The membrane topography of the diphtheria toxin T domain linked to the a chain reveals a transient transmembrane hairpin and potential translocation mechanisms.
The diphtheria toxin T domain helps translocate the A chain of the toxin across membranes. To gain insight into translocation, the membrane topography of key residues in T domain attached to the A chain (AT protein) was compared to that in the isolated T domain using fluorescence techniques. This study demonstrates that residues in T domain hydrophobic helices (TH5-TH9) tended to be less exposed to aqueous solution in the AT protein than in the isolated T domain. Under conditions in which the loop connecting TH5 to TH6/7 is located stably on the cis (insertion) side of the membrane in the isolated T domain, it moves between the cis and trans sides of the membrane in the AT protein. This is indicative of the formation of a dynamic, transient transmembrane hairpin topography by TH5-TH7 in the AT protein. Since TH8 and TH9 also form a transmembrane hairpin, this means that TH5-TH9 may form a cluster of transmembrane helices. These helices have a nonpolar surface likely to face the lipid bilayer in a helix cluster and a surface rich in uncharged hydrophilic residues which in a helix cluster would likely be facing inward (and perhaps be pore-lining). This uncharged hydrophilic surface could play a crucial role in translocation, interacting transiently with the translocating A chain. A similar motif can be found in, and may be important for, other protein translocation systems.
Wang
Jie
J
Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, New York 11794-5215, USA.
London
Erwin
E
D004167:Diphtheria Toxin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D017434:Protein Structure, Tertiary / Q000235:genetics / Q000502:physiology; D021381:Protein Transport / Q000235:genetics / Q000502:physiology*; D013050:Spectrometry, Fluorescence
D004167:Diphtheria Toxin; D008051:Lipid Bilayers