Expression of human pI(Cln) and ClC-6 in Xenopus oocytes induces an identical endogenous chloride conductance

Abstract

pI(Cln) is a protein that induces an outwardly rectifying, nucleotide- sensitive chloride current (I(Cln)) when expressed in Xenopus oocytes, but its precise function (plasma-membrane anion channel versus cytosolic regulator of a channel) remains controversial. We now report that a chloride current identical to I(Cln) is induced when Xenopus oocytes are injected with human ClC-6 RNA. Indeed, both the pI(Cln) and the ClC-6 induced current are outwardly rectifying, they inactivate slowly at positive potentials and have an anion permeability sequence NO 3/ - > I - > Cl - > gluconate. Cyclamate, NPPB, and extracellular cAMP block the induced currents. The success rate of current expression is significantly increased when the injected Xenopus oocytes are incubated at a higher temperature (24 or 37 °C) prior to the analysis. In addition, the I(Cln) current was detected in 6.2% of noninjected control Xenopus oocytes. We therefore conclude that the I(Cln) current in Xenopus oocytes corresponds to an endogenous conductance that can be activated by expression of structurally unrelated proteins. Furthermore, functional, biochemical, and morphological observations did not support the notion that pI(Cln) resides in the plasma membrane either permanently or transiently after cell swelling. Thus, it is unlikely that pI(Cln) forms the channel that is responsible for the I(Cln) current in Xenopus oocytes.