Biosynthesis of high density lipoprotein by chicken liver: intracellular transport and proteolytic processing of nascent apolipoprotein A-1.

Abstract

To study the in vivo processing and secretion of Apolipoprotein A-I (Apo A-I), young chickens were administered individual L-[3H]amino acids intravenously and the time of intracellular transport of nascent Apo A-I from rough endoplasmic reticulum (RER) to the Golgi apparatus was measured. Within 3 to 9 min there was maximal incorporation of radioactivity into Apo A-I in both the RER and the Golgi cell fractions. By contrast, the majority of radioactive albumin was also present in the RER by 3 to 9 min, but did not reach peak amounts in the Golgi fraction until 9 to 25 min. Both radioactive Apo A-I and albumin appeared in the blood at about the same time (between 20 and 30 min). NH2-terminal amino acid sequence analysis of nascent intracellular Apo A-I showed that it contains a pro-hexapeptide extension identical to that of human Apo A-I. After 30 min of administration of radioactive amino acids radioactive Apo A-I was isolated by immunoprecipitation from the liver and serum. NH2-terminal sequence analysis of 20 amino acids indicated that chicken liver contained an equal mixture of nascent pro-Apo A-I and fully processed Apo A-I, whereas the serum only contained processed Apo A-I. Further studies showed that the RER only contained pro-Apo A-I, whereas a mixture of pro-Apo A-I and processed Apo A-I was found in the Golgi complex. These results indicate that, in chicken hepatocytes, there is a more rapid transport of Apo A-I than of albumin from the RER to the Golgi cell fractions, and that Apo A-I remains in the Golgi apparatus for a longer period of time before it is secreted into the blood. In addition these studies show that the in vivo proteolytic processing of chicken pro-Apo A-I to Apo A-I occurs in the Golgi cell fractions.

Polyclonal antibodies against formaldehyde-modified apolipoprotein A-I. An approach to circumventing fixation-induced loss of antigenicity in immunocytochemistry.

“…These observations correlate with results of a study on chicken hepatocytes, 53 which concluded that apo A-l is synthesized in the ER, transported to the Golgi very rapidly, and remains there for a longer period before secretion into the blood.…”

Apolipoprotein CII from Chicken (Gallus domesticus)

“…A similar cleavage site, GlnHis, is present in chicken apoAI (40). More than 90% of the apoAI in chicken plasma is cleaved which demonstrates that the proteolytic machinery is operational in chickens (31).…”

Quality control in the apoA-I secretory pathway

“…Whereas VLDL particles have been readily isolated from the Golgi (9, 10), efforts to identify HDL particles along the secretory pathway have had little success (9). Although these findings provided little support for the intracellular assembly of nascent HDL, a number of other studies supported the concept of an intracellular apoA-I lipidation pathway (11)(12)(13)(14)(15)(16). With the discovery of the ABCA1 transporter, many of these concepts concerning nascent HDL assembly have been revised, and recent studies support the idea of both ABCA1-dependent and -independent mechanisms responsible for apoA-I lipidation (17,18).…”