Formation of cytoplasmic turns between two closely spaced transmembrane helices during membrane protein integration into the ER membrane

The helical hairpin, two closely spaced transmembrane helices separated by a short turn, is a recurring structural element in integral membrane proteins, and may serve as a compact unit that inserts into the membrane en bloc. Previously, we have determined the propensities of the 20 natural amino acids, when present in the middle of a long hydrophobic stretch, to induce the formation of a helical hairpin with a lumenally exposed turn during membrane protein assembly into the endoplasmic reticulum membrane. Here, we present results from a similar set of measurements, but with the turn placed on the cytoplasmic side of the membrane. We find that a significantly higher number of turn-promoting residues need to be present to induce a cytoplasmic turn compared to a lumenal turn, and that, in contrast to the lumenal turn, the positively charged residues Arg and Lys are the strongest turn-promoters in cytoplasmic turns. These results suggest that the process of turn formation between transmembrane helices is different for lumenal and cytoplasmic turns.