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

Protein-tyrosine phosphatases (PTPs) are vital for regulating tryosine phos phorylation in many processes, including growth and differentiation(1,2). T he regulation of receptor-like PTP (RPTP) activity remains poorly understoo d, but based on the crystal structure of RPTP alpha domain 1 we have propos ed that dimerization can negatively regulate activity, through the interact ion of an inhibitory 'wedge' on one monomer with the catalytic cleft of dom ain 1 in the other monomer(3,4). Here we show that dimerization inhibits th e activity of a full-length RPTP in vivo. We generated stable disulphide-bo nded full-length RPTP alpha homodimers by expressing mutants with single cy steines at different positions in the ectodomain juxtamembrane region. Expr ession of wild-type RPTP alpha and Phe135Cys and Thr141Cys mutants in RPTP alpha-null mouse embryo cells increased dephosphorylation and activity of T yr529 in the protein tyrosine kinase c-Src; in contrast, expression of a Pr o137Cys mutant did not. Mutation of Pro 210/211 to leucine in the inhibitor y wedge of the Pro137Cys mutant restored its ability to activate c-Src, ind icating that dimerization may inhibit full-length RPTP alpha activity in a manner stereochemically consistent with RPTP alpha crystal structures(3). O ur results suggest that RPTP alpha activity can in principle be negatively regulated by dimerization in vivo.