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 domai n of the canine Sec61 gamma protein, an essential protein from the tra nslocation pore of secretory proteins across the endoplasmic reticulum membrane, were performed using two-dimensional proton NMR spectroscop y. This canine domain is one of the smallest domains within the homolo gous protein family and may thus constitute the minimal functional str ucture. The peptide was solubilized in pure aqueous solution or in the presence of dodecylphosphocholine micelles mimicking a membrane-solut ion interface. In pure aqueous solution, the peptide is remarkably unf olded. Forming a stable complex with dodecylphosphocholine micelles, i t acquires a well-defined alpha-helix-loop-alpha-helix secondary struc ture, with the first helix, highly amphipathic, lying at the micelle s urface. The loop comprising four residues is delimited by two flanking helix-capping structures, highly conserved in the whole homologous pr otein family. No tertiary structure, which could have been revealed by interhelix NOE contacts, was observed. From these experimental result s and using general arguments based on sequence information and knowle dge of peptide-membrane interactions, a structure of the entire Sec61 gamma protein in membrane bilayers is proposed.