Membrane Protein Folding: Modulating the Interactions between Transmembrane Alpha-helices

• Main Author: Ng, Derek
• Other Authors: Deber, Charles M.
• Language: en_ca
• Published: 2013
• Subjects: Membrane protein; TM alpha helices; Helix interactions; PLP; Proteolipid protein; Membrane; Bilayer; Detergent; 0307; 0317; 0487
• Online Access: http://hdl.handle.net/1807/43682

The fundamental process by which an alpha-helical membrane protein attains its ultimate structure has previously been depicted as two energetically distinct stages where (1) the transmembrane (TM) segments are first threaded into the membrane bilayer as stable alpha-helices; and then (2) laterally interact to form the correct tertiary and/or quaternary structures. Central to the second stage of this model is the presence of amino acid sequence motifs in the TM segments that provide interaction-compatible surfaces through which the TM alpha-helices interact. Although these ideas have proven to be pivotal to the progress of the membrane protein folding field, a growing number of examples indicates that a variety of additional factors work together to dictate the ultimate interaction fate of TM embedded segments. In this context, we expand on these factors and explore other properties that can modulate the association of TM alpha-helices. A peptide model of myelin proteolipid protein (PLP) TM4 is capable of TM helix-helix interactions in SDS and biological membranes. Increasing the side chain volumes of two disease relevant residues (Ala242 and A248) reduces peptide self-association, indicating that these sites mediate TM helix packing through van der Waals interactions. Examination of the PLP TM2 alpha-helix shows that it is also capable of self-association and that its dimeric state depends on the presence or absence of residues at its C-terminus. Specifically, this sensitivity was attributed to changes in local hydrophobicity; a decrease in hydrophobicity likely reduces detergent-peptide interactions, which disrupts peptide alpha-helicity and the effectiveness of a nearby interaction compatible surface. We take advantage of this finding to determine the feasibility of coupling helix-helix interactions to an external factor such as pH. Our results indicate that pH can indeed modulate the dimerization state of the TM2 peptide and does so through the change in protonation state of Glu88. Increasing our knowledge of the variables contributing to TM helix-helix interactions provides valuable insights into membrane protein folding and how mutations can compromise this process. This knowledge will allow us to expand our arsenal of approaches to counter membrane protein misassembly--and ultimately human disease.