This protein family includes Immune protein Tsi2 (also known as Anti-toxin protein Tsi2) from Pseudomonas aeruginosa and similar sequences predominantly found in Proteobacteria. Tsi2 is a immunity protein that plays a role in preventing early activation of toxin Tse2, which is transported via the type VI secretion system (T6SS). It binds to a large surface of Tse2 and thereby occludes the active site to specifically inhibit Tse2. Tsi2 adopts a dimeric configuration. Each monomer consists of two large alpha-helices arranged as an anti-parallel coiled-coil connected by a short turn [1-3].

1: Structural basis for type VI secretion effector recognition by a cognate immunity protein. Li M, Le Trong I, Carl MA, Larson ET, Chou S, De Leon JA, Dove SL, Stenkamp RE, Mougous JD; PLoS Pathog. 2012;8:e1002613. PMID:22511866

2: Crystal structure of Pseudomonas aeruginosa Tsi2 reveals a stably folded superhelical antitoxin. Zou T, Yao X, Qin B, Zhang M, Cai L, Shang W, Svergun DI, Wang M, Cui S, Jin Q; J Mol Biol. 2012;417:351-361. PMID:22310046

3: The Structure of the Toxin and Type Six Secretion System Substrate Tse2 in Complex with Its Immunity Protein. Robb CS, Robb M, Nano FE, Boraston AB; Structure. 2016;24:277-284. PMID:26749446