How membrane surface affects protein structure

Abstract

The immediate environment of the negatively charged membrane surface is characterized by decreased dielectric constant and pH value. These conditions can be modeled by water-alcohol mixtures at moderately low pH. Several globular proteins were investigated under these conditions, and their conformational behavior in the presence of phospholipid membranes was determined, as well as under conditions modeling the immediate environment of the membrane surface. These proteins underwent conformational transitions from the native to a molten globule-like state. Increased flexibility of the protein structure facilitated protein functioning. Our experimental data allow understanding forces that affect the structure of a protein functioning near the membrane surface (in other words, in the membrane field). Similar conformational states are widely reported in the literature. This indicates that the negatively charged membrane surface can serve as a moderately denaturing agent in the cell. We conclude that the effect of the membrane field on the protein structure must be taken into account.

Structural and dynamic characterization of the Golgi Reassembly and Stacking Protein (GRASP) in solution

“…Indeed, a pronounced decrease in the secondary structure content of CnGRASP is observed by increasing or decreasing pH values ( Figure 22D), with the maximal structural content close to the pH used in the working buffer solution. However, since this behavior is not significantly different from those observed for other well-structured proteins [158,197], changes in ±2 units from the neutral pH do not do not affect significantly the CnGRASP structure, even though this protein is membrane associated and the pH can be more acidic at the lipid/water interface [210]. The very broad CD spectra obtained at pHs 4 and 5 ( Figure 22D) reflect protein aggregation, since these pH values are close to the theoretical pI of CnGRASP (~ 4.9).…”

“…Variations in ε can partially denature well-structured proteins [214,215], increase the secondary structure content of many different IDPs [91,190,216,217] and induce, in some well-folded proteins, a transformation to a molten globulelike state in aqueous solution under mild denaturing conditions [210,218]. The latter phenomenon is hypothesized to be important for protein translocation across the membrane [219,220,221].…”

“…Because of this specific localization, it is expected that disturbances in the physicochemical parameters induced by biological membranes can have a great influence on GRASP structure and function. Membranes may act as denaturing agents for some proteins in the cell [210,211,212] due mainly to changes in the dielectric constant (ε) and interfacial pH values, i.e, the "membrane field". Typically, the hydrophobic core of biological membranes, comprised by the lipid acyl chains, presents very low ε values (around 2-4), whereas the bulk solvent exhibits ε of ~ 80 [210,213].…”

“…Membranes may act as denaturing agents for some proteins in the cell [210,211,212] due mainly to changes in the dielectric constant (ε) and interfacial pH values, i.e, the "membrane field". Typically, the hydrophobic core of biological membranes, comprised by the lipid acyl chains, presents very low ε values (around 2-4), whereas the bulk solvent exhibits ε of ~ 80 [210,213]. Therefore, a dielectric gradient is observed at the membrane/water interface, which can be modelled by an exponentially increasing function from ε = 2-4 at the lipid bilayer to 80 at approximately 5-6 nm from the interface [213].…”

“…Because regular proteins are of this order of size, the ε gradient can have a great impact on protein structure close to membrane surface [210]. Variations in ε can partially denature well-structured proteins [214,215], increase the secondary structure content of many different IDPs [91,190,216,217] and induce, in some well-folded proteins, a transformation to a molten globulelike state in aqueous solution under mild denaturing conditions [210,218].…”