Ubiquitin-proteasome-dependent degradation of apolipoprotein B100 in vitro

Apolipoprotein B100 (apoB) is a large secretory protein that forms very low density lipoprotein in liver. An in vitro degradation assay was developed using rabbit reticulocyte (RR) lysate in order to investigate the mechanism of intracellular degradation of newly synthesized apoB by the ubiquitin-proteasome pathway. [3H]apoB, isolated from [3H]leucine pulsed/chased Hep G2 cells, was degraded 51% when incubated for 2 h at 37°C in an assay mixture that included RR lysate (source of the ubiquitin conjugation system and proteasome) and an exogenous ATP regenerating system. ApoB degradation was ATP-dependent and degradation fragments were not observed suggesting that the very large apoB molecule was extensively degraded. ApoB degradation was decreased to 50% when potent proteasome inhibitors, clasto-lactacystin β-lactone (10 μM) or MG-132 (50 μM), were added to the reaction mixture, but was not affected by the cysteine protease inhibitor, E-64, or the serine protease inhibitor, phenylmethylsulfonyl fluoride. ApoB degradation was inhibited by the mutant ubiquitin protein K48R and by ubiquitin aldehyde, an inhibitor of ubiquitin-protein isopeptidases. During incubation ubiquitination of apoB increased even as apoB was being degraded. These results suggest that in vitro degradation of apoB, a large secretory protein that is normally found in the endoplasmic reticulum (ER) lumen or associated with the ER membrane, was proteasome-dependent and involved both ubiquitination and deubiquitination steps.