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 pathway include the common Z variant (Glu342-->Lys) and the rarer alpha 1-antitrypsin Siiyama (Ser53-->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 Siiyama variants of alpha 1-antitrypsin using Xenopus oocytes. Siiyama 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 Phe51-->Leu abolished completely the intracellular blockage of Siiyama 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 P11/12 alanines by valines. Thus a decrease in Z and Siiyama alpha 1-antitrypsin retention was observed with mutations which either closed the A sheet or decreased loop mobility at the loop hinge region.
Sidhar
S K
SK
Department of Physiology and Pharmacology, University of Southampton, United Kingdom.
Lomas
D A
DA
Carrell
R W
RW
Foreman
R C
RC
D000818:Animals; D001692:Biological Transport; D001704:Biopolymers; D006801:Humans; D007930:Leucine / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D009865:Oocytes; D010649:Phenylalanine / Q000235:genetics / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D014982:Xenopus laevis; D000515:alpha 1-Antitrypsin / Q000235:genetics / Q000378:metabolism*; D019896:alpha 1-Antitrypsin Deficiency
D001704:Biopolymers; D000515:alpha 1-Antitrypsin; D010649:Phenylalanine; D007930:Leucine