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



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9778 Biochemistry 1999 38 9778 9782 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 Sa a f 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 stoptransfer 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



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