ALPHA(1)-ANTITRYPSIN MMALTON (PHE(52)-DELETED) FORMS LOOP-SHEET POLYMERS IN-VIVO - EVIDENCE FOR THE C-SHEET MECHANISM OF POLYMERIZATION

The Z (Glu(342) --> Lys) and Siiyama (Ser(53) --> Phe) deficiency vari ants of alpha(1)-antitrypsin result in the retention of protein in the endoplasmic reticulum of the hepatocyte by loop-sheet polymerization in which the reactive center loop of one molecule is inserted into a b eta-pleated sheet of a second. We show here that antitrypsin Mmal-ton (Phe(52)-deleted), which is associated with the same liver inclusions, is also retained at an endoglycosidase H-sensitive stage of processin g in the Xenopus oocyte and spontaneously forms polymers in vivo. Thes e polymers, obtained from the plasma of an Mmalton/QO (null) bolton he terozygote, were much shorter than other antitrypsin polymers and cont ained a reactive center loop-cleaved species. Monomeric mutant antitry psin was also isolated from the plasma. The monomeric component had a normal unfolding transition on transverse urea gradient gel electropho resis and formed polymers in vitro more readily than M, but less readi ly than Z, antitrypsin, The A beta-sheet accommodated a reactive cente r loop peptide much less readily than Z antitrypsin, which in turn was less receptive than native M antitrypsin. The nonreceptive conformati on of the A sheet in antitrypsin Mmalton had little effect on kinetic parameters, the formation of SDS-stable complexes, the S to R transiti on, and the formation of the latent conformation. Comparison of the re sults with similar findings of short chain polymers associated with th e antithrombin variant Rouen VI (Bruce, D., Perry, D., Borg, J.-Y., Ca rrell, R. W., and Wardell, M. R. (1994) J. Clin. Invest. 94, 2265-2274 ) suggests that polymerization is more complicated than the mechanism proposed earlier. The Z, Siiyama, and Mmalton mutations favor a confor mational change in the antitrypsin molecule to an intermediate between the native and latent forms. This would involve a partial overinserti on of the reactive loop into the A sheet with displacement of strand 1 C and consequent loop-C sheet polymerization.