The biosynthesis of rat alpha 1-antitrypsin.

The biosynthesis of rat al-antitrypsin (al-AT) has been studied in vitro in a cell-free system and in primary cultures of hepatocytes.The mRNA translation product for al-antitrypsin has a 24-amino acid residue long NHz-terminal signal peptide and its amino acid sequence was found to be (Met)-Ala-Pro-Ser-Ile-Ser- Arg-Gly-Leu-Leu-Leu-Leu-Ala-Gly-Leu-X-Cys-Leu-Ala-Pro-Ser-Phe-Leu-Ala.The signal peptide contains a central block of hydrophobic residues similar to those found for other known signal peptides.The mRNA translation product has an apparent M,. = 43,000.When translation was performed in the presence of dog pancreas microsomal vesicles a M, = 50,000 was obtained for the segregated and core-glycosylated product.This was identical with the intracellular counterpart detected in pulse-chase experiments.Both these forms were degraded by endo-/l-N-acetylglucosaminidase H and the degradations indicated that rat al-antitrypsin has three oligosaccharide side chains.Terminally glycosylated endo -/3-Nacetylglucosaminidase -resistant and neuraminidase-sensitive al-antitrypsin was also found intracellularly prior to secretion.The M, of this species was 52,000 whereas that of al-antitrypsin synthesized in the presence of tunicamycin was 41,000, indicating an apparent M, = 11,000 for the terminally glycosylated oligosaccharide side chains.The protein synthesized in the presence of tunicamycin was secreted, although more slowly than was its glycosylated counterpart, indicating that glycosylation is not a prerequisite for export of this protein.