A new method to model membrane protein structure based on silent amino acid substitutions

The importance of accurately modeling membrane proteins cannot be overstated, in lieu of the difficulties in solving their structures experimentally. Often, however, modeling procedures (e.g., global searching molecular dynamics) generate several possible candidates rather then pointing to a single model. Herein we present a new approach to select among candidate models based on the general hypothesis that silent amino acid substitutions, present in variants identified from evolutionary conservation data or mutagenesis analysis, do not affect the stability of a native structure but may destabilize the non-native structures also found. The proof of this hypothesis has been tested on the alpha-helical transmembrane domains of two homodimers, human glycophorin A and human CD3-zeta, a component of the T-cell receptor. For both proteins, only one structure was identified using all the variants. For glycophorin A, this structure is virtually identical to the structure determined experimentally by NMR. We present a model for the transmembrane domain of CD3-zeta that is consistent with predictions based on mutagenesis, homology modeling, and the presence of a disulfide bond. Our experiments suggest that this method allows the prediction of transmembrane domain structure based only on widely available evolutionary conservation data.