Locating phospholamban in co-crystals with Ca2+-ATPase by cryoelectron microscopy

Phospholamban (PLB) is responsible for regulating Ca2+ transport by Ca2+-ATPase across the sarcoplasmic reticulum of cardiac and smooth muscle. This regulation is coupled to beta -adrenergic stimulation, and dysfunction has been associated with end-stage heart failure. PLB appears to directly bind to Ca2+-ATPase, thus slowing certain steps in the Ca2+ transport cycle. We have determined 3D structures from co-crystals of PLB with Ca2+-ATPase by cryoelectron microscopy of tubular co-crystals at 8-10 Angstrom resolution. Specifically, we have used wild-type PLB, a monomeric PLB mutant (L37A), and a pentameric PLB mutant (N27A) for co-reconstitution and have compared resulting structures with three control structures of Ca2+ -ATPase alone. The overall molecular shape of Ca2+-ATPase was indistinguishable in the various reconstructions, indicating that PLB did not have any global effects on Ca2+-ATPase conformation. Difference maps reveal densities which we attributed to the cytoplasmic domain of PLB, though no difference densities were seen for PLB's transmembrane helix. Based on these difference maps, we propose that a single PLB molecule interacts with two Ca (2+)-ATPase molecules. Our model suggests that PLB may resist the large domain movements associated with the catalytic cycle, thus inhibiting turnover.