Mechanistic Basis for Enhanced Strigolactone Sensitivity in KAI2 Triple Mutant.

Striga hermonthica is a parasitic weed that destroys billions of dollars' worth of staple crops every year. Its rapid proliferation stems from an enhanced ability to me-tabolize strigolactones (SLs), plant hormones that direct root branching and shoot growth. Striga's SL receptor, Sh HTL7, bears more similarity to the staple crop kar-rikin receptor KAI2 than to SL receptor D14, though KAI2 variants in plants like Arabidopsis thaliana show minimal SL sensitivity. Recently, studies have indicated that a small number of point mutations to HTL7 residues can confer SL sensitivity to At KAI2. Here, we analyze both wild-type At KAI2 and SL-sensitive mutant Var64 through all-atom, long-timescale molecular dynamics simulations to determine the ef-fects of these mutations on receptor function at a molecular level. We demonstrate that the mutations stabilize SL binding by about 2 kcal/mol. They also result in a doubling of the average pocket volume, and eliminate the dependence of binding on certain pocket conformational arrangements. While the probability of certain non-binding SL-receptor interactions increases in the mutant compared with the wild-type, the rate of binding also increases by a factor of ten. All these changes account for the increased SL sensitivity in mutant KAI2, and suggest mechanisms for increasing functionality of host crop SL receptors.