MEMBRANE PROTEIN FOLDING: THE ROLE OF AMINO ACID SEQUENCE IN SPECIFYING THE STABILITY AND CONFORMATION OF TRANSMEMBRANE OLIGOMERS

Proteins carry out essential processes of the cell, and to function properly they must adopt and maintain their native fold. Both thermodynamic and structural data for soluble protein have provided insight into the forces that drive folding in an aqueous environment. However, structural and energetic characterization for membrane proteins has lagged behind that of soluble proteins. Therefore, membrane protein model systems have been extremely valuable to understand the interactions that stabilize and specify membrane protein conformation. Early thermodynamic studies on membrane proteins demonstrated that interactions between helices promote and specify the native fold; and a large scale mutagenesis of a transmembrane dimer, Glycophorin A (GpA), showed that the amino acid sequence on a single face of the helix conferred stability to the dimer. When the NMR structure was solved, the motif highlighted by mutagenesis was found at the dimer interface. Therefore, preferential interactions at the dimer interface may stabilize and specify the native fold of a membrane protein. When this thesis study was commenced, interactions in a single sequence motif were thought to provide the driving force for association in the GpA dimer. However, in our studies, we have demonstrated that this motif, albeit critical for strong dimerization, is neither necessary nor sufficient to drive association. This work highlights that the entire sequence context provides the framework for the stability and specificity of transmembrane helix-helix interactions. The correlation of structural models to energetic measurements strongly suggests that local iii packing defects may be responsible for the perturbation upon amino acid substitution. However, upon experimental examination of structural changes, packing defects may also initiate global conformation change in the dimer and therefore these mutations would perturb interactions between the helices and between the helices and solvating lipids. The information derived from these studies on a model membrane protein dimer was also applied to transmembrane segments that are involved in vesicle fusion. The association of these proteins, syntaxin and synaptobrevin, may be essential to the process of fusion. Both homo-dimerization and hetero-dimerization of these proteins is weak but is still modulated by the amino acid sequence. In this case, a dynamic equilibrium for protein-protein interactions may be critical for biological function. Therefore, amino acid sequences can encode both stable and transient protein interactions that are biologically relevant. Thus, when we study membrane protein structure and interactions, it is necessary to consider the forces that both drive and repel folding, since it is the equilibrium of these forces that establishes biologically functional proteins