Viral and cellular small integral membrane proteins can modify ion channels endogenous to Xenopus oocytes.

A slowly activated, inward current could be evoked from Xenopus oocytes in response to application of a strong (approximately -190 mV) hyperpolarizing pulse. However, a much lesser hyperpolarization (approximately -130 mV) was able to evoke a similar current from oocytes that expressed the cellular proteins IsK and phospholemman, the synthetic protein SYN-C, and the NB protein of influenza B virus. All of these currents were carried principally by Cl-, and they had similar blocker profiles. The time course (the function of time that described the current increase during a hyperpolarizing voltage-clamp pulse, i.e., activation kinetics) varied from one batch of oocytes to another, but did not vary within each batch with the type of protein expressed. This slowly activated, inward current evoked by hyperpolarization to approximately -130 mV required the expression of a characteristic, minimum level of each of the proteins IsK, SYN-C, and NB. However, not every integral membrane protein expressed in oocytes allowed substantial inward currents to be generated at -130 mV. Oocytes that expressed large amounts of the M2 protein of influenza A virus, which is known to possess an intrinsic cation channel activity, did not display a Cl- current when hyperpolarized to -130 mV. These results suggest that expression of any of the four proteins-IsK, phospholemman, SYN-C, or NB- acts as an activator of an endogenous Cl- conductance.