The synthesis and secretion of
apolipoprotein B-100 (apoB-100) have been studied in a human
hepatoma cell line, the Hep G2 cells. The time needed for the synthesis of
apoB-100 was estimated to be 14 min, which corresponds to a translation rate of approximately 6
amino acids/s.
ApoB-100 was compared with
albumin and
alpha 2-macroglobulin as to the distribution between the membrane and the
luminal content in the endoplasmic reticulum (ER) and the Golgi apparatus. The results suggested that
apoB-100 approximately followed the distribution of these secretory
proteins in the Golgi, while the ratios between the percent membrane-bound
apoB-100 and percent membrane-bound
albumin or
alpha 2-macroglobulin were 3-4:1 in the ER. This may suggest that
apoB-100 occurs in a membrane-associated form in ER prior to the integration in the
lipoproteins. Pulse-chase studies combined with subcellular fractionation was used to investigate the kinetics for the intracellular transfer of
apoB-100. A 3-min pulse of [35S]
methionine was followed by an increase in
apoB-100 radioactivity in the ER during the first 10-15 min of chase. The following 10-15 min of chase were characterized by linear decrease in
apoB-100 radioactivity with a decay rate of approximately 6%/min. The residence kinetics for
apoB-100 in the ER differed from that of
transferrin and probably also from that of
albumin. By comparing the time for the pulse maximum in ER with that in the denser Golgi fractions the time needed for the transfer between ER and Golgi could be estimated to be 10 min. The time needed for the secretion of newly synthesized
apoB-100 was estimated to be 30 min. This indicates that the transfer of the
protein through the Golgi apparatus to the extracellular space requires 20 min.