Structural insights into bacterial dimethylsulfoniopropionate import by BCCT-family transporters

Dimethylsulfoniopropionate (DMSP) is a ubiquitous marine organosulfur compound central to microbial stress responses, chemotaxis, and nutrient cycling. Its catabolism produces dimethylsulfide (DMS), a climate-active gas, and plays a key role in the global sulfur cycle. However, the molecular basis of DMSP import, underpinning its microbial metabolism, remains poorly understood. Here, we identify and characterize the BCCT-family transporter DddT from Psychrobacter sp. D2, a marine gamma-proteobacterium that utilizes DMSP as a carbon source. DddT is essential for DMSP uptake and functions as a Na⁺-coupled symporter driven by the transmembrane sodium gradient. Using cryo-electron microscopy, we determined DddT structures in multiple conformational states, revealing its Na⁺-dependent transport mechanism involving two sodium ions, one coordinated by a previously uncharacterized binding site. Sequence analysis shows that DddT-like proteins with conserved sodium-binding features are widespread in marine bacteria, suggesting this Na⁺-coupled transport mechanism represents a broadly conserved feature of the BCCT family. Our findings provide mechanistic insights into sodium-driven substrate uptake and marine sulfur cycling.