8.A.91.  The TRAP Transporter TatT Protein Component (TatT) family

The bacterial acquisition of metabolites is largely facilitated by transporters with unique substrate scopes. The tripartite ATP-independent periplasmic (TRAP) transporters (TC# 2.A.56) comprise a large family of bacterial proteins that facilitate the uptake of a variety of small molecules. It has been reported that some TRAP systems encode a fourth protein, the T component, TatT). TatT is predicted to be a periplasmic-facing lipoprotein that enables the uptake of metabolites from the outer membrane. However, no substrates were previously revealed for any TatT, and their functional role(s) remained enigmatic. Zhai et al. 2023 identified a homolog in Methylococcus capsulatus that binds to sterols, and they reported two additional homologs that bind long-chain fatty acids. Their bioinformatics, quantitative analyses of protein-ligand interactions, and high-resolution crystal structures suggested that TatTs might facilitate the trafficking of hydrophobic or lipophilic substrates and thereby represent a new class of bacterial lipid and fatty acid transporters (Zhai et al. 2023). This hypothesis must remain speculative.