Abstract
Co-expression of the auxiliary β1 subunit with the pore forming α subunit of BK dramatically alters apparent calcium sensitivity. Investigation of the mechanism underlying the increase in calcium sensitivity of BK in smooth muscle has concentrated on the energetic effect of β1′s interaction with α. We take a novel approach, exploring whether β1 modification of calcium sensitivity reflects altered interaction between the channel protein and surrounding lipids. We reconstituted hSlo BK α and BK α+β1 channels into two sets of bilayers. One set contained POPE with POPS, POPG, POPA and POPC, where the length of acyl chains is constant, but surface charge differs. The second set is a series of neutral bilayers formed from DOPE with phosphatidylcholines (PCs) of varying acyl chain lengths: C (14∶1), C (18∶1), C (22∶1) and C (24∶1), and with brain sphingomyelin (SPM), in which surface charge is constant, but bilayer thickness varies. The increase in calcium sensitivity caused by the β1 subunit was preserved in negatively charged lipid bilayers but not in neutral bilayers, indicating that modification of apparent Ca2+ sensitivity by β1 is modulated by membrane lipids, requiring negatively charged lipids in the membrane. Moreover, the presence of β1 reduces BK activity in thin bilayers of PC 14∶1 and thick bilayers containing SPM, but has no significant effect on activity of BK in PC 18∶1, PC 22∶1 and PC 24∶1 bilayers. These data suggest that auxiliary β1 subunits fine-tune channel gating not only through direct subunit-subunit interactions but also by modulating lipid-protein interactions.
Highlights
Large conductance, Ca2+-activated K+ channels (BK) play a crucial role in the regulation of arterial tone, where they facilitate a negative feedback mechanism that opposes vasoconstriction [1,2]
We found that the increase of apparent Ca2+ sensitivity by the b1 subunit is more dramatic in bilayers composed of negatively charged lipids than in bilayers made of neutral lipids, suggesting lipid modulation of the increase of apparent Ca2+ sensitivity in the presence of b1
Smooth muscle BK channels are more sensitive to Ca2+, when compared with BK channels from brain and skeletal muscle
Summary
Ca2+-activated K+ channels (BK) play a crucial role in the regulation of arterial tone, where they facilitate a negative feedback mechanism that opposes vasoconstriction [1,2]. Further investigation demonstrated that b1 has two major effects on channel gating energetics: 1) b1 reduces the intrinsic closedopen equilibrium and 2) shifts the open channel voltage sensor activation to negative membrane potentials [12]. In bilayers made of neutral lipids of increasing acyl chain length (from PC 14:1 to PC 24:1), b1 has little effect on the single channel activities in bilayers of PC 18:1, PC 22:1 and PC 24:1, but reduces channel activity in the thin bilayer of PC 14:1 and the thick bilayer of SPM Once again, this suggests an important role for the lipid environment in the effects of b1 on BK Ca2+ sensitivity
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