His-ADPR: Revealing The Chemical Milestones of Immune Signal Evolution

The evolutionary arms race between life and pathogens drives diversification in immune system signaling mechanisms. Recent research has found that the TIR protein of the bacterial type II Thoeris defense system can produce a novel “hybrid” immune signaling molecule—histidine-ADP-ribose (His-ADPR). This molecule, formed by the direct linkage of an amino acid and a nucleotide, challenges the traditional view that TIR enzymes generate only pure nucleotide derivatives. This signal is specifically recognized by the Macro domain of an effector protein, triggering the transmembrane domain to disrupt the membrane for defense. The study further reveals that phages can evade immunity by expressing “signal sponge” proteins that bind and sequester His-ADPR. This offensive-defensive pressure drives TIR enzymes to continuously expand their “chemical arsenal” of signaling molecules. The discovery not only confirms the shared biochemical core of bacterial TIR signaling molecules (based on NAD⁺ modification), but also highlights their remarkable chemical plasticity and evolutionary innovative capacity. It provides a new perspective for understanding the origin and diversity of immune signaling.