KIR signaling is regulated by electrostatic interaction of its cytosolic tail with the plasma membrane despite being neutral polyampholyte.

Abstract

Many receptors signal upon phosphorylation of tyrosine-based motifs present in their cytosolic tail. Intrinsic disorder appears common among such cytosolic tails. Studies on CD3ζ and CD3ε tails, which are disordered and polybasic, suggested regulation of phosphorylation through accessibility of tyrosines, governed by electrostatic interactions with membrane anionic lipids. We noticed characteristics of intrinsic disorder and previously unappreciated features in tyrosine-based motif-bearing cytosolic tails of, especially, inhibitory receptors. These tails are neutral or acidic polyampholytes, with acidic and basic residues linearly segregated. To explore roles of these electrostatic features, we studied inhibitory killer-cell immunoglobulin-like receptor (KIR). KIR cytosolic tail is disordered neutrally-charged polyampholyte, wherein juxtamembrane and membrane-distal stretches are basic, and the intervening stretch is acidic. Despite lacking net charge, it interacted electrostatically with the plasma membrane. Juxtamembrane stretch was crucial for overall binding, which sequestered tyrosines in membrane, and restrained their constitutive phosphorylation. Ligand binding to KIR was sufficient to release its tail from the plasma membrane, initiating signaling. This tail release occurred independently of KIR polymerization, clustering, and tyrosine phosphorylation, but required the acidic stretch. Signaling strength of KIR was dictated by plasma membrane interaction of its tail. These results revealed an electrostatic protein-lipid interaction that is unusual in being governed by segregated clusters of acidic and basic residues in disordered region of protein. Allowing electrostatic interaction irrespective of net charge, segregated clusters of oppositely charged residues made both binding and unbinding modules inherent to receptor tail, which could enable the interaction to act as an independent signaling switch.