This review summarizes the results of our investigations of actin unfolding-refolding and presents the notion that protein unfolding pathway, the number and the appearance order of intermediate states do not dependent on denaturing agents. To place our concept in the context of current knowledge of protein folding, we review in brief the development of general ideas of protein folding mechanisms, paying special attention to some key points of this process. Thus we focus on the characteristics of amino acid sequences that provide the existence of protein native structure, and on the interactions that compensate the increase of free energy due to the decrease of entropy on the way from multitude unfolded conformations to unique native state. In particular, we emphasize that ordered structures can arise both due to intramolecular and intermolecular interactions which lead to the formation of native and misfolded (associates, amorphous aggregates amyloid and amyloid-like fibrils) states, respectively.