Transmembrane topography of plasma membrane constituents in mung bean (Vigna radiata L.) hypocotyl cells. II. The large scale asymmetry of surface peptides.

The large scale asymmetry in surface (poly)peptides of the plasma membrane (PM) of mung bean (Vigna radiata L.) hypocotyl cells was investigated by protease and 1 M KCl treatments of PM vesicles obtained by an aqueous two-phase partition technique. Proteases only slightly reduced the protein content of right-side-out PM vesicles and the treatment with 1 M KCl resulted in the dissociation of only a few peripheral proteins from the outer surface of right-side-out PM vesicles, indicating that few surface peptides including peripheral proteins existed on the outer surface. From experiments of the re-partitioning of endomembrane vesicles removed from surface peptides, it was found that the surface peptide content is a factor determining the partitioning, and the hypothesis that sterols are asymmetrically distributed across higher plant PM was proposed. We speculate that asymmetrical properties between the outer and the inner surfaces of plant PM, especially in partitioning in the two-phase system, derive from the asymmetry of the bulk of surface peptides and PM sterols. The comparatively low hydrophilicity of the outer surface of the PM would be important for the partitioning of right-side-out PM vesicles in the upper phase of the two-phase system.