ИСТИНА

Actin is the most widespread protein in nearly all eukaryotic cells.Actin is the most common in the muscle cells, as a key component of muscle contraction. Being anelement of the cytoskeleton of eukaryotic cells, thisproteinplaysacrucialroleinsuchvital cellular processesascellularmotility, intracellulartransport, celldivision, themaintenanceofcellularmorphology, etc. In cells actin has been found in two main states: the monomeric form (G-actin) and the polymeric form (F-actin).The native G-actin is stabilized by binding of one ATP molecule and one Mg2+ion (or Ca2+in vitro).Removing of bivalent cation and/or ATP from G-actin leads to formation oftheso called "inactivated actin" (I-actin), that is not able to formF-actin.I-actin is supramolecular monodisperse aggregate comprised of 14-16 subunits.I-actin can be obtained as the result of heating, chemical denaturants or chelating agent treatment or even spontaneously during the storageG-actin.Thus, the native G-actin has a kinetically stable but thermodynamically unstable structure which, in the absence of Ca2+ or other bivalent metal ions, spontaneously converts to the thermodynamically stable I-state.[1] Earlier it has been supposed that macromolecular crowding affects the maintenance of native statestructure and formation of F-actin under polymerization conditions. [2] Using intrinsic fluorescence, we showed, that dextran 70 does not affect G-actin structure, while poly(ethylene glycol) 8000 (PEG 8000) induced the significant increase of fluorescent anisotropy and parameter A (A= I320/I365, where I320 and I365 are the fluorescence intensities at 320 and 356 nm), which characterizes fluorescence spectra position. Recorded changes in intrinsic fluorescence parameters might be an indirect evidence of actin polymerization, which occurs in spite of the absence of polymerization conditions (namelyhigh ionic strengths).Examination of the influence of crowding agents (dextran 70 and PEG 8000) on actin denaturation process revealed that both selected crowding agents dextran 70 and PEG 8000 stabilize the native actin structure decelerating the denaturation processes, but do not stop it.Furthermore, dextran 70 stabilizes the actin structure in inactivated state preventing the complete denaturation of the protein.We alsoshowed that fluorescence characteristics, such as form and position of fluorescence spectrum as well as the value of fluorescence anisotropy, of inactivated actin in diluted conditions and in crowded mediumare practically the same