Understanding Protein Organization in Membranes via Thermodynamic Characterization of Transmembrane Helix Insertion and Association

Abstract

The insertion and association of membrane proteins are important in several cellular processes. Most of these processes were thought to be protein-driven, but increasing evidence points towards a large role of the membrane. For instance, the “lipophobic” contribution to insertion, analogous to the hydrophobic effect, has been suggested to contribute to the association of membrane peptides. However, the main driving forces have not been thermodynamically quantified. Here, we study the insertion of a polyalanine peptide into a lipid bilayer and estimate the free energy of insertion, as well as the lipophobic component. Free energy calculations have been performed using a coarse-grain force-field for each of the individual coarse-grain beads and polyalanine peptides of increasing length. As expected, the charged and polar moieties have the least favorable free energy of insertion, and the highest lipophobic component. A length-dependence is observed in the polyalanine peptides with the lipophobic component increasing non-linearly with peptide length. The effect of membrane fluidity has been tested by varying temperature and lipid type. The lipophobic contribution increases with decreasing membrane fluidity, although the total free energy of insertion is variable. The results are an important step in estimating the membrane effects in protein insertion and association.