Identification of the homophilic binding site of the receptor protein tyrosine phosphatase PTP mu.

The receptor-type protein tyrosine phosphatase PTP mu comprises an extracellular segment containing a MAM domain, an immunoglobulin domain and four fibronectin type III repeats, a transmembrane segment, and two intracellular PTP domains. We have previously shown that PTP mu binds homophilically, i.e. PTP mu on the surface of one cell binds to PTP mu on an apposing cell, and that the extracellular segment alone is sufficient for homophilic binding. In this study we report that in MvLu cells PTP mu is proteolytically processed into two noncovalently associated fragments, one comprising most of the extracellular segment (approximately 100 kDa) and the other containing predominantly the transmembrane and intracellular portions (approximately 100 kDa). We have also identified the homophilic binding site within the extracellular segment. We have generated, expressed, and purified various fragments of the extracellular segment of PTP mu and have used fluorescent beads (Covaspheres) coated with these fragments in three binding assays: (i) measurement of bead aggregation, (ii) binding of beads to surfaces of dishes coated with purified PTP mu, or (iii) binding to MvLu cells. Only beads coated with recombinant fragments that contained the immunoglobulin domain underwent aggregation or bound to surfaces displaying PTP mu, suggesting that neither the MAM domain nor the fibronectin type III repeats bound homophilically in these assays. The fragment containing the Ig domain alone bound as well as any other Ig domain-containing fragment, suggesting that the Ig domain is both necessary and sufficient for homophilic binding under these conditions.