Serine protease that displays ATP-independent proteolytic activity towards peptides and unfolded proteins and ATP-dependent activity for the cleavage of folded proteins. Mg(2+). Exhibits no peptide cleavage activity without Mg(2+), regardless of the presence or the absence of ATP. Can also use other divalent cations such as Ni(2+), Ca(2+), Mn(2+) and Co(2+). ADP inhibits the peptide cleavage activity of LonTk, but the enzyme retains 57% activity in comparison to the condition with the addition of ATP. Optimum pH is 9 for the ATP-independent peptide cleavage activity and for ATPase activity. Optimum temperature is 70 degrees Celsius for the ATP-independent peptide cleavage activity, and 95 degrees Celsius for ATPase activity. Homohexamer. Organized in a ring with a central cavity (By similarity). Possesses a two-domain structure consisting of an N-terminal ATPase domain belonging to the AAA(+) superfamily and a C-terminal protease domain. The ATPase domain likely acts as a molecular chaperone functioning in the unfolding of protein structures, along with ATP hydrolysis. Belongs to the peptidase S16 family. Archaeal LonB subfamily.