Determination of the molecular reach of the protein tyrosine phosphatase SHP-1

Immune receptor signalling proceeds by the binding of enzymes to their cytoplasmic tails before they catalyse reactions on substrates within reach. Studies of binding and catalysis have led to a binding- induced allosteric activation model for enzymes, such as SHP-1, whose catalytic activity increases upon recruitment to inhibitory receptors, such as PD-1. However, the impact of molecular reach is poorly understood. Here, we use surface plasmon resonance to measure binding, catalysis, and molecular reach for tethered SHP-1 reactions. We find a molecular reach for SHP-1 (13.0 nm) that is smaller than a maximum stretch estimate (20.4 nm) but larger than an estimate from crystal structure of the active conformation (5.3 nm), suggesting that SHP-1 explores a spectrum of active conformations. The molecular reach confines SHP-1 to a small membrane-proximal volume near its substrates leading membrane recruitment to increase its activity by a 1000-fold increase in concentration with a smaller 2-fold activity increase by PD-1 binding-induced allostery. Lastly, we use mathematical modelling to quantify the degree of co-clustering required for PD-1-SHP-1 complexes to reach their substrates.