Pacaud and Uriel described an
enzyme from Escherichia coli ("
protease I") that hydrolyzes acetyl
phenylalanine naphthyl
ester (
APNE). We examined the possible involvement of this
enzyme in intracellular protein degradation, its subcellular distribution, and its proteolytic activity. Although the
APNE-hydrolyzing activity is localized primarily in the periplasm, proteolytic activity against
casein was found in the periplasm, membrane, and cytoplasm with similar specific activities. The
APNE-hydrolyzing
enzyme did not appear to contribute to the proteolytic activity of the periplasm. A mutant deficient in
APNE-hydrolyzing activity lacked all activity in the periplasm but showed a slight percentage of residual activity in the cytoplasm. Extracts of such cells were normal in their ability to hydrolyze
casein. The mutant was indistinguishable from wild-type cells in its rate of protein degradation during growth or
glucose starvation and in the ability to rapidly degrade
puromycin-containing
polypeptides.
Nitrogen starvation, which increased
protein breakdown severalfold, affected neither the total amount nor the distribution of
APNE-hydrolyzing activity. The mutant showed no defect in its ability to cleave small
phenylalanine-containing
peptides released during protein degradation. The mutant and wild-type cells are equally able to hydrolyze exogenously supplied phenylalanyl
peptides. These experiments suggest that the
APNE-hydrolyzing
enzyme is not required for protein degradation and that "
protease I" is probably not a
protease.