The Protein Quality Control (PQC) system plays a leading role in maintaining the safety of the cellular proteome in all natural kingdoms. This review summarizes information about the structural and functional characteristics of molecular chaperones and energy-dependent proteases that form the PQC system, emphasizing the crucial role of proteins belonging to the AAA+ superfamily, with the focus on ATP-dependent Lon proteases as a special family in the PQC. Similarities and differences among the enzymes of individual Lon subfamilies are discussed in detail using up-to-date data, elucidating the structural features and unique mechanisms of functioning of these proteins.