Homophilic binding of PTPμ, a receptor-type protein tyrosine phosphatase, can mediate cell-cell aggregation

Abstract

The receptor-like protein tyrosine phosphatase, PTPμ, displays structural similarity to cell-cell adhesion molecules of the immunoglobulin superfamily. We have investigated the ability of human PTPμ to function in such a capacity. Expression of PTPμ, with or without the PTPase domains, by recombinant baculovirus infection of Sf9 cells induced their aggregation. However, neither a chimeric form of PTPμ, containing the extracellular and transmembrane segments of the EGF receptor and the intracellular segment of PTPμ, nor the intracellular segment of PTPμ expressed as a soluble protein induced aggregation. PTPμ mediates aggregation via a homophilic mechanism, as judged by lack of incorporation of uninfected Sf9 cells into aggregates of PTPμ-expressing cells. Homophilic binding has been demonstrated between PTPμ-coated fluorescent beads (Covaspheres) and endogenously expressed PTPμ on MvLu cells. Additionally the PTPμ-coated beads specifically bound to a bacterially expressed glutathione-S-transferase fusion protein containing the extracellular segment of PTPμ (GST/PTPμ) adsorbed to petri dishes. Covaspheres coated with the GST/PTPμ fusion protein aggregated in vitro and also bound to PTPμ expressed endogenously on MvLu cells. These results suggest that the ligand for this transmembrane PTPase is another PTPμ molecule on an adjacent cell. Thus homophilic binding interactions may be an important component of the function of PTPμ in vivo.