The Defective Secretion of a Naturally Occurring α‐1‐Antichymotrypsin Variant with a Frameshift Mutation

Abstract

A newly found vuriunt 11-1 -antichymotrypsin (ACT), ACT Tsehara-2, has a deletion of two bases (AA) at codon 391 near the carboxyl terminus. This framcshift mutation caused a change i n the amino acid sequence and generated 1 0 extra arnino acids (408 amino acids total) ITsuda, M., Sei, Y., Matsurnoto, M., Kamiguchi, H., Yamamoto, Y., Shinohara, Y., Igarashi, T, & Yarnamura, M. (1992) ffum. Genet. 91, 467-4681, Thc seruin AC'I' levels in three unrelated hctcrozygotes with this mutant ACT gene were 37'%:, 49% and 54% that of thc normal individuals. To examine thc reduced serum levels, the normal ACT and the mutant ACT created by site-directed tnutagenesis were transfected into COS-7 cells for comparison. The value for the retention rate (intracellular A C T h t a l ACT) w pparently highcr in the cells expressing mutant ACT Isehonr-2 than those bearing the normal gene. I n the pulse-chase expcrimcnls, the secretion of the synthesized mutant ACT into the mediurn was riot observed, whereas the normal AC'I' was mostly secreted as a 64-kDa form. The endoglycosidasr H digestion and an electron microscopic analysis indicated that the retained mutant ACT was prcscnt in the cndoplasmic reticulum. Thcsc results provide thc biochemical basis for the decreased serum ACT level of individuals with ACT J . S F~U~L I -2 , and suggest the importance of the carboxyl-terminal region for its sccrction.Keywords: serpin ; (1-1 mtichymotrypsin deficiency ; deletion vui-iant ; endoplasnlic reticulum ; frarneshift mutation.(1-1 -antichymotrypsin (ACT) is an acute-phase reactant glycoprotein with a molecular mass of 64 kDa, which is synthesized primarily in hepatocytes and subsequently secreted into thc blood circulation. In addition to its protease inhibitory activities 11 -41, ACT is reported to be involved in inany biological processes including the modulation of immunological and cellular functions 15-71. Jn fact, we have found that ACT has the ability to bind to DNA, thereby inhibiting DNA polymerase LI and DNA priinasc iri vitro 18-lo]. It was also reported that ACT is present in senile plaque in the brain, particularly in the hippocampus of patients with Alzheimer's disease, whose ACT conccntration in cerebrospinal fluid is increased [ l l -151. A recent paper indicates that ACT promotes the assembly of Alzheirner / I protein into filiinicnts 1161. Thus, ACT might be involved i n the regulation of biological or pathological proccsses; the change in the quality 01-quantity of ACT would result in some diseases. A naturally occurring ACT variant which inodulates the quality of the protein is not been available, allhough the mutation of the reactivc site of ACT produced by site-directed mutagenesis alters its properties [17][18][19]. Recently, two fainilia1 forms of ACT deficiency havc been reported to be associated with chronic obCorrr~~~,onrlEncf to M,

The Delicate Balance Between Secreted Protein Folding and Endoplasmic Reticulum-Associated Degradation in Human Physiology

“…It should be mentioned that in addition to the serpinopathies described here, three other mutant forms of serpins exist that are ER retained (23,503). Defects in these proteins lead to thyroxine binding globulin deficiency, alpha-1-antichimotrypsin deficiency, and type I hereditary angioedema.…”

Traffic Jam: A Compendium of Human Diseases that Affect Intracellular Transport Processes

“…hypofibrinogenemia (55 -58) Some mutants are retained in the ER of the liver, where Lung disease, liver ACT Alpha-1-antichymotrypsin (ACT) the enzyme is produced. This leads to defects of the liver disease deficiency (57,59) and also of the lung where it is required for proper lung function.…”