Apolipoprotein B48-membrane interactions. Absence of transmembrane localization in nonhepatic cells

Apolipoprotein B (apoB) is essential for the hepatic assembly and secretion of triglyceride-rich very low density lipoproteins. Recent studies have revealed that in both hepatic and nonhepatic cells a large percentage of newly synthesized apoB polypeptides engage in transmembrane interactions with the endoplasmic reticulum (ER). These apoB-membrane interactions have been implicated in the processes of lipoprotein assembly and regulation. To identify domains of apoB that are responsible for its transmembrane localization, overlapping 300 amino acid segments of human apoB48 (amino-terminal 48% of apoB) and two control proteins, human complement component C3 and mouse CD4, were appended to the amino-terminal 77 amino acids of a soluble secretory precursor protein and expressed in COS-1 cells. While the integral membrane protein CD4 conferred predictable transmembrane orientation on the hybrid protein, as evidenced by its partial protease accessibility in intact microsomes, all of the apoB-containing proteins and the soluble secretory control, C3, were fully protease-resistant, consistent with their complete translocation into the ER. To determine if conformational properties of apoB are responsible for its transmembrane interactions with the ER, proteins containing the entire amino-terminal approximately 50% of apoB (apoB50) were expressed in COS-1 cells. Irrespective of whether targeting and translocation initiation were directed by a heterologous signal peptide or the native apoB signal peptide, apoB50 appeared to undergo complete membrane translocation into a protease-inaccessible compartment. These results demonstrate that the amino-terminal 50% of apoB lacks autonomous signals or properties that can fully block ER membrane translocation or promote any other form of stable transmembrane assembly in nonhepatic cells.