Mechanisms of PTPσ-Mediated Presynaptic Differentiation.

Formation of synapses between neurons depends in part on binding between axonal and dendritic cell surface synaptic organizing proteins, which recruit components of the developing presynaptic and postsynaptic specializations. One of these presynaptic organizing molecules is protein tyrosine phosphatase sigma (PTP sigma). Although the protein domains involved in adhesion between PTPs and its postsynaptic binding partners are known, the mechanisms by which it signals into the presynaptic neuron to recruit synaptic vesicles and other necessary components for regulated transmitter release are not well understood. One attractive candidate to mediate this function is liprin-alpha, a scaffolding protein with well-established roles at the synapse. We systematically mutated residues of the PTP sigma intracellular region (ICR) and used the yeast dihydrofolate reductase (DHFR) protein complementation assay to screen for disrupted interactions between these mutant forms of PTPs and its various binding partners. Using a molecular replacement strategy, we show that disrupting the interaction between PTPs and liprin-alpha, but not between PTP sigma and itself or another binding partner, caskin, abolishes presynaptic differentiation. Furthermore, phosphatase activity of PTP sigma and binding to extracellular heparan sulfate (HS) proteoglycans are dispensable for presynaptic induction. Previous reports have suggested that binding between PTP sigma and liprin-alpha is mediated by the PTP sigma membrane-distal phosphatase-like domain. However, we provide evidence here that both of the PTP sigma phosphatase-like domains mediate binding to liprin-alpha and are required for PTP sigma-mediated presynaptic differentiation. These findings further our understanding of the mechanistic basis by which PTP sigma acts as a presynaptic organizer.