The separation of protein based on their age, for the study of protein degradation in Escherichia coli

Density labelling with 2H2O has been used in association with isopycnic centrifugation to isolate proteins of known age from cultures of Escherichia coli. The physical properties of protein samples of known age have been examined to detect a correlation between specific properties and susceptibility to degradation. No evidence of a correlation between size, charge, thermodynamic properties or amino acid composition, on one hand, and susceptibility to degradation, on the other hand, was observed following step-down conditions of growth. However, the SH content of proteins in E. coli does appear to be correlated with their susceptibility to degradation.