THE CHLORIDE PUMP - A CL--TRANSLOCATING P-TYPE ATPASE

Three widely documented mechanisms of chloride transport across plasma membranes are anion-coupled antiport, sodium-coupled symport, and an electrochemical coupling process. No direct genetic evidence has yet b een provided for primary active chloride transport despite numerous re ports of cellular Cl--stimulated adenosine triphosphate (ATP)ases coex isting in the same tissue with uphill chloride transport that could no t be accounted for by the three common chloride transport processes. C l--stimulated ATPases are a common property of practically all biologi cal cells, with the major location being of mitochondrial origin. It a lso appears that plasma membranes are sites of Cl--stimulated ATPase a ctivity. Recent studies of Cl--stimulated ATPase activity and chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electroche mical gradient across plasma membranes. Further studies, especially fr om a molecular biological perspective, are required to confirm a direc t transport role to plasma membrane-localized Cl--stimulated ATPases.