Figure 3. Model of PIN-ABCB-TWD1 interaction. In this hypothetical model, TWD1 functions in recruiting PID for ABCB1 linker phosphorylation resulting in inhibition of auxin efflux (-). In the absence of TWD1, PID is thought to phosphorylate ABCB1 at an unknown site resulting in activation (+). The opposite is found for PIN/ABCB-mediated efflux (-) but at the moment it is unclear if this is the consequence of PIN or ABCB phosphorylation. Functional domains of TWD1 are in blue (FKBD), red (TPR), yellow (calmodulin-binding domain) and gray (in-plane membrane anchor). Question marks indicate uncertainties that are currently under investigation.