Solution Conformation of the Extracellular Domain of the Human Tumor Necrosis Factor Receptor Probed by Raman and UV-Resonance Raman Spectroscopy: Structural Effects of an Engineered PEG Linker

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Abstract
The solution structure of the Escherichia coli-expressed extracellular domain, residues 12-172, of the human 55 kDa type I tumor necrosis factor receptor (TNFR) has been probed by Raman (514.5 nm) and ultraviolet-resonance Raman (244 nm) excitations. The Raman spectra have been collected from both the free TNFR domain and an engineered "dumbbell-like" derivative, consisting of two mutant receptor moieties linked by a 20 kDa polyethylene glycol (PEG) tether. The results demonstrate a TNFR secondary structure which is rich in beta-sheet and deficient in alpha-helix, consistent with the reported X-ray crystal structure of baculovirus expressed receptor complexed with factor beta [Banner, D. W., D'Arcy, A., Janes, W., Gentz, R., Schoenfeld, H.-J., Broger, C., Loetscher, H., & Lesslauer, W. (1993) Cell 73, 431-445]. Conversely, the solution structure of TNFR differs from the crystal structure in its distribution of disulfide rotamers and in the orientation of its unique indole side chain (tryptophan-107). These differences are attributed, respectively, to N-terminal truncation and factor binding in the TNFR crystal structure. The tryptophan configuration, which is easily monitored in both Raman and UVRR spectra, is proposed as a potential signal of receptor/factor recognition and binding. Application of the Raman probes to the engineered TNFR dumbbell, which is of interest as a potential therapeutic, shows that TNFR moieties of the dumbbell exhibit secondary structures and side chain environments which are indistinguishable from those of the native, wild-type moiety. The results suggest that the PEGylated dumbbell may function as an effective TNFR drug delivery system without the consequence of a deleterious antigenic response.

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