Glycosylation-deficient mutations in tissue-nonspecific alkaline phosphatase impair its structure and function and are linked to infantile hypophosphatasia
Tissue-nonspecific alkaline phosphatase (TNSALP) is a membrane glycoprotein with a proposed role in bone mineralization. Indeed, mutations in TNSALP have been identified in patients with hypophosphatasia (HPP), a genetic disease characterized by hypomineralization of bone and teeth and a deficiency in serum ALP activity. TNSALP has five potential N-glycosylation sites at N140, N230, N271, N303 and N430 by standard nomenclature. A mutation at one of these sites, N430, was recently detected in a patient with infantile HPP. Using site-directed mutagenesis, we demonstrated that TNSALP has five N-glycans in transfected COS-1 cells and that individual single N-glycan deletion mutants of TNSALP retain the dimeric structure required for ALP activity, excluding the possibility that any single N-glycan plays a vital role in the structure and function of TNSALP. However, we found that TNSALP (N430Q) and TNSALP (N430E) mutants, but not a TNSALP (N430D) mutant, failed to form dimers. The TNSALP (N430S) mutant linked to infantile HPP was glycosylation-defective and unable to dimerise, similar to TNSALP (N430Q) and TNSALP (N430E) mutants; therefore, TNSALP (N430S) was established as a severe allele without strong ALP activity. By contrast to individual single N-glycan deletion mutants, TNSALP devoid of all five N-glycans was present to a much lesser extent than wild-type TNSALP in transfected cells, possibly reflecting its instability. A comprehensive analysis of a series of multiple N-glycan depletion mutants in TNSALP revealed that three N-glycans on N230, N271 and N303 were the minimal requirement for the structure and function of TNSALP and a prerequisite for its stable expression in a cell.