MUTATIONS WHICH IMPEDE LOOP SHEET POLYMERIZATION ENHANCE THE SECRETION OF HUMAN ALPHA(1)-ANTITRYPSIN DEFICIENCY VARIANTS

alpha(1)-Antitrypsin plasma deficiency variants which form hepatic inclusion bodies within the endoplasmic path way include the common Z variant (Glu(342) --> Lys) and the rarer alpha(1)-antitrypsin S-iiyama (Ser(53) --> Phe). It has been proposed that retention of both abnormal proteins is accompanied by a common mechanism of loop-sheet polymerization with the insertion of the reactive center loop of one molecule into a beta-pleated sheet of another. We have compared the biosynthesis, glycosylation, and secretion of normal, Z and S-iiyama variants of alpha(1)-antitrypsin using Xenopus oocytes. S-iiyama and Z alpha(1)-antitrypsin both duplicated the secretory defect seen in hepatocytes that results in decreased plasma alpha(1)-antitrypsin levels. Digestion with endoglycosidase H localized both variants to a pre-Golgi compartment. The mutation Phe(51) --> Leu abolished completely the intracellular blockage of S-iiyama alpha(1)-antitrypsin and reduced significantly the retention of Z alpha(1)-antitrypsin. The secretory properties of M and Z alpha(1)-antitrypsin variants containing amino acid substitutions designed to decrease loop mobility and sheet insertion were investigated. A reduction in intracellular levels of Z alpha(1)-antitrypsin was achieved with the replacement of P-11/12 alanines by valines. Thus a decrease in Z and S-iiyama alpha(1)-antitrypsin retention was observed with mutations which either closed the A sheet or decreased loop mobility at the loop hinge region.