Four-turn α-helical segment prevents surface expression of the auxiliary hβ2 subunit of BK-type channel

Abstract

Large conductance, voltage- and Ca2+-activated K+ (BK) channels encoded by the mslo α and β2 subunits exist abundantly in rat chromaffin cells, pancreatic β cells, and DRG neurons. The extracellular loop of hβ2 acting as the channel regulator influences the rectification and toxin sensitivity of BK channels, and the inactivation domain at its N terminus induces rapid inactivation. However, the regulatory mechanism, especially the trafficking mechanism of hβ2, is still unknown. With the help of immunofluorescence and patch clamp techniques, we determine that the hβ2 subunit alone resides in the endoplasmic reticulum, suggesting that trafficking mechanism of hβ2 differs from that of hβ1 opposite to what we predicted previously. We further demonstrate that a four-turn α helical segment at the N terminus of hβ2 prevents the surface expression of hβ2, that is, the helical segment itself is a retention signal. Using the c-Myc epitope-tagged extracellular loop of hβ2, we reveal that the most accessible site by antibody is located at the middle of the extracellular loop, which might provide clues to understand how the auxiliary β subunits regulates the toxin sensitivity and the rectification of BK-type channels. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.