Role of cholesterol in substrate recognition by $$\gamma$$-secretase

Abstract$$\gamma$$ γ -Secretase is an enzyme known to cleave multiple substrates within their transmembrane domains, with the amyloid precursor protein of Alzheimer’s Disease among the most prominent examples. The activity of $$\gamma$$ γ -secretase strictly depends on the membrane cholesterol content, yet the mechanistic role of cholesterol in the substrate binding and cleavage remains unclear. In this work, we used all-atom molecular dynamics simulations to examine the role of cholesterol in the initial binding of a direct precursor of $$\beta$$ β -amyloid polypeptides by $$\gamma$$ γ -secretase. We showed that in cholesterol-rich membranes, both the substrate and the enzyme region proximal to the active site induce a local membrane thinning. With the free energy methods we found that in the presence of cholesterol the substrate binds favorably to the identified exosite, while cholesterol depletion completely abolishes the binding. To explain these findings, we directly examined the role of hydrophobic mismatch in the substrate binding to $$\gamma$$ γ -secretase, showing that increased membrane thickness results in higher propensity of the enzyme to bind substrates. Therefore, we propose that cholesterol promotes substrate binding to $$\gamma$$ γ -secretase by increasing the membrane thickness, which leads to the negative hydrophobic mismatch between the membrane and binding partners.

TatA and TatB generate a hydrophobic mismatch that is important for function and assembly of the Tat translocon in Escherichia coli

The Tat system translocates folded proteins across energy-transducing prokaryotic membranes. In the bacterial model system Escherichia coli, the three components TatA, TatB, and TatC assemble to functional translocons. TatA and TatB both possess an N-terminal transmembrane helix (TMH) that is followed by an amphipathic helix (APH). The TMHs of TatA and TatB generate a hydrophobic mismatch with only 12 consecutive hydrophobic residues that span the membrane. We shortened or extended this stretch of hydrophobic residues in either TatA, TatB, or both, and analyzed effects on transport functionality and translocon assembly. The wild type length functioned best but was not an absolute requirement, as some variation was tolerated. Length-variation in TatB clearly destabilized TatBC-containing complexes, indicating that the 12-residues-length is crucial for Tat component interactions and translocon assembly. Metal tagging transmission electron microscopy revealed the dimensions of TatA assemblies, which prompted molecular dynamics simulations. These showed that interacting TMHs of larger TatA assemblies can thin the membrane together with laterally aligned tilted APHs that generate a deep V-shaped groove. The conserved hydrophobic mismatch may thus be important for membrane destabilization during Tat transport, and the exact length of 12 hydrophobic residues could be a compromise between functionality and proton leakage minimization.

Hydrophobic mismatch effect is a key factor in protein transport on the Tat pathway

The twin-arginine translocation (Tat) pathway transports folded proteins across membranes in bacteria, thylakoid, plant mitochondria, and archaea. In most species, the active Tat machinery consists of three independent subunits, TatA, TatB and TatC. TatA and TatB from all bacterial species possess short transmembrane alpha-helices (TMHs), both of which are only fifteen residues long in E. coli. Such short TMHs cause a hydrophobic mismatch between Tat subunits and the membrane bilayer. Here, by modifying the length of the TMHs of E. coli TatA and TatB, we access the functional importance of the hydrophobic mismatch in the Tat transport mechanism. Surprisingly, both TatA and TatB with as few as 11 residues in their respective TMHs are still able to insert into the membrane bilayer, albeit with a decline in membrane integrity. Three different assays, both qualitative and quantitative, were conducted to evaluate the Tat activity of the TMH length mutants. Our experiments indicate that the TMHs of TatA and TatB appear to be evolutionarily tuned to 15 amino acids, with activity dropping off with any modification of this length. We believe our study supports a model of Tat transport utilizing localized toroidal pores that form when the membrane bilayer is thinned to a critical threshold. In this context, the 15-residue length of the TatA and TatB TMHs can be seen as a compromise between the need for some hydrophobic mismatch to allow the membrane to reversibly reach the threshold thinness required for toroidal pore formation, and the permanently destabilizing effect of placing even shorter helices into these energy-transducing membranes.

