Alterations of inter-domain flexibility in actin monomers during cyclophosphamide treatment

The actin is one of the main component of the eukaryotic cytoskeleton. The continuous rearrangement of actin filaments is provided by the different complexes with divalent cations (Ca 2+ or Mg 2+ ) and nucleotides (ATP, ADP). In the medical routine, cyclophosphamide (CP) is applied as cytostatic and it was shown that in vivo muscle filament system was changed by the CP treatment and it has direct interaction with actin monomers as well. The evolutionary importance of physical links between domains is one of the most interesting question to understand the multi-domain development of protein functions. Here, we analyse the thermal stability modifier act of inter-domain links in proteins, monitored by DSC, with the concept of that how did the nucleotide binding cleft between the two main domains of actin monomers affect the activation energy of domains if it was blocked or released by CP binding or dissociation, respectively. We investigated the importance of inter-domain linkers on the thermodynamic properties of actin. Ca 2+ and Mg 2+ bound G-actin can be stabilized by CP binding or polymerization. CP treatment of Ca 2+ -F actin lacks the structural integrity of the more flexible polymer and shows same stability as CP bound monomers. However, Mg 2+ -F actin did not show any kinetic response to the CP treatment. We can assume that the inter-domain linker of actin reduces the stability of the domains which leads to a more reactive and variable structure as a thermodynamic advantage for the development of a multi-domain protein can be blocked by CP treatment.