Cell Surface Expression of Receptor Protein Tyrosine Phosphatase RPTPIx Is Regulated by Cell--Cell Contact

RPTPIx is a t ransmembrane protein tyrosine phosphatase with an adhesion molecule-like ectodomain. It has recently been shown that RPTPIx mediates homophilic interactions when expressed in insect cells. In this study, we have examined how RPTPIx may function as a cell contact receptor in mink lung epithelial cells, which express RPTPIx endogenously, as well as in transfected 3T3 cells. We find that RPTPI x 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 RPTPIx is restricted to regions of tight cell-cell contact. RPTPIx surface expression increases significantly with increasing cell density. This density-induced upregulation of RPTPIx is independent of its catalytic activity and is also observed when transcription is driven by a constitutive promoter , indicating that modulation of RPTPIx surface expression occurs posttranscriptionally. Based on our results, we propose the following model of RPTPIx function: In the absence of cell--cell contact, newly synthesized RPTPIx molecules are rapidly cleared from the cell surface. Cell-cell contact causes RPTPIx to be trapped at the surface through homophilic binding, resulting in accumulation of RPTPIx at intercellular contact regions. This contact-induced clustering of RPTPIx may then lead to tyrosine dephosphorylation of intracellular substrates at cell-cell con-