Structural determination of membrane proteins by NMR spectroscopy remains a challenge, especially for helical membrane proteins. Here we report the NMR assignment and secondary structure of a 31 kDa helical membrane protein, the C-terminal domain of Stt3p. The C-terminal domain of Stt3p has been proposed to be the catalytic domain of yeast oligosaccharyl transferase (OT), a multisubunit membrane-associated enzyme complex catalyzing N-glycosylation, which is an essential and highly conserved protein modification. NMR assignment is the first critical step in the determination of the high-resolution solution structure and further structure-function studies.