Structural Features of Membrane Proteins

Cytochrome c oxidase--structure, function, and physiology of a redox-driven molecular machine

Cytochome c oxidase is the terminal member of the electron transport chains of mitochondria and many bacteria. Providing an efficient mechanism for dioxygen reduction on the one hand, it also acts as a redox-linked proton pump, coupling the free energy of water formation to the generation of a transmembrane electrochemical gradient to eventually drive ATP synthesis. The overall complexity of the mitochondrial enzyme is also reflected by its subunit structure and assembly pathway, whereas the diversity of the bacterial enzymes has fostered the notion of a large family of heme-copper terminal oxidases. Moreover, the successful elucidation of 3-D structures for both the mitochondrial and several bacterial oxidases has greatly helped in designing mutagenesis approaches to study functional aspects in these enzymes.

Identification of a polar region in transmembrane domain 6 that regulates the function of the G protein-coupled alpha-factor receptor

The alpha-factor pheromone receptor (Ste2p) of the yeast Saccharomyces cerevisiae belongs to the family of G protein-coupled receptors that contain seven transmembrane domains (TMDs). Because polar residues can influence receptor structure by forming intramolecular contacts between TMDs, we tested the role of the five polar amino acids in TMD6 of the alpha-factor receptor by mutating these residues to nonpolar leucine. Interestingly, a subset of these mutants showed increased affinity for ligand and constitutive receptor activity. The mutation of the most polar residue, Q253L, resulted in 25-fold increased affinity and a 5-fold-higher basal level of signaling that was equal to about 19% of the alpha-factor induced maximum signal. Mutation of the adjacent residue, S254L, caused weaker constitutive activity and a 5-fold increase in affinity. Comparison of nine different mutations affecting Ser254 showed that an S254F mutation caused higher constitutive activity, suggesting that a large hydrophobic amino acid residue at position 254 alters transmembrane helix packing. Thus, these studies indicate that Gln253 and Ser254 are likely to be involved in intramolecular interactions with other TMDs. Furthermore, Gln253 and Ser254 fall on one side of the transmembrane helix that is on the opposite side from residues that do not cause constitutive activity when mutated. These results suggest that Gln253 and Ser254 face inward toward the other TMDs and thus provide the first experimental evidence to suggest the orientation of a TMD in this receptor. Consistent with this, we identified two residues in TMD7 (Ser288 and Ser292) that are potential contact residues for Gln253 because mutations affecting these residues also cause constitutive activity. Altogether, these results identify a new domain of the alpha-factor receptor that regulates its ability to enter the activated conformation.