Structural dynamics of sphingosine kinase 1 regulation and inhibition.

Sphingosine kinase 1 (SK1) generates sphingosine-1-phosphate, a bioactive lipid implicated in cancer and other diseases. Despite its clinical importance, the structural and dynamic basis of SK1 regulation and inhibition remains poorly understood. Using integrated spectroscopic and computational approaches, we uncover conformational transitions that govern substrate entry, catalysis, and inhibitor binding. Phosphorylation of Ser225 reconfigures the regulatory loop and reshuffles salt bridges, priming SK1 for membrane engagement and catalytic activity. We identify a previously uncharacterized catalytic intermediate with a distinct conformation and a highly dynamic lipid-binding loop 1 (LBL-1), sensitive to potent inhibitors such as PF-543. These inhibitors not only stabilize non-catalytic states but also induce LBL-mediated dimerization, blocking membrane binding and substrate access. Our findings reveal a multilayered regulatory mechanism driven by structural flexibility and establish a novel inhibitory paradigm. This framework provides critical insight into SK1 regulation and a foundation for developing next-generation SK1-targeted therapeutics.