Temperature and induction effects on the degradation rate of an abnormal beta-galactosidase in Escherichia coli.
Intracellular protein degradation was investigated using an unstable fragment of Escherichia coli beta-galactosidase, the CSH11 mutant, as a model protein. This abnormal protein was expressed from a single copy gene in the chromosome and is converted to a detectable degradable intermediate. The in vivo degradation rates of both beta-galactosidase fragments were measured using pulse-chase radioactive labeling techniques, and their intracellular concentrations were determined using alpha-complementation assays. In the physiological range of 30 to 37 degrees C, the apparent degradation rate constant for the CSH11 fragment follows Arrhenius behavior; while the intermediate's apparent degradation rate constant is nearly unchanged. However, above 37 degrees C the degradation rates of both fragments increase significantly. Analysis of the labeled intermediate's rate of change above 40 degrees C reveals that the CSH11 fragment is being degraded by a second pathway which does not produce the intermediate. When the induction level of the abnormal beta-galactosidase was varied the degradation rates of both fragments behaved similarly, but they unexpectedly decreased with increasing IPTG concentration. The two parallel degradation pathways for CSH11 apparently operated at only the lower IPTG levels. The measured degradation rates did not correlate directly with the intracellular concentration of abnormal proteins.
Department of Chemical Engineering, California Institute of Technology, Pasadena 91125.
M J
MJ
Kosinski
J E
JE
Bailey
D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D009154:Mutation; D013696:Temperature; D001616:beta-Galactosidase / Q000378:metabolism*
D001616:beta-Galactosidase