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Secondary structure, orientation, and oligomerization of phospholemman, a cardiac transmembrane protein

Department of Biological Sciences, The University of Warwick, Coventry CV4 8HE, United Kingdom

(RECEIVED October 10, 2005; FINAL REVISION January 5, 2006; ACCEPTED February 3, 2006)

Human phospholemman (PLM) is a 72-residue protein, which is expressed at high density in the cardiac plasma membrane and in various other tissues. It forms ion channels selective for K⁺, Cl^–, and taurine in lipid bilayers and colocalizes with the Na⁺/K⁺-ATPase and the Na⁺/Ca²⁺-exchanger, which may suggest a role in the regulation of cell volume. Here we present the first structural data based on synthetic peptides representing the transmembrane domain of PLM. Perfluoro-octaneoate-PAGE of reconstituted proteoliposomes containing PLM reveals a tetrameric homo-oligomerization. Infrared spectroscopy of proteoliposomes shows that the PLM peptide is completely -helical, even beyond the hydrophobic core residues. Hydrogen/deuterium exchange experiments reveal that a core of 20–22 residues is not accessible to water, thus embedded in the lipid membrane. The maximum helix tilt is 17° ± 2° obtained by attenuated total reflection infrared spectroscopy. Thus, our data support the idea of ion channel formation by the PLM transmembrane domain.

Keywords: secondary structure; oligomerization; membrane proteins; infrared spectroscopy; peptide; phospholemman

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