Sequence and Structure of the Membrane-Associated Peptide of Glycophorin A

Glycophorin A (GPA) has been reconstituted into dimyristoylphosphatidylcholine vesicles and digested with proteinase K to identify the membrane domain and to characterize its structure and orientation. After digestion of the inner and outer domain of GPA by protease action restricted to the aqueous phase, a protected peptide migrates on an electrophoresis gel as a 7.5-kDa dimer (His66-Ile95). The secondary structure and orientation in a lipid bilayer of the 7.5-kDa dimer have been studied by Fourier transform infrared spectroscopy. Our proteolytic and spectroscopic data are in agreement with a topological model in which the His66-Glu72 peptide adopts a beta-sheet conformation and is oriented parallel to the lipid-water interface and the Ile73-Ile95 domain is helical and oriented parallel to the lipid acyl chains, in a transmembrane configuration. Digestion of the domain protruding to the outside of the liposome generates "head-head" and "head-tail" dimers of 16 and 38 kDa, respectively. This observation is discussed in terms of the specificity of the dimer formation process.