NMR conformational study of the cytoplasmic domain of the canine Sec61 gamma protein from the protein translocation pore of the endoplasmic reticulum membrane

Conformational studies of the synthesized N-terminal cytoplasmic domain of the canine Sec61 gamma protein, an essential protein from the translocation pore of secretory proteins across the endoplasmic reticulum membrane, were performed using two-dimensional proton NMR spectroscopy. This canine domain is one of the smallest domains within the homologous protein family and may thus constitute the minimal functional structure. The peptide was solubilized in pure aqueous solution or in the presence of dodecylphosphocholine micelles mimicking a membrane-solution interface. In pure aqueous solution, the peptide is remarkably unfolded. Forming a stable complex with dodecylphosphocholine micelles, it acquires a well-defined alpha-helix-loop-alpha-helix secondary structure, with the first helix, highly amphipathic, lying at the micelle surface. The loop comprising four residues is delimited by two flanking helix-capping structures, highly conserved in the whole homologous protein family. No tertiary structure, which could have been revealed by interhelix NOE contacts, was observed. From these experimental results and using general arguments based on sequence information and knowledge of peptide-membrane interactions, a structure of the entire Sec61 gamma protein in membrane bilayers is proposed.