Ca2+ activation of hSlo K+ channel is suppressed by N-terminal GFP tag

The human slow poke (hSlo) K+ channel was tagged with GFP (green fluorescen t protein) at the N-terminus of its a-subunit. The fusion protein was expre ssed transiently in HEK293 cells; it formed functional voltage-gated channe ls as shown by whole cell patch-clamp measurements. However, the tag lowere d the voltage dependence of gating and it suppressed the typical left-shift of gating by intracellular binding of Ca2+. The location of the GFP-tagged N-terminus was confirmed to be on the extracellular side by application of a monoclonal antibody to nonpermeabilized cells. Structural interpretation s of the effects are discussed.