The transport of apolipoprotein B (apoB) between the endoplasmic reticulum (ER) 
and Golgi was studied in puromycin-synchronized HepG2 cells, using an antibody 
that could distinguish between apoB in ER and Golgi compartments. In cells with 
normal ER-to-Golgi transport, both albumin and apoB colocalized throughout the 
ER and appeared as intense, compact signals in Golgi. When ER-to-Golgi transport 
was blocked with brefeldin A, apoB and albumin remained colocalized in the ER 
network and three-dimensional constructed images showed more intense signals for 
both proteins in a central, perinuclear region of the ER. When protein synthesis 
was stopped in cells with brefeldin A-inhibited ER-to-Golgi transport, apoB 
degradation was visualized as a homogeneous decrease in fluorescence signal 
intensity throughout the ER that could be slowed with clasto-lactacystin 
beta-lactone, a proteasome inhibitor. Incubation of cells with CP-10447, an 
inhibitor of microsomal triglyceride transfer protein, inhibited apoB, but not 
albumin, transport from ER to Golgi. Nanogold immunoelectron microscopy of 
digitonin-permeabilized cells showed proteasomes in close proximity to the 
cytosolic side of the ER membrane. Thus, newly synthesized apoB is localized 
throughout the entire ER and degraded homogeneously, most likely by neighboring 
proteasomes located on the cytosolic side of the ER membrane. Although albumin 
is colocalized with apoB in the ER, as expected, it was not targeted for 
ER-associated proteasomal degradation.