A Membrane-Bound Archaeal Lon Protease Displays ATP-Independent Proteolytic Activity towards Unfolded Proteins and ATP-Dependent Activity for Folded Proteins

Abstract

ABSTRACT In contrast to the eucaryal 26S proteasome and the bacterial ATP-dependent proteases, little is known about the energy-dependent proteolysis in members of the third domain, Archaea . We cloned a gene homologous to ATP-dependent Lon protease from a hyperthermophilic archaeon and observed the unique properties of the archaeal Lon. Lon from Thermococcus kodakaraensis KOD1 (Lon Tk ) is a 70-kDa protein with an N-terminal ATPase domain belonging to the AAA + superfamily and a C-terminal protease domain including a putative catalytic triad. Interestingly, a secondary structure prediction suggested the presence of two transmembrane helices within the ATPase domain and Western blot analysis using specific antiserum against the recombinant protein clearly indicated that Lon Tk was actually a membrane-bound protein. The recombinant Lon Tk possessed thermostable ATPase activity and peptide cleavage activity toward fluorogenic peptides with optimum temperatures of 95 and 70°C, respectively. Unlike the enzyme from Escherichia coli , we found that Lon Tk showed higher peptide cleavage activity in the absence of ATP than it did in the presence of ATP. When three kinds of proteins with different thermostabilities were examined as substrates, it was found that Lon Tk required ATP for degradation of folded proteins, probably due to a chaperone-like function of the ATPase domain, along with ATP hydrolysis. In contrast, Lon Tk degraded unfolded proteins in an ATP-independent manner, suggesting a mode of action in Lon Tk different from that of its bacterial counterpart.