Novel aggregate formation of a frame‐shift mutant protein of tissue‐nonspecific alkaline phosphatase is ascribed to three cysteine residues in the C‐terminal extension

Abstract

In the majority of hypophosphatasia patients, reductions in the serum levels of alkaline phosphatase activity are caused by various missense mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. A unique frame-shift mutation due to a deletion of T at cDNA number 1559 [TNSALP (1559delT)] has been reported only in Japanese patients with high allele frequency. In this study, we examined the molecular phenotype of TNSALP (1559delT) using in vitro translation/translocation system and COS-1 cells transiently expressing this mutant protein. We showed that the mutant protein not only has a larger molecular size than the wild type enzyme by approximately 12 kDa, reflecting an 80 amino acid-long extension at its C-terminus, but that it also lacks a glycosylphosphatidylinositol anchor. In support of this, alkaline phosphatase activity of the cells expressing TNSALP (1559delT) was localized at the juxtanucleus position, but not on the cell surface. However, only a limited amount of the newly synthesized protein was released into the medium and the rest was polyubiquitinated, followed by degradation in the proteasome. SDS/PAGE and analysis by sucrose-density-gradient analysis indicated that TNSALP (1559delT) forms a disulfide-bonded high-molecular-mass aggregate. Interestingly, the aggregate form of TNSALP (1559delT) exhibited a significant enzyme activity. When all three cysteines at positions of 506, 521 and 577 of TNSALP (1559delT) were replaced with serines, the aggregation disappeared and instead this modified mutant protein formed a noncovalently associated dimer, strongly indicating that these cysteine residues in the C-terminal region are solely responsible for aggregate formation by cross-linking the catalytically active dimers. Thus, complete absence of TNSALP on cell surfaces provides a plausible explanation for a severe lethal phenotype of a homozygote hypophosphatasia patient carrying TNSALP (1559delT).