TRANSLOCATIONAL PAUSING OF APOLIPOPROTEIN-B CAN BE REGULATED BY MEMBRANE LIPID-COMPOSITION

One potential mechanism by which apolipoprotein (apo) B secretion is r egulated is via transient pausing during translocation across the endo plasmic reticulum membrane, We have previously shown that translocatio n and secretion of full-length and truncated variants of apoB 100 are impaired in hepatocytes in which microsomal membranes are enriched in the phospholipid phosphatidyl-monomethylethanolamine (PMME), We have n ow investigated whether or not the decreased translocation of apoB is the result of altered membrane lipid composition having an impact on t ranslocational pausing. Our experiments showed that less in vitro tran slated apoB-15 (the N-terminal 15% of human apoE-100) was translocated into the lumen of PMME-enriched microsomes than of control microsomes , Proteinase K treatment of the translocation products yielded discret e N-terminal fragments of apoB indicating that both types of microsoma l membranes contained translocationally paused nascent chains. Similar ly, apoB generated from a truncated mRNA lacking a stop codon was also found to be translocationally paused. However, restarting of transloc ation after translocational pausing was impaired in PMME enriched, but not in control, microsomes. These data suggest that secretion of apoB -containing lipoproteins can be regulated by membrane lipid compositio n at the level of translocational pausing.