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Functional Diversity of Mammalian BK K+ channels - Alternative Splicing and Modulatory Beta Subunits.

 

Asser Nyander Poulsen

Summary

The Big Conductance Ca2+ activated K+ channel (BK) is not one, but a group of functionallydifferent isoforms due to co-expression of modulatory β-subunits and alternative splicing of thepore-forming α-subunit. While β-subunit expression readily explains the phenotype of BK channelsin e.g. smooth muscle and neurons, the physiological footprint of α-subunit alternative splicing isless clear (perhaps except for the Strex variant).We have looked into the distribution of α splice variants in rat vascular and neuronal tissues andrevealed a distinct distribution of splicing when comparing neuronal to most other tissue types[Poulsen et al. 2009].The majority of BK in neuronal tissue contains a 27 a.a. insert in the C-terminal part and many alsoan 8 a.a. insert in the extracellular loop between trans-membrane segments S1 and S2. Two smallinserts of 4 and 3 a.a. are also found. In other tissues, the insert-less Zero or the STREX variants aredominating.Two BK clones containing the nerve-specific inserts were obtained (Slo27,3 and X1) and uponexpression in Xenopus oocytes the X1 variant showed reduced current and faster activation speed.By using intermediary constructs, we found that the 8 a.a. insert caused the reduction in currentseen with variant X1, while the faster activation could be attributed mostly to the 27 a.a. insert.The β2 subunit is expressed in neuronal tissues and is known to induce inactivating BK currents.Surprisingly, when β2 is co-expressed with X1 no inactivation is observed as seen with Zero orSlo27,3. The lack of inactivation was caused neither by the 8 or 27 a.a. inserts, but rather by thesmall 4 a.a. insert present in the C-terminal of the X1 variant.Two rare transcript variants were also cloned from meningeal tissue, containing 92 or 188 bp insertsdownstream of the channel pore region. The insert sequences contain stop-codons and leads totruncation of the channel protein. These truncated channels failed to generate current whenexpressed in oocytes despite positive protein expression. One variant X2+92 was co-expressed withZero, and appeared to induce a slowing of activation of Zero but no dominant negative effect. Thissuggests a role of the “silent” variants as modulators of other variants.All together the results illustrate the extent and complexity of α-subunit splice variants and suggesta role in determining BK channel function in interplay with β-subunits.