Biochemical characterization of osteo-testicular protein tyrosine phosphatase and its functional significance in rat primary osteoblasts

Rat osteo-testicular protein tyrosine phosphatase (OST-PTP), expressed in o steoblasts and testis, is a receptor-like transmembrane protein with two ta ndemly repeated phosphatase domains in the cytoplasmic region. In this repo rt, we show that the first domain (CD1) is enzymatically active and appears to be influenced by the catalytically inactive second domain (CD2). The ac tivity of CD1 is specific to phosphorylated tyrosine, Full-length OST-PTP p rotein expressed in COS cells has a molecular mass of approximately 185 kDa , and immunoprecipitates of this protein using OST-PTP-specific antisera sh ow strong tyrosine phosphatase activity. Expression of OST-PTP mRNA in prim ary rat calvarial osteoblasts is temporally regulated, and peak expression is found at approximately day 15, which correlated well with the appearance of OST-PTP protein and its associated tyrosine phosphatase activity. Treat ment of osteoblasts in culture with antisense oligonucleotides directed aga inst the 5' untranslated region of OST-PTP results in abrogation of differe ntiation, confirming the functional importance of OST-PTP expression in ost eoblast development.