Phospholamban is a phosphoprotein regulator of cardiac sarcoplasmic reticulum which is phosphorylated in response to β-adrenergic stimulation. Previous results have shown that phospholamban forms Ca²⁺-selective channels in lipid bilayers. The channel-forming domain has been localized to amino acid residues 26-52, which form a stable pentameric, helical structure. The specific residues responsible for stabilizing the pentameric membrane domain of phospholamban have been identified by mutational analysis. Residues 26-52 were individually mutated to Ala or Phe, and the ability of the resulting mutant to form a pentamer or other oligomer was assessed by SDS-polyacrylamide gel electrophoresis analysis. Replacement of Leu³⁷, Ile⁴⁰, Leu⁴⁴, Ile⁴⁷, or Leu⁵¹ by Ala prevented pentamer formation, indicating their essential involvement in the oligomeric assembly. The heptad repeats, and 3-4-residue spacing of the essential amino acids suggest that residues 37-52 adopt a pentameric coiled-coil structure stabilized by a leucine zipper motif formed by the close packing of Leu³⁷, Ile⁴⁰, Leu⁴⁴, Ile⁴⁷, and Leu⁵¹. The resulting symmetric structure contains a central pore defined by the hydrophobic surface of the five stabilizing leucine zippers, which are oriented to the interior and form the backbone of the pentamer.