Molecular biology and electrophysiology of calcium-activated potassium channels from lens epithelium

PURPOSE: We structurally and functionally characterized the alpha and beta subunits of the human lens epithelium Ca(++)-activated potassium channel (BK). METHODS: The two subunits were sequenced following RT-PCR with multiple primer pairs. The subunits were cloned using a PCR approach and were expressed in tsA-201 cells for patch clamp recording. Green fluorescence protein-channel subunit fusion proteins were characterized by patch clamping and were imaged by fluorescence microscopy. RESULTS: Alpha subunits alone make a large single-channel conductance, potassium-selective channel with modest Ca++ sensitivity. Beta subunits alone make no channel but, when coexpressed with the alpha subunit, make a channel with increased Ca++ sensitivity, although still less than for natural channels of this type. GFP-BK subunit fusion proteins continue to function and result in a fluorescing channel, which can be localized by fluorescence microscopy. The alpha subunit codes for a "minimal" BK channel in that none of its potential alternative splicing sites contains an "extra" exon. CONCLUSIONS: The Ca(++)-activated potassium channel known as BK has the nucleotide sequences of its alpha and beta subunits represented in messenger RNA of cultured human lens epithelium. It is the first identified channel, to date, which imparts internal Ca++ dependence to lens epithelial potassium conductance.