Processing of proapolipoprotein AI requires specific conformation.
Apolipoprotein AI of human high-density lipoproteins is secreted by hepatocytes as a proapolipoprotein with a N-terminal hexapeptide sequence (Arg-His-Phe-Trp-Gln-Gln-) which differs from the prosequence of rat apolipoprotein AI (Trp-Asp-Phe-Trp-Gln-Gln). The two proteins have in common the unusual cleavage site -Gln-Gln-Asp-Glu-. It is hydrolysed by a specific serum proteinase with the release of mature apo AI. We synthesized a model substrate for the study of the final processing of pro-apo AI by the serum proteinase. It is an undecapeptide embracing the human pro-hexapeptide sequence and the first five N-terminal residues of apo AI, covalently linked to a hydrophilic resin. The N-terminal arginine residue was 3H-labelled. [formula; see text] This sequence was not cleaved by human serum under the conditions under which rat serum processes the pro-form of apo AI secreted by rat hepatocytes. Pepsin and chymotrypsin fragmented the undecapeptide at sites characteristic for these proteinases. We conclude that the proteolytic cleavage at the specific site (-Gln-Gln-Asp-Glu-) requires the correct conformation in addition to the specific amino-acid sequence.
D000595:Amino Acid Sequence; D016632:Apolipoprotein A-I; D016633:Apolipoprotein A-II; D001054:Apolipoproteins A / Q000032:analysis / Q000302:isolation & purification*; D055598:Chemical Phenomena; D002621:Chemistry; D002918:Chymotrypsin; D006801:Humans; D006868:Hydrolysis; D007009:Hypolipoproteinemias / Q000097:blood; D010434:Pepsin A; D010455:Peptides / Q000032:analysis; D011487:Protein Conformation; D011498:Protein Precursors / Q000032:analysis / Q000302:isolation & purification*; D012116:Resins, Plant
D016632:Apolipoprotein A-I; D016633:Apolipoprotein A-II; D001054:Apolipoproteins A; D010455:Peptides; D011498:Protein Precursors; D012116:Resins, Plant; D002918:Chymotrypsin; D010434:Pepsin A