Relative stability of intracellular proteins in bacterial cells.

The relative stabilities of soluble and membrane proteins were examined in growing Escherichia coli cells. In contrast to mammalian cells, we found no correlations between the isoelectric points or molecular weights of E. coli proteins and their degradative rates. The soluble proteins with short half-lives tended to be degraded preferentially in vitro by trypsin or chymotrypsin. The stability of membrane proteins in vivo was correlated with in vitro sensitivity to chymotrypsin but not to trypsin. In the total membrane fraction, endogenous proteolytic activity varied with growth conditions. This activity was inhibited by o-phenanthroline, EDTA and dithiothreitol suggesting that one or more metallo-proteinases were present. Membrane proteinase activity was also inhibited by phenethyl alcohol, a membrane perturbant. The abundance of the membrane proteins that were most labile in vivo was dependent on growth conditions. The most labile protein accumulated in the outer membrane with an inverse relationship to growth rate.