Phospholamban, a 52-residue membrane protein, associates to form a pen
tameric complex of five long alpha-helices traversing the sarcoplasmic
reticulum membrane of cardiac muscle cells. The transmembrane domain
of the protein is largely hydrophobic, with only three cysteine residu
es having polar side chains, yet it functions as a Ca2+-selective ion
channel. In this report, infrared spectroscopy is used to probe the co
nformation of the three cysteine side chains and to establish whether
the free S-H groups form intrahelical hydrogen bonds in the pentameric
complex. Vibrational spectra of a transmembrane peptide were obtained
which corresponded to the transmembrane domain of wild-type phosphola
mban and three peptides each containing a cysteine double right arrow
alanine substitution. The observed S-H frequencies argue that each of
the sulfhydryl groups is hydrogen-bonded to an i-4 backbone carbonyl o
xygen. Electrostatic calculations on a model of phospholamban based on
molecular dynamics and mutagenesis studies, show that the sulfhydryl
groups may significantly contribute to the electrostatic potential fie
ld of the protein.