The Extracellular Loop Of The Auxiliary Beta(1)-Subunit Is Involved In The Regulation Of Bkca Channel Mechanosensitivity

The large conductance Ca2+-activated potassium (BKCa) channel is activated by stretch. The stress-regulated exon (STREX) in alpha-subunits is known to affect the mechanosensitivity of BKCa channels. However, in human colonic smooth muscle cells (HCoSMCs), we found that the alpha-subunits without STREX (ZERO-BK) and beta(1)-subunits could be detected yet the cells were mechanosensitive. Whether the beta(1)-subunit is involved in the regulation of BKCa mechanosensitivity is unclear. In the present study, ZERO-BK and beta(1)-subunits were individually expressed or coexpressed in HEK293 cells and cell-attached patch-clamp techniques were used to measure BKCa currents defining mechanosensitivity. Single-channel patch-clamp recordings from HEK293 cells cotransfected with ZERO-BK and beta(1)-subunits showed stretch sensitivity, but there was no mechanosensitivity in HEK293 cells transfected only with ZERO-BK. We also showed that expression of the beta(1)-subunit could increase mechanosensitivity of the STREX-type alpha-subunits (STREX-BK). To identify the domain in beta(1)-subunits responsible for affecting stretch sensitivity, we expressed beta(1)-Loop(Del) (chimeric beta(1)-subunits without the extracellular loop) or beta(1)-C Term(Del) (chimeric beta(1)-subunits without COOH terminus) with ZERO-BK in HEK293 cells. The data showed that deletion of the extracellular loop but not the COOH terminus of beta(1)-subunits virtually abolished the mechanosensitivity. These results suggest that the extracellular loop of the beta(1)-subunit is involved in the regulation of BKCa channel mechanosensitivity and that role is independent of STREX.