Title: “Proton-driven alternating access and broad-specificity in Spns lipid transporters.”
Spns lipid transporters are critical for transporting sphingosine-1-phosphate (S1P) and lysolipids across cellular membranes. S1P regulates the growth, survival, and migration of cells with profound proangiogenic effects. In humans, Spns2 acts as the main S1P transporter in endothelial cells, making it a potential drug target for modulating S1P export and signaling. Using an integrated approach in lipid membranes, Dr. Dastvan’s team combined double electron-electron resonance (DEER) spectroscopy with molecular dynamics simulations to identify unknown conformational states of two close bacterial homologs of the human Spns proteins from Hyphomonas neptunium (HnSpns) and Mycobacterium smegmatis (MsSpns). By defining their proton- and substrate-coupled conformational dynamics, his team’s systematic study reveals conserved residues critical for protonation steps and their regulation, as well as how sequential protonation of these proton switches coordinates the conformational transitions. The study unveiled two distinct proton-coupled alternating access mechanisms for the two bacterial homologs, enabled by identical conserved residues. For HnSpns, a noncanonical ligand-dependent alternating access was revealed, in the absence of an obvious outward-facing conformational state. A conserved periplasmic salt bridge keeps the transporter in a closed conformation, while proton-dependent conformational dynamics are significantly enhanced on the periplasmic side, providing a pathway for ligand exchange. Furthermore, his team’s study suggests a previously underappreciated role for the Spns lipid transporters as broad-specificity efflux pumps that argue for their involvement in multidrug resistance (MDR), beyond their activity in lysolipid transport and signaling. Using a similar integrated approach to define the transport mechanism of other Spns family members, including human Spns2 and their prokaryotic homologs, Dr. Dastvan will identify the key commonalities and differences in their mechanisms, highlighting the mechanistic flexibility that enables their diverse function and transformative therapeutic potential.
