Cytosolic hsp70 facilitates the ubiquitination and proteasomal degradation of a secretory protein, apoproteinb100

Regulation of apoprotein B100 (apoB) secretion from human hepatoma cells involves controlled degradation of newly synthesized apoB in the endoplasmic reticulum (ER) Our previous studies indicate that nascent apoB interacts with the cytosolic chaperone, heat shock protein 70 (HspVO). This is indicative of arrested apoB translocational status. We demonstrated that increased levels of HspVO, either induced by Herbimycin A (HA) treatment or by transient transfection with hsp70 cDNA, were associated with increased apoB degradation and decreased secretion. Both basal and Hsp70stimuiated apoB degradation were blocked by a proteasome-specific inhibitor, laclacystin In addition, nascent apoB was conjugated with ubiquitin The amount of ubiquitin-conjugated apoB accumulated when cells were treated with lactacystin. Increasing Hsp70 levels by HA treatment or by transfection resulted in increased apoB ubiquitination. Finally, the truncated N-terminus 50% of apoB expressed in CHO cells was found in a ubiquitinconjugated state, whereas the truncated N-terminal 16% of apoB showed minimal ubiquitination. The difference in ubiquitination paralleled their abilities to interact with H.sp70. We propose that Hsp70 facilitates apoB degradation by promoting apoB ubiquitination and proteasomal proteolysis.