Proline-induced disruption of a transmembrane alpha-helix in its natural environment

alpha-Helix formation in globular proteins has been studied both theoretica lly and experimentally for decades, while a lack of both high-resolution st ructures and suitable experimental techniques has hampered the study of hel ices in membrane proteins. We have developed a new experimental approach, g lycosylation mapping, where the active site of the lumenally exposed endopl asmic reticulum enzyme oligosaccharyl transferase is used as a point of ref erence against which the position of a transmembrane segment in the membran e can be measured. Here, we report an initial analysis of the helix-breakin g properties of proline residues inserted in a transmembrane helix. We find that proline residues can break a transmembrane helix, but only when inser ted near the end, and only when the helix is sufficiently long. The glycosy lation mapping technique may be generally useful for determining the positi on of transmembrane helices in the membrane. (C) 1998 Academic Press.