this case the control of the structure's size is totally apical and it is controlled in some way by gene expression, that is, by the total quantity of protein monomer synthesized in any given moment . </P> <P> The actin cytoskeleton in vivo is not exclusively composed of actin, other proteins are required for its formation, continuance and function . These proteins are called actin - binding proteins (ABP) and they are involved in actin's polymerization, depolymerization, stability, organisation in bundles or networks, fragmentation and destruction . The diversity of these proteins is such that actin is thought to be the protein that takes part in the greatest number of protein - protein interactions . For example, G - actin sequestering elements exist that impede its incorporation into microfilaments . There are also proteins that stimulate its polymerization or that give complexity to the synthesizing networks . </P> <Ul> <Li> Thymosin β - 4 is a 5 kDa protein that can bind with G - actin - ATP in a 1: 1 stoichiometry; which means that one unit of thymosin β - 4 binds to one unit of G - actin . Its role is to impede the incorporation of the monomers into the growing polymer . </Li> <Li> Profilin, is a cytosolic protein with a molecular weight of 15 kDa, which also binds with G - actin - ATP or - ADP with a stoichiometry of 1: 1, but it has a different function as it facilitates the replacement of ADP nucleotides by ATP . It is also implicated in other cellular functions, such as the binding of proline repetitions in other proteins or of lipids that act as secondary messengers . </Li> </Ul> <Li> Thymosin β - 4 is a 5 kDa protein that can bind with G - actin - ATP in a 1: 1 stoichiometry; which means that one unit of thymosin β - 4 binds to one unit of G - actin . Its role is to impede the incorporation of the monomers into the growing polymer . </Li>

Where are actin filaments found within the cell