Helical integrity and microsolvation of transmembrane domains from Flaviviridae envelope glycoproteins

Charged and polar amino acids in the transmembrane domains of integral membrane proteins can be crucial for protein function and also promote helix-helix association or protein oligomerization. Yet, our current understanding is still limited on how these hydrophilic amino acids are efficiently translocated from the Sec61/SecY translocon into the cell membrane during the biogenesis of membrane proteins. In hepatitis C virus, the putative transmembrane segments of envelope glycoproteins E1 and E2 were suggested to heterodimerize via a Lys-Asp ion-pair in the host endoplasmic reticulum. Therefore in this work, we carried out molecular dynamic simulations in explicit lipid bilayer and solvent environment to explore the stability of all possible bridging ion-pairs using the model of H-segment helix dimers. We observed that, frequently, several water molecules penetrated from the interface into the membrane core to stabilize the charged and polar pairs. The hydration time and amount of water molecules in the membrane core depended on the position of the charged residues as well as on the type of ion-pairs. Similar microsolvation events were observed in simulations of the putative E1-E2 transmembrane helix dimers. Simulations of helix monomers from other members of the Flaviviridae family suggest that these systems show similar behaviors. Thus this study illustrates the important contribution of water microsolvation to overcome the unfavorable energetic cost of burying charged and polar amino acids in membrane lipid bilayers. Also, it emphasizes the novel role of bridging charged or polar interactions stabilized by water molecules in the hydrophobic lipid bilayer core that has an important biological function for helix dimerization in several envelope glycoproteins from the family of Flaviviridae viruses.