Formation of helical hairpins during membrane protein integration into theendoplasmic reticulum membrane. Role of the N and C-terminal flanking regions

The helical hairpin, two closely spaced transmembrane helices separated by a short turn, is a common structural element in integral membrane proteins. Previous studies on the sequence determinants of helical hairpin formation have focussed on the role of polar and charged residues placed centrally i n a long stretch of hydrophobic residues, and have yielded a "propensity sc ale" for the relative efficiency with which different residues promote the formation of helical hairpins. In this study, we shift our attention to the role of charged residues flanking the hydrophobic stretch. Clusters of cha rged residues are known to hinder membrane translocation, and thus flanking charged residues may conceivably force a long hydrophobic segment to form a helical hairpin even if there are no or only weakly turn-promoting residu es in the hydrophobic stretch. We indeed find that Lys and, more surprising ly, Asp residues strongly affect helical hairpin formation when placed next to a poly-Leu-based transmembrane segment. We also find that a cluster of four consecutive Lys residues can affect the efficiency of helical hairpin formation even when placed similar to 30 residues downstream of the hydroph obic stretch. These observations have interesting implications for the way we picture membrane protein topogenesis within the context of the endoplasm ic reticulum (ER) translocon. (C) 2001 Academic Press.