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Cell surface expression of receptor protein tyrosine phosphatase RPTP mu is regulated by cell-cell contact
M F Gebbink1, G C Zondag, G M Koningstein
1Division of Cellular Biochemistry, The Netherlands Cancer Institute, Amsterdam.
Abstract:
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.
Insights
Protein tyrosine phosphatase mu (RPTP mu) acts as a cell contact receptor, accumulating at cell junctions. This accumulation, driven by homophilic binding and cell density, is regulated post-transcriptionally and may facilitate intracellular signaling.
Area of Science:
- Cell biology
- Molecular biology
- Biochemistry
Background:
- RPTP mu is a transmembrane protein tyrosine phosphatase.
- It possesses an adhesion molecule-like ectodomain.
- RPTP mu mediates homophilic interactions in insect cells.
Purpose of the Study:
- Investigate RPTP mu's function as a cell contact receptor in epithelial and transfected cells.
- Determine the regulation of RPTP mu surface expression.
- Propose a model for RPTP mu's role in cell-cell interactions.
Main Methods:
- Immunofluorescence analysis
- Cell culture (mink lung epithelial cells, transfected 3T3 cells)
- Analysis of protein half-life and posttranslational modifications
Main Results:
- RPTP mu has a short half-life (3-4 hours) and is cleaved into subunits.
- Surface expression is restricted to cell-cell contact regions and increases with cell density.
- Density-induced upregulation is post-transcriptional and independent of catalytic activity.
Conclusions:
- RPTP mu functions as a cell contact receptor, accumulating at intercellular junctions via homophilic binding.
- Cell-cell contact traps RPTP mu at the surface, leading to clustering.
- This clustering may initiate intracellular signaling through tyrosine dephosphorylation at cell contacts.