Cell surface expression of receptor protein tyrosine phosphatase RPTP mu is regulated by cell-cell contact.
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.
Gebbink
M F
MF
Division of Cellular Biochemistry, The Netherlands Cancer Institute, Amsterdam.
Zondag
G C
GC
Koningstein
G M
GM
Feiken
E
E
Wubbolts
R W
RW
Moolenaar
W H
WH
D016475:3T3 Cells / Q000166:cytology / Q000502:physiology; D000818:Animals; D001483:Base Sequence; D002450:Cell Communication / Q000502:physiology*; D002452:Cell Count; D018076:DNA, Complementary; D015870:Gene Expression / Q000502:physiology; D000882:Haplorhini; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008907:Mink; D008969:Molecular Sequence Data; D009154:Mutation / Q000502:physiology; D011485:Protein Binding / Q000502:physiology; D017027:Protein Tyrosine Phosphatases / Q000096:biosynthesis / Q000502:physiology* / Q000648:ultrastructure; D051381:Rats; D015398:Signal Transduction / Q000502:physiology; D014162:Transfection; D015854:Up-Regulation / Q000502:physiology
D018076:DNA, Complementary; D008565:Membrane Proteins; D017027:Protein Tyrosine Phosphatases