Proteins that pump ions across membranes are essential to proper function of cells. Transmembrane calcium transporters, for example, are key to muscle contraction, among other activities. But proteins embedded in phospholipid membranes are difficult to crystallize and study structurally. A contrast X-ray technique has now enabled a new view of a Ca2+-ATPase pump, demonstrating that the pump’s reaction cycle involves a rocking motion facilitated by amino acid-phospholipid interactions (Nature 2017, DOI: 10.1038/nature22357). A team led by the University of Tokyo’s Chikashi Toyoshima studied Ca2+-ATPase, also known as SERCA1, in a phospholipid bilayer by placing crystals in contrast media of different concentrations and comparing the resulting X-ray diffraction intensities. They found that the protein tilts back and forth as it pumps calcium through the membrane, staying anchored in place through interactions between positively charged arginine and lysine residues and negatively charged phospholipid head groups on either side of the membrane. The interactions allow for large protein movements while keeping hydrophobic residues within the bilayer. A belt of hydrophobic tryptophan residues may serve to sense the water-lipid boundary, and a trio of two tryptophans and a lysine-phospholipid interaction may serve as a pivot point for tilting.