Characterization of phosphate transport, and isolation of cDNAs encoding the mitochondrial Pi transporter and pyrophosphate transporter from chicken growth plate chondrocytes

In vertebrates, about 80% of the total inorganic phosphate (Pi) in the body is stored in bone. Pi is utilized by osteoblasts to form bone, and by growth plate chondrocytes to mineralize the matrix during endochondral ossification. Two types of Pi transporters have been found to be expressed in chicken growth plate chondrocytes. One is strictly sodium-dependent with its pH preference at 6.8, and the other is not strictly sodium-dependent and exhibits alkaline pH preference. We named this transporter growth plate cartilage specific phosphate transporter (GPT). Sodium-dependent Pi transporter is expressed in the early stage of the cultured cells, while GPT is especially expressed during chondrocyte calcification and is stimulated by the presence of high phosphate concentrations in the medium.; A cDNA segment similar to the human Glvr1 cDNA has been cloned. This cDNA is expressed in the in the resting and proliferative zones in the growth plate tissue. The related gene is very possibly the sodium-dependent Pi transporter gene. A full-length cDNA encoding mitochondrial Pi transporter was also cloned. The entire clone is 1312 base pairs in length with 5′- and 3′-untranslated regions of 64 and 174 base pairs, respectively. It encodes a mature protein consisting of 313 amino acids (aa), preceded by a 44 aa presequence enriched in basic residues. There is 87.3% identity between the chicken phosphate transporter and the bovine transporter in their amino acid residues. Hydropathy analysis suggested that they consist of 6 putative transmembrane domains. In situ hybridization studies demonstrate this transporter is expressed in proliferative and hypertrophic zones in the chicken growth plate.; A cDNA encoding pyrophosphate transporter was cloned from chicken growth plate. This cDNA has 82% identical to human progressive ankylosis ( ank) cDNA. It encodes a 493 aa protein, which is one more amino acid than its human counterpart. The expected molecular mass and isoelectric point of the PPi transporter are 54.6 kDa and 8.45, respectively. The predicted protein contains six potential glycosylation sites and multiple putative phosphorylation sites. Hydropathy analysis revealed 8 hydrophobic stretches; most are 23 residues long, as would be expected for membrane-spinning regions in an integral multi-pass transmembrane protein.