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The Aromatic Residues Trp and Phe Have Different Effects on the Positioning of a Transmembrane Helix




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The Aromatic Residues Trp and Phe Have Different Effects on the Positioning of a Transmembrane Helix in the Microsomal Membrane Paula Braun and Gunnar von Heijne* Department of Biochemistry, Stockholm UniVersity, S-106 91 Stockholm, Sweden ReceiVed April 23, 1999; ReVised Manuscript ReceiVed June 3, 1999 ABSTRACT: We have examined the effect of Trp and Phe residues on the positioning of a poly-Leu transmembrane helix relative to the microsomal membrane by employing a previously described “glycosylation mapping” technique [Nilsson, I. M., Sääf, A., Whitley, P., Gafvelin, G., Waller, C., and von Heijne, G. (1998) J. Mol. Biol. 284, 1165-1175]. Both Trp and Phe tend to push the transmembrane helix into the membrane when inserted in positions flanking the poly-Leu stretch, and Trp (but not Phe) pulls the transmembrane helix toward the lipid-water interface when inserted inside the poly-Leu segment. Thus, the preference of Trp for the lipid-water interface previously suggested on the basis of biophysical studies of model peptides can also be observed for a bona fide transmembrane helix in a biological membrane. We further show that a sufficiently long poly-Trp segment functions as an efficient stop- transfer sequence during protein translocation across the microsomal membrane, despite the preference of Trp residues for the lipid-water interface region. Statistical analysis of the few existing high-resolution structures of R-helical integral membrane proteins shows that the occurrence of aromatic residues in transmembrane R-helices is remarkably position-dependent (1). Trp and Tyr are enriched near the ends of the helices, suggesting that they interact favorably with the lipid headgroups. Phe, in contrast, is more abundant in the central core region of the transmembrane helices. A number of recent biophysical studies have investigated the apparent affinity of aromatic residues for the lipid-water interface or central hydrophobic regions of phospholipid membranes. Data for small model peptides show that the partitioning of Trp into the interface of lipid bilayers is highly favorable (2, 3), and NMR measurements with Trp analogues lacking polar groups in the indol ring demonstrate that partitioning is not much affected by these groups (4). Hence, both aromaticity, which is thought to favor partitioning into the electrostatically complex interface environment, and unfavorable interactions of the flat, rigid Trp side chain with the hydrocarbon core seem to be responsible for the residue’s preference for the interface (4). Other in vitro studies have addressed the effect of Trp on the membrane insertion and orientation of synthetic model peptides (5) and its role in hydrophobic matching between transmembrane peptides and lipid bilayers (6). While in vitro studies with various model systems thus support a role for Trp and Tyr residues in positioning transmembrane helices relative to the lipid bilayer, it has so far not been possible to study this phenomenon in a biological membrane. Here, we have compared the effect of Trp and Phe residues on the positioning of a poly-Leu transmembrane helix in microsomal membranes derived from dog pancreas endoplasmic reticulum (ER),1 using the recently introduced “glycosylation mapping” technique. This technique allows any transmembrane segment to be positioned ...





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