Forced Transmembrane Orientation of Hydrophilic Polypeptide Segments in Multispanning Membrane Proteins

In a current model of integration of multispanning membrane proteins into the endoplasmic reticulum, it is proposed that the transmembrane segments show alternating translocation initiation and stop-transfer functions. Here, we present evidence for a mode of cotranslational insertion in which an internal signal-anchor sequence with N exo/C cyt topology confers a transmembrane disposition onto a preceding hydrophilic segment, resulting in a topology where the hydrophilic segment apparently can slip back and forth across the membrane. Our results demonstrate that hydrophobicity is not, as hitherto thought, an absolute requirement for the formation of a transmembrane segment, and suggest that integral membrane proteins may contain hydrophilic transmembrane segments with a considerable freedom to move in relation to the membrane.