Breaking the camel’s back: proline-induced turns in a model transmembrane helix

We have constructed model membrane proteins with hydrophobic segments of the general composition Leu29 Val-Leu n where n = 10 and 20, and have analyzed their transmembrane topology when inserted into microsomal membranes. These hydrophobic segments span the membrane once, even though they are twice as long as normal transmembrane helices. Strikingly, a single proline residue introduced near the center of the Leu39 Val hydrophobic stretch induces the formation of two transmembrane segments separated by a tight turn. These results have implications for our understanding of membrane protein assembly in the endoplasmic reticulum, and for the development of techniques for predicting membrane protein topology.