CLONING OF HUMAN PEX CDNA - EXPRESSION, SUBCELLULAR-LOCALIZATION, ANDENDOPEPTIDASE ACTIVITY

Mutations in the PEX gene are responsible for X-linked hypophosphatemi c rickets, To gain insight into the role of PEX in normal physiology w e have cloned the human full-length cDNA and studied its tissue expres sion, subcellular localization, and peptidase activity. We show that t he cDNA encodes a 749-amino acid protein structurally related to a fam ily of neutral endopeptidases that include neprilysin as prototype. By Northern blot analysis, the size of the full-length PEX transcript is 6.5 kilobases, PEX expression, as determined by semiquantitative poly merase chain reaction, is high in bone and in tumor tissue associated with the paraneoplastic syndrome of renal phosphate wasting. PEX is gl ycosylated in the presence of canine microsomal. membranes and partiti ons exclusively in the detergent phase from Triton X-114 extractions o f transiently transfected COS cells. Immunofluorescence studies in A29 3 cells expressing PEX tagged with a c-myc epitope show a predominant cell-surface location for the protein with its COOH-terminal domain in the extracellular compartment, substantiating the assumption that PEX , like other members of the neutral endopeptidase family, is a type II integral membrane glycoprotein, Cell membranes from cultured COS cell s transiently expressing PEX efficiently degrade exogenously added par athyroid hormone-derived peptides, demonstrating for the first time th at recombinant PEX can function as an endopeptidase, PEX peptidase act ivity may provide a convenient target for pharmacological intervention in states of altered phosphate homeostasis and in metabolic bone dise ases.