THE DEFECTIVE SECRETION OF A NATURALLY-OCCURRING ALPHA-1-ANTICHYMOTRYPSIN VARIANT WITH A FRAMESHIFT MUTATION

A newly found variant alpha-1-antichymotrypsin (ACT), ACT Isehara-2, h as a deletion of two bases (AA) at codon 391 near the carboxyl terminu s. This frameshift mutation caused a change in the amino acid sequence and generated 10 extra amino acids (408 amino acids total) [Tsuda, M. , Sei, Y., Matsumoto, M., Kamiguchi, H., Yamamoto, Y., Shinohara, Y., Igarashi, T. st Yamamura, M. (1992) Hum. Genet. 91, 467-468]. The seru m ACT levels in three unrelated heterozygotes with this mutant ACT gen e were 37%, 49% and 54% that of the normal individuals. To examine the reduced serum levels, the normal ACT and the mutant ACT created by si te-directed mutagenesis were transfected into COS-7 cells for comparis on. The value for the retention rate (intracellular ACT/total ACT) was apparently higher in the cells expressing mutant ACT Isehara-2 than t hose bearing the normal gene. In the pulse-chase experiments, the secr etion of the synthesized mutant ACT into the medium was not observed, whereas the normal ACT was mostly secreted as a 64-kDa form. The endog lycosidase H digestion and an electron microscopic analysis indicated that the retained mutant ACT was present in the endoplasmic reticulum. These results provide the biochemical basis for the decreased serum A CT level of individuals with ACT Isehara-2, and suggest the importance of the carboxyl-terminal region for its secretion.