Modulating Transmembrane α-Helix Interactions through pH

Abstract

The folding and assembly of α-helical integral membrane proteins into functional molecules is largely mediated by the lateral interactions of its transmembrane (TM) α-helices where sequence motifs often maximize van der Waals packing and/or polar interactions at sites of helix-helix contact. These interactions play a major role in determining the correct formation of tertiary and quaternary structure for these hydrophobic proteins. We have previously shown that a Lys-tagged TM2 α-helix of myelin proteolipid protein (PLP), of parent sequence, KKKK-AFQYVIYGTASFFFLYGALLLAEGF-KKKK, is capable of self-dimerization in the membrane mimetic detergent sodium dodecylsulfate (SDS). In the present work, we use protein gel electrophoresis, circular dichroism, and fluorescence resonance energy transfer to demonstrate that the oligomeric state of this peptide can be modulated by pH. Specifically, as the pH is increased, the peptide shifts from dimer to monomer. Mutational analysis reveals that this functionality is due to the C-terminal glutamic acid. Our overall results emphasize the potential influence of surrounding residues on the interaction strength of TM helix-helix contact sites that mediate oligomerization and may have implications for the control of biologically relevant events involving helix-helix interactions in some membrane proteins.