Amphipathic Helices as Mediators of the Membrane Interaction of Amphitropic Proteins, and as Modulators of Bilayer Physical Properties

Abstract

The amphipathic helix (AH) motif is used by a subset of amphitropic proteins to accomplish reversible and controlled association with the interfacial zone of membranes. Functioning as more than mere membrane anchoring domains, amphipathic helices can serve as autoinhibitory domains to suppress the protein activity in its soluble form, and as sensors or modulators of membrane curvature. Thus amphipathic helices can both respond to and modulate membrane physical properties. These and other features are illustrated by the behavior of CTP: phosphocholine cytidylyltransferase (CCT), a key regulatory enzyme in PC synthesis. A comparison of the physico-chemical features of CCT's AH motif and 10 others reveals similarities and several differences. The importance of these parameters to the particulars of the membrane interaction and to functional consequences requires more systematic exploration. The membrane partitioning of amphitropic proteins with AH motifs can be regulated by various strategies including changes in membrane lipid composition, phosphorylation, ligand-induced conformational changes, and membrane curvature. Several amphitropic proteins that control budding or tubule formation in cells have AH motifs. The insertion of the hydrophobic face of these amphipathic helices generates an asymmetry in the lateral pressure of the two leaflets resulting in an induction of positive curvature. Curvature induction or stabilization may be a universal property of AHA proteins, not just those involved in budding, but this possibility requires further demonstration.

Structure and topology of monomeric phospholamban in lipid membranes determined by a hybrid solution and solid-state NMR approach

“…In fact, the L-shaped membrane architecture of PLN is reminiscent of the structures and topologies of the FXYD proteins (a family of membrane associated proteins that serve as subunits to Na,K-ATPases) (40), the fd coat protein (responsible for viral assembly) (56), the VpU protein from HIV-1 (57), and the M2 channel from the inf luenza A virus (58). Amphipathic helix motifs are known to be functionally important in a number of capacities, ranging from stabilizing protein structures to sensing changes in the physical properties of the membrane (59,60).…”

Structural Basis for Autoinhibition of CTP:Phosphocholine Cytidylyltransferase (CCT), the Regulatory Enzyme in Phosphatidylcholine Synthesis, by Its Membrane-binding Amphipathic Helix

“…Replacement with other residues (even conservative arginine), cripples k cat /K m by 5 orders of magnitude (9). 4 The membrane binding domain contains an inducible amphipathic helix (domain M) that detects physical properties of a PC-deficient membrane, such as high negative charge density and lipid packing stress (11)(12)(13)(14). An encounter with such a membrane surface triggers insertion of the hydrophobic face of the amphipathic helix (15) and transduces a conformational change in the catalytic domain associated with an increase in k cat /K m of nearly 3 orders of magnitude (16 -18).…”

Structural basis for membrane binding and catalytic activation of the peripheral membrane enzyme pyruvate oxidase from Escherichia coli

“…Proteins that adhere directly to the biological membrane are termed amphitropic proteins and can attach to the bilayer through interaction of amphipathic helices, hydrophobic loops, ions, or covalently attached lipids (1,2). In many cases studied, these proteins exhibit a very low basal membrane affinity, becoming recruited to the membrane from the cytosol only after a conformational transition or electrostatic switch that not only triggers membrane binding but may also initiate or elevate biological activity (3).…”