2.A.22.6.1
Na⁺/Amino acid transporter 1, SIT1/IMINO (SLC6A20). Transports imino acids such as proline (Km=0.2 mM), pipecolate, and N-methylated amino acids such as MeAIB and sarcosine (Na⁺-dependent, Cl^--stimulated, pH-independent, voltage-dependent) (Li⁺, but not H⁺ can substitute for Na⁺) (Takanaga et al., 2005). It is a 2Na⁺/1Cl^--proline cotransporter (Bröer et al., 2009). To identify new inhibitors of the proline transporter SIT1, its expression in Xenopus laevis oocytes was optimized. Trafficking of SIT1 was augmented by co-expression of angiotensin-converting enzyme 2 (ACE2) in oocytes, but there was no strict requirement for co-expression of ACE2. A pharmacophore-guided screen identified tiagabine as a potent non-competitive inhibitor of SIT1 (Bröer et al. 2024).  The cryo-EM structure of ACE2-SIT1 bound with tiagabine was determined. The inhibitor binds close to the orthosteric proline binding site with its size extends into the cytosolic vestibule. This causes the transporter to adopt an inward-open conformation, in which the intracellular gate is blocked. This study provides the first structural insight into inhibition of SIT1 and generates tools for a better understanding of the ACE2-SIT1 complex (Bröer et al. 2024).