Dimerization inhibits the activity of receptor-like protein-tyrosine phosphatase-α

Protein-tyrosine phosphatases (PTPs) are vital for regulating tryosine phosphorylation in many processes, including growth and differentiation(1,2). The regulation of receptor-like PTP (RPTP) activity remains poorly understood, but based on the crystal structure of RPTP alpha domain 1 we have proposed that dimerization can negatively regulate activity, through the interaction of an inhibitory 'wedge' on one monomer with the catalytic cleft of domain 1 in the other monomer(3,4). Here we show that dimerization inhibits the activity of a full-length RPTP in vivo. We generated stable disulphide-bonded full-length RPTP alpha homodimers by expressing mutants with single cysteines at different positions in the ectodomain juxtamembrane region. Expression of wild-type RPTP alpha and Phe135Cys and Thr141Cys mutants in RPTP alpha-null mouse embryo cells increased dephosphorylation and activity of Tyr529 in the protein tyrosine kinase c-Src; in contrast, expression of a Pro137Cys mutant did not. Mutation of Pro 210/211 to leucine in the inhibitory wedge of the Pro137Cys mutant restored its ability to activate c-Src, indicating that dimerization may inhibit full-length RPTP alpha activity in a manner stereochemically consistent with RPTP alpha crystal structures(3). Our results suggest that RPTP alpha activity can in principle be negatively regulated by dimerization in vivo.