Fig. 9: Model of PCAT1 conformational cycle.

The coupled cycle is initiated by binding of Sub either to the PCAT1 bound to ATP/Mg²⁺ in the IF conformation (1a) or to ADP-bound PCAT1 in the OC conformation (1b). The coupled cycle proceeds through processing of Sub (2) followed by ATP hydrolysis (3) to populate an OF conformation that releases leaderless Sub to the extracellular side or the outer leaflet of the membrane. Subsequently, Mg²⁺ dissociation (4b) could trap the transporter in an occluded conformation to reduce futile ATP hydrolysis. Alternatively, Mg²⁺-ADP dissociation with Mg²⁺-ATP binding (4a) could facilitate an IF conformation in an uncoupled cycle without Sub binding where ATP hydrolysis (5a) followed by dissociation of Mg²⁺ (5b) results in the OC conformation.