Protein degradation in animal and bacterial cells is dependent upon ATP [ 1,2].
In bacteria [3-61 and mammalian mitochondria [7], this energy dependence arises in large part from the involvement of a novel type of proteinase whose function is directly coupled to ATP hydrolysis [2,3]. The best-characterized of these ATPdependent enzymes is protease La from Escherichiu coli [3-71. This unusual enzyme is both a serine proteinase and an ATPase [4,5]. In fact the rate of protein degradation is directly proportional to the rate of ATP consumption [3-51. Protein substrates stimulate ATP cleavage by protease La, but nonhydrolyzable proteins or small peptide substrates have no effect on this ATPase activity [4,5]. Previous studies [5] have shown that ATP hydrolysis is required for the degradation of proteins to acid-soluble products, but the hydrolysis of small peptides requires only the binding of a nucleotide to the enzyme. For example, nonhydrolyzable ATP analogues can support hydrolysis of oligopeptides but not protein degradation [5]. On this basis, a multistep model of protein degradation has been proposed in which ATP binding activates the enzyme for each round of proteolysis (Fig. 1) [5].
Since peptide bond cleavage in small peptides does not require ATP hydrolysis [5], the latter process must serve another important function in the degradation of large polypeptides. For example, ATP consumption may promote the release of the polypeptide products from the enzyme; alternatively, ATP hydrolysis may permit the translocation of the enzyme along the substrate to the next cleavage site [5]. One prediction of such mechanisms is that in the presence of a nonhydrolyzable ATP analogue, a single cleavage or only a limited number of cleavages should occur. Previous studies of the function of protease La have measured the generation of acidsoluble peptides and therefore would not have detected limited proteolysis that resulted in large acid-precipitable fragments.
Timothy Edmunds's present address is Damon Biotech,