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1-subunits increase surface expression of a large-conductance Ca2+-activated K+ channel isoform
1 Biology and Biochemistry, University of Houston, Houston, Texas, United States
* To whom correspondence should be addressed. E-mail: sdryer{at}uh.edu.
Auxiliary (beta) subunits of large-conductance Ca2+-activated K+ (BKCa) channels regulate the gating properties of the functional channel complex. Here we show that an avian 
1-subunit also stimulates the trafficking of BKCa channels to the plasma membrane in HEK293T cells and in a native population of developing vertebrate neurons. One C-terminal variant of BKCa 
-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 BKCa 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 BKCa channels, as reported previously. Consistent with this, over-expression 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, over-expression of the N-terminal domain by itself is sufficient to stimulate trafficking of VEDEC channels to the plasma membrane, whereas over-expression of either the cytoplasmic C-terminal domain, or the extracellular loop domain did not affect surface expression of VEDEC.
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