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Biophysical Journal 84:1263-1271 (2003)
© 2003 The Biophysical Society

Role of Side-Chain Conformational Entropy in Transmembrane Helix Dimerization of Glycophorin A

Wei Liu*, Evan Crocker{dagger}, David J. Siminovitch{ddagger} and Steven O. Smith*

* Department of Biochemistry and Cell Biology and {dagger} Department of Physics and Astronomy, Center for Structural Biology, SUNY Stony Brook, Stony Brook, New York 11794-5115 USA and {ddagger} Department of Physics, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada

Correspondence: Address reprint requests to Steven O. Smith, Dept. of Biochemistry and Cell Biology, Center for Structural Biology, SUNY Stony Brook, Stony Brook, NY 11794-5115. Tel.: 631-632-1210; Fax: 631-632-8575; E-mail: steven.o.smith{at}sunysb.edu.

Dimerization of the transmembrane domain of glycophorin A is mediated by a seven residue motif LIxxGVxxGVxxT through a combination of van der Waals and hydrogen bonding interactions. One of the unusual features of the motif is the large number of ß-branched amino acids that may limit the entropic cost of dimerization by restricting side-chain motion in the monomeric transmembrane helix. Deuterium NMR spectroscopy is used to characterize the dynamics of fully deuterated Val80 and Val84, two essential amino acids of the dimerization motif. Deuterium spectra of the glycophorin A transmembrane dimer were obtained using synthetic peptides corresponding to the transmembrane sequence containing either perdeuterated Val80 or Val84. These data were compared with spectra of monomeric glycophorin A peptides deuterated at Val84. In all cases, the deuterium line shapes are characterized by fast methyl group rotation with virtually no motion about the C{alpha}-Cß bond. This is consistent with restriction of the side chain in both the monomer and dimer due to intrahelical packing interactions involving the ß-methyl groups, and indicates that there is no energy cost associated with dimerization due to loss of conformational entropy. In contrast, deuterium NMR spectra of Met81 and Val82, in the lipid interface, reflected greater motional averaging and fast exchange between different side-chain conformers.




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