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

Abstract

Immune receptor signalling proceeds by the binding (or tethering) of enzymes to their cytoplasmic tails before they catalyse reactions on substrates within reach. This is the case for the enzyme SHP-1 that, upon tethering to the inhibitory receptor PD-1, dephosphorylates membrane substrates to suppress T cell activation. Precisely how tethering regulates SHP-1 activity is incompletely understood. Here, we use surface plasmon resonance to measure binding, catalysis, and molecular reach for PD-1 tethered SHP-1 reactions. We find that the reach of PD-1--SHP-1 complexes is dominated by the 13.0 nm reach of SHP-1 itself. This is longer than an estimate from the structure of the allosterically active conformation (5.3 nm), suggesting that SHP-1 explores multiple active conformations. Using modelling, we show that when uniformly distributed, PD-1--SHP-1 complexes can only reach 15% of substrates but this increases to 90% when they are co-clustered. When within reach, we show that membrane recruitment increases the activity of SHP-1 by a 1000-fold increase in local concentration. The work highlights how molecular reach regulates the activity of membrane-recruited SHP-1 with insights applicable to other membrane-tethered reactions. Significance statementImmune receptors transduce signals by recruiting (or tethering) cytoplasmic enzymes to their tails at the membrane. When tethered, these enzymes catalyse reactions on other substrates to propagate signalling. Precisely how membrane tethering regulates enzyme activity is incompletely understood. Unlike other tethered reactions, where the enzyme tethers to the substrate, the substrate in this case is a different receptor tail. Therefore, the ability of the receptor-tethered enzyme to reach a substrate can be critical in controlling reaction rates. In this work, we determine the molecular reach for the enzyme SHP-1 and the receptor PD-1 to which it can tether, and show how molecular reach controls receptor signalling.