Heterologous expression of apolipoprotein B carboxyl-terminal truncates: A model for the study of lipoprotein biogenesis

Expression of proteins in cells that lack specific chaperones, or subunits required for correct assembly results in the degradation of these proteins early on after synthesis, probably in the endoplasmic reticulum (ER) or an ER-related compartment. We have explored heterologous expression as a model to study the complex process involved in the biogenesis of apolipoprotein B (apoB)-containing lipoproteins. As an initial step, a series of carboxyl- terminal truncated apoB in COS cells were expressed to characterize this system. ApoB proteins ranging from apoB-13 to apoB-41 were expressed. Truncates larger than apoB-29 were completely retained and degraded within the cells; hence these cells have a 'secretory defect' for the larger truncates. Degradation of these apoB proteins is likely to be in the endoplasmic reticulum or ER-related compartment, as shown by sensitivity to endo β-N-acetylglucosaminidase, by lack of an effect on the degradation kinetics by brefeldin A treatment, and by ImmunoGold EM. Degradation in COS cells was not affected by a variety of protease inhibitors, including N- acetyl-leucyl-leucyl-norleucinal (ALLN). Addition of oleate to the culture medium did not alter their metabolic fate. In comparison to the larger truncates, apoB proteins less than apoB-29 were partially secreted; the majority of these, too, were also retained and degraded intracellularly in a similar compartment. Secreted apoB-17 protein was shown to have been processed correctly in the Golgi and was secreted as a minimally lipidated protein. This model, based on the expression of apoB truncates larger than B- 29 in COS cells, may therefore allow for the identification and study of the requirements of specific factors that may be required for both apoB secretion and lipoprotein biogenesis by in vivo complementation of this 'secretory' defect.