Side-chain Contributions to Membrane Protein Structure and Stability

• Published in: Journal of molecular biology 2004-01, Vol.335 (1), p.297-305
• Main Authors: Faham, Salem, Yang, Duan, Bare, Emiko, Yohannan, Sarah, Whitelegge, Julian P., Bowie, James U.
• Format: Article
• Language: English
• Subjects: Amino Acids - chemistry; bacteriorhodopsin; Bacteriorhodopsins - chemistry; helix kink; hydrogen bond; Membrane Proteins - chemistry; Membrane Proteins - genetics; Models, Molecular; Protein Denaturation - genetics; protein folding; Protein Structure, Secondary; Protein Structure, Tertiary; Thermodynamics; van der Waals
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2003.10.041

The molecular forces that stabilize membrane protein structure are poorly understood. To investigate these forces we introduced alanine substitutions at 24 positions in the B helix of bacteriorhodopsin and examined their effects on structure and stability. Although most of the results can be rationalized in terms of the folded structure, there are a number of surprises. (1) We find a remarkably high frequency of stabilizing mutations (17%), indicating that membrane proteins are not highly optimized for stability. (2) Helix B is kinked, with the kink centered around Pro50. The P50A mutation has no effect on stability, however, and a crystal structure reveals that the helix remains bent, indicating that tertiary contacts dominate in the distortion of this helix. (3) We find that the protein is stabilized by about 1 kcal/mol for every 38 Å 2 of surface area buried, which is quite similar to soluble proteins in spite of their dramatically different environments. (4) We find little energetic difference, on average, in the burial of apolar surface or polar surface area, implying that van der Waals packing is the dominant force that drives membrane protein folding.