title: Terminal residue hydrophobicity modulates transmembrane helix-helix interactions.
abstract: Central to the formation of tertiary structure in membrane protein folding is the presence of amino acid sequence motifs (such as "small-XXX-small" segments) in the TM segments that promote interaction-compatible surfaces through which the TM α-helices interact. Here, we sought to elucidate additional factors that may work in tandem to dictate the ultimate interaction fate of TM-embedded segments. In this context, we used proteolipid protein (PLP), the major protein from central nervous system myelin for which mutant-dependent non-native oligomerization has been implicated in neurological disorders, to explore the specific effects of TM boundary residues (the membrane entry and exit points), keying on the secondary structure and self-association of peptides corresponding to the PLP TM2 α-helix (wild-type sequence ⁶⁶AFQYVIYGTASFFFLYGALLLAEGF⁹⁰). Using gel electrophoresis, circular dichroism, and Förster resonance energy transfer in the membrane-mimetic detergent sodium dodecyl sulfate (SDS), we found that mutation of F90 to residues such as A, I, L, or V maintains the onset of TM2-TM2 dimerization, whereas mutation to E, G, Q, N, S, or T abrogates dimer formation. We attribute this sensitivity to changes in local hydrophobicity, viz., a decrease in hydrophobicity reduces local lipid-peptide interactions, which in turn disrupts peptide α-helicity and hence the effectiveness of an incipient interaction-compatible surface. Our results show that the secondary structure and oligomeric state of PLP TM2 Lys-tagged peptides are significantly modulated by the specific nature of their C-terminal boundary residue, thus providing insight as to how point mutations, particularly where they produce disease states, can compromise the folding process.
authors.affiliation: Division of Molecular Structure & Function, Research Institute, Hospital for Sick Children , Toronto, Ontario M5G 0A4, Canada.
authors.forename: Derek P
authors.initials: DP
authors.lastname: Ng
authors.forename: Charles M
authors.initials: CM
authors.lastname: Deber
mesh_terms: D019943:Amino Acid Substitution; D002942:Circular Dichroism; D003902:Detergents / Q000737:chemistry; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D031541:Fluorescence Resonance Energy Transfer; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D008051:Lipid Bilayers / Q000737:chemistry* / Q000378:metabolism; D008823:Micelles; D008958:Models, Molecular; D018991:Myelin Proteolipid Protein / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017354:Point Mutation; D017510:Protein Folding; D054730:Protein Interaction Domains and Motifs; D011499:Protein Processing, Post-Translational; D055550:Protein Stability; D017433:Protein Structure, Secondary; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry
chemical_list: D003902:Detergents; D008051:Lipid Bilayers; D008823:Micelles; D018991:Myelin Proteolipid Protein; C472930:PLP1 protein, human; D010446:Peptide Fragments; D012967:Sodium Dodecyl Sulfate