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1-Subunits Increase Surface Expression of a Large-Conductance Ca²⁺-Activated K⁺ Channel Isoform

Department of Biology and Biochemistry, University of Houston, Houston, Texas

Submitted 4 January 2007; accepted in final form 18 February 2007

Auxiliary (beta) subunits of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels regulate the gating properties of the functional channel complex. Here we show that an avian 1-subunit also stimulates the trafficking of BK_Ca channels to the plasma membrane in HEK293T cells and in a native population of developing vertebrate neurons. One C-terminal variant of BK_Ca -subunits, called the VEDEC isoform after its five last residues, is largely retained in intracellular compartments when it is heterologously expressed in HEK293T cells. A closely related splice variant, called QEERL, shows high levels of constitutive trafficking to the plasma membrane. Co-expression of 1-subunits with the VEDEC isoform resulted in a large increase in surface BK_Ca channels as assessed by cell-surface biotinylation assays, whole cell recordings of membrane current, and confocal microscopy in HEK293T cells. Co-expression of 1-subunits slowed the gating kinetics of BK_Ca channels, as reported previously. Consistent with this, overexpression of 1-subunits in a native cell type that expresses intracellular VEDEC channels, embryonic day 9 chick ciliary ganglion neurons, resulted in a significant increase in macroscopic Ca²⁺-activated K⁺ current. Both the cytoplasmic N- and C-terminal domains of avian 1 are able to bind directly to VEDEC and QEERL channels. However, overexpression of the N-terminal domain by itself is sufficient to stimulate trafficking of VEDEC channels to the plasma membrane, whereas overexpression of either the cytoplasmic C-terminal domain or the extracellular loop domain did not affect surface expression of VEDEC.

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