FOLDING AND STABILITY OF THE Z-IIYAMA AND S-IIYAMA GENETIC-VARIANTS OF HUMAN ALPHA(1)-ANTITRYPSIN

Z (Glu(342) --> Lys) and S-iiyama (Ser(53) --> Phe) genetic variations of human alpha(1)-antitrypsin (alpha(1)-AT) cause a secretion blockag e in the hepatocytes, leading to alpha(1)-AT deficiency in the plasma, Using in vitro folding analysis, we have shown previously that these mutations interfere with the proper folding of polypeptides. To unders tand the fundamental cause for the secretion defect of the Z and S-iiy ama variants of alpha(1)-AT, we investigated in vivo folding and stabi lity of these variant alpha(1)-AT using the secretion system of yeast Saccharomyces cerevisiae, Various thermostable mutations suppressing t he folding block of the Z variant in vitro corrected the secretion def ect as well as the intracellular degradation in the yeast secretion sy stem, Significantly, the extent of suppression in the secretion defect of Z protein was proportional to the extent of suppression in the fol ding defect, assuring that the in vivo defect associated with the Z va riant is primarily derived from the folding block. In contrast, the fo lding and secretion efficiency of S-iiyama was not much improved by th e same mutations, In addition, none of the rarely secreted S-iiyama al pha(1)-AT carrying the stabilizing mutations for the wild type and Z v ariant were active, It appears that the major defect in S-iiyama varia nt is the loss of stability in contrast to the kinetic block of foldin g in the Z variant.