The Second Catalytic Domain of Protein Tyrosine Phosphatase δ (PTPδ) Binds to and Inhibits the First Catalytic Domain of PTPς

The LAR family protein tyrosine phosphatases (PTPs), including LAR, PTPδ, and PTPς, are transmembrane proteins composed of a cell adhesion molecule-like ectodomain and two cytoplasmic catalytic domains: active D1 and inactive D2. We performed a yeast two-hybrid screen with the first catalytic domain of PTPς (PTPς-D1) as bait to identify interacting regulatory proteins. Using this screen, we identified the second catalytic domain of PTPδ (PTPδ-D2) as an interactor of PTPς-D1. Both yeast two-hybrid binding assays and coprecipitation from mammalian cells revealed strong binding between PTPς-D1 and PTPδ-D2, an association which required the presence of the wedge sequence in PTPς-D1, a sequence recently shown to mediate D1-D1 homodimerization in the phosphatase RPTPα. This interaction was not reciprocal, as PTPδ-D1 did not bind PTPς-D2. Addition of a glutathione S-transferase (GST)–PTPδ-D2 fusion protein (but not GST alone) to GST–PTPς-D1 led to ~50% inhibition of the catalytic activity of PTPς-D1, as determined by an in vitro phosphatase assay against p-nitrophenylphosphate. A similar inhibition of PTPς-D1 activity was obtained with coimmunoprecipitated PTPδ-D2. Interestingly, the second catalytic domains of LAR (LAR-D2) and PTPς (PTPς-D2), very similar in sequence to PTPδ-D2, bound poorly to PTPς-D1. PTPδ-D1 and LAR-D1 were also able to bind PTPδ-D2, but more weakly than PTPς-D1, with a binding hierarchy of PTPς-D1>>PTPδ-D1>LAR-D1. These results suggest that association between PTPς-D1 and PTPδ-D2, possibly via receptor heterodimerization, provides a negative regulatory function and that the second catalytic domains of this and likely other receptor PTPs, which are often inactive, may function instead to regulate the activity of the first catalytic domains.