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-DZ. 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.