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 in their cytosolic tail, with intrinsic disorder as a common feature. 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 many, especially, inhibitory receptors. They 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). Its cytosolic tail is a 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. The juxtamembrane stretch was crucial for overall binding, which sequestered tyrosines in the lipid bilayer and restrained their constitutive phosphorylation. Human leukocyte antigen-C ligand binding to KIR released its tail from the plasma membrane to initiate signaling. Tail release occurred independently of KIR polymerization, clustering, or tyrosine phosphorylation, but required acidic residues of the acidic stretch. Tail interaction with the plasma membrane dictated signaling strength of KIR. These results revealed an electrostatic protein–lipid interaction that is unusual in being governed by segregated clusters of acidic and basic residues in polyampholytic disordered region of protein. In contrast to previously known, segregated distribution of oppositely charged residues made both binding and unbinding modules inherent to receptor tail, which could make the interaction an independent signaling switch.