EFFECTS OF MG2+ AND ATP ON THE PHOSPHATE TRANSPORTER OF SARCOPLASMIC-RETICULUM

The extra uptake of Ca2+ by vesicles of sarcoplasmic reticulum (SR) observed in the presence of P(i), attributable to transport of P(i) by the P(i)-transporter, has been studied. It has been shown that the P(i) transporter is stimulated by ATP. Single channel conductance measurements have shown that the Cl- channel in the SR membrane is impermeable to P(i). It is suggested that the transporter could be an an ion antiporter system. Studies of uptake as a function of pH and Mg2+ concentration suggest that transport of MgHPO4 and H2PO4- are faster than transport of HPO4(2-). For oxalate and pyrophosphate, Mg2+ binding inhibits transport. It is suggested that protonation of lysine residue(s) at the anion binding site increase the rate of transport.