Molecular cloning and expression of a unique rabbit osteoclastic phosphotyrosyl phosphatase

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Abstract
Tyrosyl phosphorylation plays an important regulatory role in osteoclast formation and activity. Phosphotyrosyl phosphatases (PTPs), in addition to tyrosyl kinases, are key determinants of intracellular tyrosyl phosphorylation levels. To identify the PTP that might play an important regulatory role in osteoclasts, we sought to clone an osteoclast-specific PTP. A putative full-length clone encoding a unique PTP (referred to as PTP-oc) was isolated from a 10-day-old rabbit osteoclastic cDNA library and sequenced. A single open reading frame predicts a protein with 405 amino acid residues containing a putative extracellular domain, a single transmembrane region, and an intracellular portion. PTP-oc is structurally unique in that, unlike most known transmembrane PTPs, it has a short extracellular region (eight residues), lacks a signal peptide proximal to the N-terminus, and contains only a single 'PTP catalytic domain'. The PTP catalytic domain shows 45-50% sequence identity with the catalytic domain of human HPTP beta and with the first catalytic domain of LCA. The PTP-oc gene exists as a single copy in the rabbit genome. The corresponding mRNA (3.8 kb) is expressed in osteoclasts but not in other bone-derived cells (e.g. osteoblasts and stromal cells). The 3.8 kb PTP-oc mRNA transcript was also expressed in the rabbit brain, kidney and spleen. However, the brain and kidney, but not osteoclasts or spleen, also expressed a larger transcript (6.5 kb). The PTP catalytic domain of PTP-oc was expressed as a GST-cPTP-oc fusion protein. In vitro phosphatase assays indicated that the purified fusion protein exhibited phosphatase activities at neutral pH values toward p-nitrophenyl phosphate, phosphotyrosyl Raytide, and phosphotyrosyl histone, whereas it had no appreciable activity toward phosphoseryl casein. In summary, we have: (a) cloned and sequenced the putative full-length cDNA of a unique PTP (PTP-oc) from rabbit osteoclasts; (b) shown that the mature 3.8 kb PTP-oc mRNA was expressed primarily in osteoclasts and the spleen; and (c) shown that the PTP-oc fusion protein exhibited a phosphotyrosine-specific phosphatase activity. In conclusion, PTP-oc represents a structurally unique subfamily of transmembrane PTPs.