The N-terminal headpiece region of the lac repressor from E. coli functions as a degron. Plasmids containing the headpiece region (residues 2–38) of the E. coli lac repressor were stably transfected into cell line RJK88.13 and treated with CHX. Decay of protein expression was monitored over the indicated times by Western blotting (see “Experimental Procedures” for details). Schematic diagrams of the constructs are shown, along with representative blots. The half-lives (mean ± S.D.) are shown to the far right. A, constructs encoding LAC-hTS or a P2A mutant; B, constructs expressing eGFP fusion protein LAC-hA-eGFP or LAC-eGFP; C and D, first order decay plots of the data in A and B, respectively, from which half-lives were determined. Error bars, S.D.