RPTP mu is a transmembrane protein tyrosine phosphatase with an adhesion molecule-like ectodomain. It has recently been shown that RPTP mu mediates homophilic interactions when expressed in insect cells. In this study, we have examined how RPTP mu may function as a cell contact receptor in mink lung epithelial cells, which express RPTPmu endogenously, as well as in transfected 3T3 cells. We find that RPTP mu has a relatively short half-life (3-4 hours) and undergoes posttranslational cleavage into two noncovalently associated subunits, with both cleaved and uncleaved molecules being present on the cell surface (roughly at a 1:1 ratio); shedding of the ectodomain subunit is observed in exponentially growing cells. Immunofluorescence analysis reveals that surface expression of RPTPmu is restricted to regions of tight cell-cell contact. RPTPmu surface expression increases significantly with increasing cell density. This density-induced upregulation of RPTP mu is independent of its catalytic activity and is also observed when transcription is driven by a constitutive promoter, indicating that modulation of RPTPmu surface expression occurs posttranscriptionally. Based on our results, we propose the following model of RPTP mu function: In the absence of cell-cell contact, newly synthesized RPTP mu molecules are rapidly cleared from the cell surface. Cell-cell contact causes RPTPmu to be trapped at the surface through homophilic binding, resulting in accumulation of RPTP mu at intercellular contact regions. This contact-induced clustering of RPTPmu may then lead to tyrosine dephosphorylation of intracellular substrates at cell-cell contacts.

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