The Role of Trp in Arg-Rich Paddle Domain-Lipid Interaction

Abstract

Paddle domains in voltage-gated K+ channels exert their function to mediate voltage gating by altering their positions depending on membrane potential. Two Arg-rich paddle domains, KvAPp from the Kv channel from Aeropyrum pernix and HsapBKp from the BK channel from Homo sapiens, have been shown to be able to induce a thinning of a bilayer which may be the key of gating mechanism. However, differences between the two paddle domains were also observed, e.g., calcein leakage in large unilamellar vesicles induced by HsapBKp is more pronounced and HsapBKp can probably undergo structural transitions indicated by other spectroscopic results. The presence of a Trp in HsapBKp but not in KvAPp may be the reason for the different direct membrane effects. Therefore, in this study we have investigated the role of the Trp in the human BK channel. We have used a range of spectroscopic techniques to elucidate the role of the Trp residue by studying HsapBKp and a variant where Trp was replaced by Ala, and KvAPp and the variant where Ala was replaced by Trp. As a comparison, we also investigated a model transmembrane peptide, KALP21, and a variant of this peptide where an Ala residue in the middle of the hydrophobic region was substituted by Trp. The results show that a Trp in the middle of the sequence alters the structure, and changes the way that the motifs interact with lipids. A Trp promotes disruption of fast-tumbling small bicelles as well as magnetically aligned bicelles. The presence of a Trp residue explains the differences by which the Arg-rich motifs from the Kv and BK interact with the membrane.