Oligomerization of Transmembrane Alpha-Helices Modulated By C-terminal Boundary Residues

Abstract

In studies of the structural biology of membrane proteins, the success of strategies based on the “divide-and-conquer” approach, where peptides are used to model the individual transmembrane (TM) α-helices of membrane proteins, depends upon the correct identification of the membrane-embedded TM α-helix amino acid sequence within the full-length protein. In the present work, we examine the effects of excluding or including TM boundary residues on the intrinsic properties of the TM2 α-helix of myelin proteolipid protein (PLP). Using protein gel electrophoresis, circular dichroism, and fluorescence resonance energy transfer in the membrane-mimetic detergent sodium dodecylsulfate (SDS) to study parent sequence KKKK-61AFQYVIYGTASFFFLYGALL-LAEG89-KKKK - along with analogs containing an additional wild type Phe-90, Phe-90 and Tyr-91, and a hydrophobic mutant Leu-90 - we demonstrate that the removal of a single amino acid from the C-terminus of this TM segment is sufficient to change its intrinsic properties, with TM2 61-89 displaying only a monomeric form, but with principally dimers arising for the other three peptides. The findings suggest that deletion of critical C-terminal residue(s) tends to re-position the helix terminus toward the membrane-aqueous interface, and emphasize the potential influence of boundary residues on TM properties when utilizing peptides as models for TM α-helices. These finding may implicate a role for such residues in membrane protein folding and assembly.