Model for the function of Arg188 in OsSUT1. (A) Upper: known interaction between Arg144 of LacY and the galactose ring of lactose . Lower: Suggested interaction between Arg188 of OsSUT1 and the glucose ring of sucrose. (B) Kinetic model for a SUT protein. C represents the SUT protein, S represents sucrose, and H is the proton. The dashed arrows between the two [CH] statuses indicate H+ flows across through SUTs without transporting sucrose. In the OsSUT1(R188K) mutant, sucrose binds to the transporter from the apoplastic side, but cannot be released to the cytoplastic side, keeping the mutated protein in the [CHS] apoplastic status (step 3).