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

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Abstract
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 α‐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.

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