Vesicle packing requires a large energy source to store large numbers of neurotransmitters into a small vesicular space at high concentrations. VMAT transport relies upon the pH and electrochemical gradient generated by a vesicular H+-ATPase for this energy source. The current model of VMAT function proposes that efflux of two protons against the H+ gradient is coupled with influx of one monoamine. The first H+ efflux generates a transporter conformation associated with a high-affinity amine-binding site in the cytosolic phase; the second H+ efflux is coupled with a second large conformational change that leads to amine transport from the cytosolic side into the vesicle, reducing amine-binding affinity.