Self-Organization of Alpha Helical Proteins in Bioinspired Membranes and Vesicles

<p><b>ABSTRACT</b></p><p>A fundamental understanding of protein-protein and protein-lipid interactions under various conditions can reveal the energy pathways in photosynthetic bacterial membranes. In this study, we examine the role of key factors such as bilayer curvature, the concentration, helical separation and hydrophobic mismatch of proteins on their self-organization in bilayers. We also develop an understanding of the physical factors underlying the aggregation of proteins. We determine the impact of bilayer curvature by comparing the aggregation of proteins in membranes and vesicles. We identify a threshold helical separation below which small, stable aggregates are observed. Large, unstable protein aggregates are observed above the threshold separation. We examine the effect of the deformations incurred by the proteins via their concentration, and show the aggregation of the proteins to arise from their deformation-induced displacement. We demonstrate the negative hydrophobic mismatch condition to favor a higher degree of protein aggregation. We adopt the Molecular Dynamics simulation technique along with a coarse-grained force field to capture the behavior spanning extensive spatiotemporal scales. Our results can guide experimental studies of bioinspired materials with structure-function properties mimicking those of photosynthetic bacterial membranes, or assist in understanding the organization of inclusions in polymeric matrices.</p>

Self-Organization of Alpha Helical Proteins in Bioinspired Membranes and Vesicles

<p><b>ABSTRACT</b></p><p>A fundamental understanding of protein-protein and protein-lipid interactions under various conditions can reveal the energy pathways in photosynthetic bacterial membranes. In this study, we examine the role of key factors such as bilayer curvature, the concentration, helical separation and hydrophobic mismatch of proteins on their self-organization in bilayers. We also develop an understanding of the physical factors underlying the aggregation of proteins. We determine the impact of bilayer curvature by comparing the aggregation of proteins in membranes and vesicles. We identify a threshold helical separation below which small, stable aggregates are observed. Large, unstable protein aggregates are observed above the threshold separation. We examine the effect of the deformations incurred by the proteins via their concentration, and show the aggregation of the proteins to arise from their deformation-induced displacement. We demonstrate the negative hydrophobic mismatch condition to favor a higher degree of protein aggregation. We adopt the Molecular Dynamics simulation technique along with a coarse-grained force field to capture the behavior spanning extensive spatiotemporal scales. Our results can guide experimental studies of bioinspired materials with structure-function properties mimicking those of photosynthetic bacterial membranes, or assist in understanding the organization of inclusions in polymeric matrices.</p>

Membrane Electrostatics Sensed by Tryptophan Anchors in Hydrophobic Model Peptides Depends on Non-Aromatic Interfacial Amino Acids: Implications in Hydrophobic Mismatch

WALPs are synthetic α-helical membrane-spanning peptides that constitute a well-studied system for exploring hydrophobic mismatch. These peptides represent a simplified consensus motif for transmembrane domains of intrinsic membrane proteins due...

Channelrhodopsin-2 Function is Modulated by Residual Hydrophobic Mismatch with the Surrounding Lipid Environment

Channelrhodopsin-2 (ChR2) is a light-gated ion channel that conducts cations of multiple valencies down the electrochemical gradient. This light-gated property has made ChR2 a popular tool in the field of optogenetics, allowing for the spatial and temporal control of excitable cells with light. A central aspect of protein function is the interaction with the surrounding lipid environment. To further explore these membrane-protein interactions, we demonstrate the role of residual hydrophobic mismatch (RHM) as a mechanistically important component of ChR2 function. We combined computational and functional experiments to understand how RHM between the lipid environment and ChR2 alters the structural and biophysical properties of the channel. Analysis of our results revealed significant RHM at the intracellular/lipid interface of ChR2 from a triad of residues. The resulting energy penalty is substantial and can be lowered via mutagenesis to evaluate the functional effects of this change in lipid-protein interaction energy. The experimental measurement of channel stability, conductance and selectivity resulting from the reduction of the RHM energy penalty showed changes in progressive H+ permeability, kinetics and open-state stability, suggesting how the modulation of ChR2 by the surrounding lipid membrane can play an important biological role and contribute to the design of targeted optogenetic constructs for specific cell types.