Multiple site-specific infrared dichroism of CD3-ζ, a transmembrane helix bundle

The structure of the transmembrane domain of CD3-ζ a component of the T-cell receptor involved in signal transduction, has been studied in its native state (a lipid bilayer) by multiple site-specific infrared dichroism. For the first time, the transmembrane domain has been labelled at multiple positions along the sequence, representing a total of 11 samples, each labelled at a different residue with an isotopically modified carbonyl group, ¹³C==¹⁸O. A strategy is outlined that, based on the above data, can yield the rotational orientation and the local helix tilt for each labelled residue, giving a detailed description of helix geometry. The results obtained indicate that the transmembrane segment is in an α-helical conformation throughout, with an average helix tilt of 12°. The N-terminal side of the helix is more tilted than the C-terminal. In an accompanying paper we describe the implementation of the infrared data in a model-building study of the CD3-ζ transmembrane complex. The model obtained is entirely consistent with results based on evolutionary conservation data. Taken together, this study represents the first step towards elucidation of the backbone structure of a transmembrane α-helical bundle by infrared spectroscopy.