Figure 9. Schematic model for phosphorylation-based regulation of Mep2 ammonium transporters.

(a) In the closed, non-phosphorylated state (i), the CTR (magenta) and ICL3 (green) are far apart with the latter blocking the intracellular channel exit (indicated with a hatched circle). Upon phosphorylation and mimicked by the CaMep2 S453D and DD mutants (ii), the region around the ExxGxD motif undergoes a conformational change that results in the CTR interacting with the inward-moving ICL3, opening the channel (full circle) (iii). The arrows depict the movements of important structural elements. The open-channel Mep2 structure is represented by archaebacterial Amt-1 and shown in lighter colours consistent with Fig. 4. As discussed in the text, similar structural arrangements may occur in plant AMTs. In this case however, the open channel corresponds to the non-phosphorylated state; phosphorylation breaks the CTR–ICL3 interactions leading to channel closure. (b) Model based on AMT transporter analogy²⁹,³³ showing how phosphorylation of a Mep2 monomer might allosterically open channels in the entire trimer via disruption of the interactions between the CTR and ICL3 of a neighbouring monomer (arrow).