Journal of Biological Chemistry
Volume 281, Issue 37, 15 September 2006, Pages 27063-27071
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Lipids and Lipoproteins
Translocation Efficiency of Apolipoprotein B Is Determined by the Presence of β-Sheet Domains, Not Pause Transfer Sequences*

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Cotranslational translocation of apoB100 across the endoplasmic reticulum (ER) membrane is inefficient, resulting in exposure of nascent apoB on the cytosolic surface of the ER. This predisposes apoB100 to ubiquitinylation and targeting for proteasomal degradation. It has been suggested that pause transfer sequences (PTS) present throughout apoB cause inefficient translocation. On the other hand, our previous study demonstrated that the translocation efficiency of apoB100 is dependent on the presence of a β-sheet domain between 29 and 34% of full-length apoB100 (Liang, J.-S., Wu, X., Jiang, H., Zhou, M., Yang, H., Angkeow, P., Huang, L.-S., Sturley, S. L., and Ginsberg, H. N. (1998) J. Biol. Chem. 273, 35216–35221); this region of apoB has no PTS. However, the effects of the β-sheet domain may require the presence of PTS elsewhere in the N-terminal region of apoB100. To further investigate the roles of PTS and β-sheet domains in the translocation of apoB100 across the ER, we transfected McArdle RH7777, HepG2, or Chinese hamster ovary cells with human albumin (ALB)/human apoB chimeric cDNA constructs: ALB/B12–17 (two PTS but no β-sheet), ALB/B29–34 (β-sheet but no PTS), ALB/B36–41 (two PTS and a β-sheet), and ALB/B49–54 (neither PTS nor a β-sheet). ALB/ALB1–40 served as a control. Compared with ALB/ALB1–40, secretion rates of ALB/B12–17, ALB/B29–34, and ALB/B36–41 were reduced. Secretion of ALB/B49–54 was similar to that of ALB/ALB1–40. However, only ALB/B29–34 and ALB/B36–41 had increased proteinase K sensitivity, ubiquitinylation, and increased physical interaction with Sec61α. These results indicate that the translocation efficiency of apoB100 is determined mainly by the presence of β-sheet domains. PTS do not appear to affect translocation, but may affect secretion by other mechanisms.

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*

This work was supported by NHLBI Grants R01 HL55638 and T32 HL07343 (to H. N. G.) and Grant R01 58541 (to E. A. F.) from the National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.