Precise packing for membrane proteins Although nonpolar amino acid side chains pack efficiently in membrane proteins, it has been difficult to determine how much this contributes to membrane protein stability. Designed membrane proteins have largely relied on other stabilizing interactions such as metal-ligand interactions and hydrogen bonds. Mravic et al. uncovered a steric packing code underlying the folding of the natural protein phospholamban, which they used to design stable membrane proteins with nonpolar interfaces. They suggest that packing of nonpolar residues plays a role in the folding and stability of many membrane proteins. Science , this issue p. 1418