Interaction and conformational dynamics of membrane-spanning protein helices

Abstract

Within 1 or 2 decades, the reputation of membrane-spanning alpha-helices has changed dramatically. Once mostly regarded as dull membrane anchors, transmembrane domains are now recognized as major instigators of protein-protein interaction. These interactions may be of exquisite specificity in mediating assembly of stable membrane protein complexes from cognate subunits. Further, they can be reversible and regulatable by external factors to allow for dynamic changes of protein conformation in biological function. Finally, these helices are increasingly regarded as dynamic domains. These domains can move relative to each other in different functional protein conformations. In addition, small-scale backbone fluctuations may affect their function and their impact on surrounding lipid shells. Elucidating the ways by which these intricate structural features are encoded by the amino acid sequences will be a fascinating subject of research for years to come.

Figure 1

Factors that are known to regulate TMD–TMD interaction.

Figure 2

Approaches and outcomes in screening combinatorial libraries for high-affinity TMDs. (A) Outline of library construction and screening. (B) Recurrent motifs as identified from different libraries where different interfacial residue patterns had been randomized with different sets of amino acids on different invariant host backgrounds. Ω, helix/helix crossing angle; aa, amino acid. The presence of GxxxG motifs in high-affinity TMDs suggests that the corresponding helix–helix pairs have negative crossing angles, even though a heptad repeat pattern underlying left-handed pairs had been randomized. In other words, parts of the heptad pattern can potentially form interfaces of right-handed structures.

Figure 3

Dynamics of membrane-embedded protein domains. (A) The activation of bitopic proteins upon binding of soluble ligands to extracellular domains has been proposed to involve the reorientation of transmembrane helices relative to each other about their long axis, reversible association/dissociation, and piston movements. (B) A comparison of X-ray structures of bovine rhodopsin and opsin reveals that TM5 elongates and moves closer to TM6 in the ligand-free and Gα-peptide (shown as space-filling representation) associated states when compared with the dark-adapted form containing cis–retinal (shown in orange).,, TM6 and TM7 are shown in green for better orientation. (C) A comparison of closed and open states of the small mechanosensitive channel MscS from E. coli indicates a large rearrangement of TMDs upon channel activation. The representations of the full structures permit a view down the pore; TM1 and TM2 of subunit A are in yellow and TM3 is in green for better orientation. The blow-up underneath the full structures shows the rearrangement of TM3 from subunits A and B after channel activation. Interfacial residues are in gray.