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


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Packing of apolar side chains enables accurate design of highly stable membrane proteins



The features that stabilize the structures of membrane proteins remain poorly understood. Polar interactions contribute modestly, and the hydrophobic effect contributes little to the energetics of apolar side-chain packing in membranes. Disruption of steric packing can destabilize the native folds of membrane proteins, but is packing alone sufficient to drive folding in lipids? If so, then membrane proteins stabilized by this feature should be readily designed and structurally characterized—yet this has not been achieved. Through simulation of the natural protein phospholamban and redesign of variants, we define a steric packing code underlying its assembly. Synthetic membrane proteins designed using this code and stabilized entirely by apolar side chains conform to the intended fold. Although highly stable, the steric complementarity required for their folding is surprisingly stringent. Structural informatics shows that the designed packing motif recurs across the proteome, emphasizing a prominent role for precise apolar packing in membrane protein folding, stabilization, and evolution.



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