2.A.133.  The RDD Na⁺(Li⁺)(K⁺)/H⁺ Antiporter (RDD) Family

The arginine-aspartate-aspartate (RDD) family, representing a category of transmembrane proteins containing one highly conserved arginine and two aspartates (RDD) are wide spread in Nature. Shao et al. 2018 characterized a member of this family from the moderate halophile Halobacillus andaensis NEAU-ST10-40^T and reported that it functions as a novel Na⁺(Li⁺, K⁺)/H⁺ antiporter. The highly conserved arginine and aspartate residues are important for the protein's cation-transporting activity.  In fact, D154, R124, R129, and D158 are indispensable for Na⁺(Li⁺, K⁺)/H⁺ antiport activity whereas neither R35 nor D42 is required for activity (Shao et al. 2018).