PinA inhibits ATP hydrolysis and energy-dependent protein degradation by Lon protease

The bacteriophage T4 PinA protein inhibited degradation of [3H]alpha-methyl casein by purified Lon protease from Escherichia coli, but inhibition was noncompetitive with respect to casein. PinA did not inhibit cleavage of the fluorogenic peptide, N-glutaryl-alanylalanylphenylalanyl-3-methoxynaphthylamide and, moreover, did not block the ability of protein substrates, such as casein, to activate cleavage of fluorogenic peptides by Lon. Thus, PinA does not block the proteolytic active site or the allosteric protein-binding site on Lon. Inhibition of basal ATPase activity was variable (50-90%), whereas inhibition of protein-activated ATPase activity was usually 80-95%. Inhibition was noncompetitive with respect to ATP. PinA did not block activation of peptide cleavage by nonhydrolyzable analogs of ATP. These data suggest that PinA does not bind at the ATPase active site of Lon and does not interfere with nucleotide binding to the enzyme. PinA inhibited cleavage of the 72-amino acid protein, CcdA, degradation of which requires ATP hydrolysis, but did not inhibit cleavage of the carboxyl-terminal 41-amino acid fragment of CcdA, degradation of which does not require ATP hydrolysis. PinA thus appears to interact at a novel regulatory or enzymatic site involved in the coupling between ATP hydrolysis and proteolysis, possibly blocking the protein unfolding or remodeling step essential for degradation of high molecular weight protein substrates by Lon.